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X
- •Foreword
- •Acknowledgments
- •Editor biographies
- •Yi Wang
- •X. Sharon Qi
- •List of contributors
- •1.2.3 Feature engineering and representation
- •1.2.4 Linear separability
- •1.2.5 Classical models
- •1.1 A brief introduction to AI
- •1.2 Machine learning basics
- •1.2.1 Learning paradigms
- •1.3 Artificial neural networks
- •1.3.1 Feed-forward neural networks
- •1.3.2 Recurrent neural networks
- •1.3.3 Convolutional neural networks
- •1.3.4 Attention
- •1.3.5 Training neural networks
- •1.3.6 Applications and use cases of deep learning
- •1.4 Model training and evaluation
- •1.4.1 Hyperparameters
- •1.4.2 Data split
- •1.4.3 Evaluation metrics
- •1.5 Generative models
- •1.5.1 Generative adversarial networks
- •1.5.2 Diffusion models
- •1.5.3 Applications and use cases
- •1.6 Ethical consideration and bias
- •1.6.1 Transparency and explainability
- •1.6.2 Bias and fairness
- •1.6.3 Data privacy violation
- •1.6.4 Risk and misuse
- •1.7 Summary
- •References
- •2.1 Introduction
- •2.1.2 Staff roles in radiation therapy
- •2.2 Overview of AI in radiation therapy
- •2.2.1 Patient evaluation and dose prescription
- •2.2.2 Treatment simulation
- •2.2.3 Contouring
- •2.2.4 Treatment planning
- •2.2.5 Quality assurance
- •2.2.6 Treatment delivery
- •2.2.7 Response assessment and toxicity management
- •2.3 Summary
- •3.1 Introduction
- •3.1.1 Introduction of clinical decision making and AI
- •3.1.2 The role of AI in clinical decision making
- •3.2 AI algorithms for clinical decision making
- •3.2.2 Radiomics
- •3.2.3 Data integration by AI
- •3.2.4 Interpretability of AI models
- •3.3 Application of AI in clinical decision making
- •3.3.1 Diagnosis and disease phenotyping
- •3.3.2 Personalized treatment
- •3.3.3 Treatment outcome and prognosis prediction
- •3.4 Challenges and future directions of AI in clinical decision making
- •3.4.1 Challenges and concerns
- •3.4.2 Future directions
- •3.5 Summary
- •References
- •4.1 Introduction
- •4.2 Imaging for treatment planning
- •4.2.1 CT simulation
- •4.2.2 4D-CT
- •4.2.3 PET/CT
- •4.2.4 MRI
- •4.3 Imaging for treatment guidance
- •4.3.1 Portal imaging
- •4.3.2 CBCT
- •4.3.3 CT-on-rail and CT-linac
- •4.3.4 MR-linac
- •4.3.5 PET-linac
- •4.4 Imaging for motion management
- •4.4.1 ExacTrac
- •4.4.2 Varian triggered imaging
- •4.4.3 4D-CBCT
- •4.4.4 Cine MRI
- •4.4.5 4D-MRI
- •4.4.6 Surface imaging
- •4.5 Imaging for treatment assessment
- •4.5.1 Contrasted CT
- •4.5.2 PET/CT
- •4.5.3 Functional MRI
- •4.6 Summary
- •5.1 Introduction to big data in radiation oncology
- •5.1.1 Overview of big data
- •5.1.2 Sources of big data in radiation oncology
- •5.1.3 Big data and AI in radiation oncology
- •5.2 Big data lifecycle in radiation oncology
- •5.2.1 Data aggregation and storage
- •5.2.2 Data sharing and security
- •5.4 The application of big data in radiation oncology
- •5.4.1 Medical image segmentation
- •5.4.2 Automatic treatment planning
- •5.4.3 Treatment response prediction
- •5.4.4 Quality assurance and patient safety
- •5.4.5 Clinical decision support
- •5.2.3 Data visualization
- •5.2.4 Knowledge creation and implementation
- •5.2.5 Data archiving and deletion
- •5.3 Big data analytics with AI
- •5.3.1 Data processing and integration
- •5.3.2 AI modeling
- •5.5 Challenges and future perspectives
- •5.6 Summary
- •Reference
- •6.1 The road to ART
- •6.1.1 3D conformal radiotherapy (3DCRT)
- •6.1.2 Intensity modulated radiotherapy (IMRT)
- •6.1.3 Image-guided radiotherapy (IGRT)
- •6.1.4 Adaptive radiotherapy (ART)
- •6.2 ART workflow and implementation
- •6.2.2 Current practice
- •6.2.3 Clinical impact
- •6.3 Considerations for implementing online ART
- •6.3.1 Time as a limiting factor
- •6.3.2 Implications for fast and reliable re-planning
- •6.3.4 Clinical considerations
- •6.4 Summary
- •7.1 Components of ART workflow
- •7.1.1 Simulation
- •7.1.2 Pre-planning
- •7.1.3 Online imaging and daily re-planning
- •7.1.4 Quality assurance
- •7.2 AI-driven ART
- •7.2.1 Simulation
- •7.2.2 Pre-planning
- •7.2.3 AI for delivery
- •7.3 Outlook and future directions
- •7.3.1 Real-time ART with AI
- •7.3.2 Dose escalation and functional adaption with AI
- •7.4 Summary
- •References
- •8.1 Introduction
- •8.2 Synthetic CT: deep learning methods
- •8.2.1 Conventional methods
- •8.2.2 U-Net
- •8.2.3 Generative adversarial networks
- •8.2.4 Denoising diffusion probabilistic model
- •8.3 Synthetic CT from CBCT
- •8.3.1 Noise and artifact reduction
- •8.3.2 Online dose calculation
- •8.3.3 Online image segmentation
- •8.4 Synthetic CT from MRI
- •8.4.1 Synthetic image accuracy
- •8.4.2 Dose calculation in MR-only radiation therapy
- •8.4.3 PET attenuation correction
- •8.4.4 Image registration
- •8.5 Discussion and outlook
- •8.6 Summary
- •References
- •9.1 AI-based image registration and segmentation for ART
- •9.1.1 Adaptive radiation therapy
- •9.2 Artificial intelligence
- •9.2.1 What is machine learning?
- •9.2.2 What is deep learning?
- •9.3 Deep learning: the basic components
- •9.3.1 Convolutional neural networks: looking at the picture
- •9.3.2 Pooling layers: keeping what matters most
- •9.3.3 Fully connected (dense) layers: bringing it all together
- •9.3.4 Activations
- •9.3.5 Loss: driving the model
- •9.3.6 Auto-encoders: remove the noise
- •9.3.7 Supervised versus unsupervised learning
- •9.3.8 Pre-trained convolutional neural networks
- •9.4 Image registration: bringing two images together
- •9.4.1 Registration similarity metrics
- •9.4.2 Types of registrations
- •9.5 AI-based image registration
- •9.5.1 Supervised learning
- •9.5.2 Unsupervised learning
- •9.5.3 Registration in ART
- •9.5.4 Commonalities in architectures
- •9.6 Image segmentation
- •9.6.1 Introduction: coloring by the numbers
- •9.6.2 Segmentation networks
- •9.6.3 Best practices
- •9.7 Summary
- •10.1 Introduction
- •10.1.1 Overview of chapter content
- •10.2 The landscape of AI-assisted dose prediction
- •10.2.1 Traditional machine learning for dose prediction
- •10.2.2 Deep learning-based dose prediction
- •10.2.3 Challenges in AI-assisted dose prediction
- •10.3 Re-planning workflows powered by AI
- •10.3.1 Deep learning for re-planning pipelines
- •10.4 Future directions of AI-assisted dose prediction and re-planning
- •10.5 Summary
- •11.1 Introduction
- •11.2.1 Imaging-based motion monitoring
- •11.2.2 Delivery system actions
- •11.2.3 Challenges for real-time ART implementation
- •11.3 AI in real-time ART workflows
- •11.3.1 Improving intrafraction motion monitoring through AI
- •11.3.2 Mitigating system latency through AI
- •11.4 AI for ART delivery: future directions
- •11.4.1 Management of non-respiratory motion
- •11.4.2 Training AI models with small or unpaired datasets
- •11.4.4 Biology-guided ART delivery
- •11.5 Summary
- •References
- •12.1 Introduction
- •12.2 Patient QA
- •12.2.1 Pre-planning QA
- •12.2.2 Pre-treatment plan QA
- •12.2.3 On-treatment QA
- •12.3 Treatment delivery systems and instruments
- •12.3.1 Machine commissioning
- •12.3.2 Machine QA
- •12.3.3 Dosimetry tool QA
- •12.4 Summary
- •References
- •13.1 Data resources for response modeling in radiotherapy
- •13.1.1 Clinical data
- •13.1.2 Imaging (radiomics)
- •13.1.3 Treatment planning (dosiomics)
- •13.1.4 Multiomics
- •13.2 Radiotherapy treatment outcome modeling
- •13.2.1 TCP/NTCP in radiotherapy
- •13.2.2 Clinical outcomes versus PROs
- •13.2.3 Machine learning response prediction
- •13.2.4 Explainability of ML response models
- •13.2.5 Sample use cases
- •13.3 AI response-based adaptive radiotherapy
- •13.3.1 Requirements and challenges
- •13.3.2 Prediction versus treatment optimization
- •13.3.3 Sample use cases
- •13.4 Challenges and recommendations
- •13.5 Summary
- •Acknowledgments
- •References
- •14.1 Overview of challenges in AI-driven ART
- •14.2 Data challenges
- •14.2.1 Data availability
- •14.2.2 Data quality
- •14.2.3 Data privacy
- •14.3 Technical challenges
- •14.3.2 Model robustness and generalizability
- •14.3.3 Model explainability and interpretability
- •14.4 Challenges associated with online and real-time workflows
- •14.4.1 Image quality
- •14.4.2 Dose calculation
- •14.4.3 Real-time ART
- •14.5 Operational challenges
- •14.5.1 Clinical validation
- •14.5.3 Staff training
- •14.5.4 User experiences
- •14.5.5 Quality management program
- •14.5.6 Financial challenges
- •14.6 Ethical, regulatory, and legal challenges
- •14.6.1 Ethical issues
- •14.6.2 Regulatory and legal issues
- •14.7 Summary
- •References
- •15.1 Clinical considerations for CT-based offline ART
- •15.1.1 Patient and site selection
- •15.1.2 Re-simulation
- •15.1.3 Re-planning
- •15.1.4 Plan summation and evaluation
- •15.1.6 Limitations and future directions
- •15.2 Clinical considerations for CBCT/CT-based online ART
- •15.2.2 Patient and site selection
- •15.2.3 Simulation
- •15.2.4 Pre-planning review
- •15.2.5 Reference planning
- •15.2.9 Limitations and future directions
- •15.3 Summary
- •References
- •16.1 Introduction
- •16.2 Overview of MRI-guided ART systems
- •16.3 MRI-guided ART workflow
- •16.4 AI applications for MRI-guided ART
- •16.4.1 Synthetic CT generation
- •References
- •16.4.2 Auto-segmentation
- •16.4.3 Image registration
- •16.4.4 Others
- •16.4.5 Future AI development and implementation
- •16.5 Summary
- •17.1 Functional PET-guided ART
- •17.1.1 PET-based functional imaging overview
- •17.1.2 From anatomy to function: the power of PET in radiation therapy
- •17.1.5 Conclusions and future prospects
- •17.2 Functional MRI-guided ART
- •17.2.1 From anatomy to function: the power of functional MRI in radiation therapy
- •17.2.4 Conclusion and future prospects
- •17.3 Summary
- •References
- •18.1 Proton ART
- •18.1.1 Clinical context and necessity
- •18.1.2 Patient populations
- •18.1.4 Rationale for AI in proton ART
- •18.2 AI in proton ART
- •18.2.1 Imaging
- •18.2.2 Deformable and rigid registration
- •18.2.3 Contour propagation
- •18.2.4 Dose calculations
- •18.2.5 Plan optimization
- •18.2.6 Other developments
- •18.3 Implementation of adaptive proton therapy
- •18.4 Summary
- •References
- •19.1 Designing clinical trials with AI
- •19.1.1 The essential role of clinical trials
- •19.1.2 Trial protocols and methodologies
- •19.1.3 AI-driven clinical trial design and execution
- •19.1.4 Incorporation of digital twins (DTs) in clinical trials
- •19.2 Implementation of AI in ongoing clinical trials
- •19.2.1 Integration with existing clinical trial frameworks
- •19.2.2 Quality assurance, compliance, and standardization
- •19.3 Case studies of AI in adaptive radiotherapy trials
- •19.3.1 Overview of guidance for advanced radiotherapy in clinical trials
- •19.3.2 AI in the radiotherapy clinical trial quality assurance processes
- •19.4 Ethical and regulatory considerations
- •19.4.1 Patient consent and data privacy
- •19.4.2 Bias, fairness, and transparency
- •19.4.3 Regulatory guidelines and compliance
- •19.5 Future directions and challenges
- •19.5.1 Emerging technologies and techniques
- •19.5.2 Alternative strategies
- •19.6 Conclusion
- •19.7 Summary
- •References
- •20.1 Risk management
- •20.1.1 Prospective risk assessments
- •20.1.2 Root cause analysis

Artificial Intelligence in Adaptive Radiation Therapy
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IOP Publishing
Artificial Intelligence in Adaptive Radiation Therapy
Yi Wang and X. Sharon Qi
Chapter 17
Functional imaging-guided adaptive
radiation therapy
Bin Han and Yu Gao
Conventional adaptive radiation therapy (ART) mainly adjusts treatment plans
based on daily anatomical changes, such as the shape and position variations of
organs at risk (OARs). However, different tumors and patients can exhibit diverse
responses to treatment, which can impact the effectiveness of radiation treatment.
Functional imaging techniques, such as positron emission tomography (PET) and
functional magnetic resonance imaging (MRI), provide crucial insights into the
tumor’s metabolism, physiology, and molecular characteristics, offering a more
comprehensive understanding than anatomy-based imaging alone. By integrating
functional imaging into radiation therapy, we can more precisely tailor treatment
plans to individual response. This holds great promise for enhancing treatment
effectiveness and improving patient outcomes. This chapter delves deeply into the
crucial role of functional imaging in refining ART, with a spotlight on PET and
MRI’s significant contributions and breakthroughs. Our analysis will showcase how
PET and MRI not only form the cornerstone for creating individualized treatment
protocols but also pave the way for groundbreaking oncological research and
innovation. We aim to illuminate the significant strides in functional imaging,
marking a new era in the precision, efficacy, and patient outcomes within radiation
therapy’s rapidly advancing domain. Sections 17.1 and 17.2 will specifically focus on
PET and MRI applications in ART, respectively.
17.1 Functional PET-guided ART
PET imaging has become a pivotal tool in the realm of adaptive radiation therapy
[1, 2], primarily due to its unmatched capability in visualizing the metabolic
activities of tumors. This functional imaging technique allows for a highly detailed
assessment of a tumor’s response to radiation therapy, making it possible to tailor
treatment plans with unparalleled precision. By highlighting areas of increased
doi:10.1088/978-0-7503-6119-4ch17 17-1 ª IOP Publishing Ltd 2025. All rights,
including for text and data mining (TDM), artificial intelligence (AI) training, and similar technologies, are reserved.

Artificial Intelligence in Adaptive Radiation Therapy
metabolic activity, positron emission tomography (PET) imaging aids in the
accurate delineation of tumors, ensuring that radiation doses are optimally targeted
to cancerous tissues while sparing adjacent healthy structures.
The application of PET imaging in adaptive radiation therapy (ART) extends to
monitoring the effectiveness of treatment over time. Clinicians utilize PET scans to
evaluate changes in a tumor’s size and metabolic activity during the course of
therapy, providing critical insights that can prompt adjustments to the treatment
plan [3]. This dynamic approach enables the adaptation of radiation doses,
optimizing therapy based on the tumor’s real-time response. Furthermore, PET
imaging facilitates dose painting [4], where varying radiation doses are applied to
different tumor regions based on their metabolic activity, thereby enhancing the
efficacy of treatment and minimizing damage to healthy tissues.
Moreover, PET imaging plays a crucial role in the post-treatment phase, serving
as a sensitive tool for early detection of cancer recurrence [5]. Its ability to identify
metabolic changes offers a significant advantage in monitoring patients for signs of
tumor regrowth, ensuring prompt intervention when necessary. Through these
applications, PET imaging in adaptive radiation therapy represents a significant
stride toward personalized cancer care, offering hope for improved treatment
outcomes and reduced side effects for patients undergoing radiation therapy.
17.1.1 PET-based functional imaging overview
PET-based functional imaging leverages PET to visualize the metabolic activities
within the body, offering crucial insights beyond what is possible with traditional
anatomical imaging techniques. By injecting radiotracers, which are substances
designed to target specific biochemical processes, PET can illuminate areas of
increased metabolic activity, such as tumors with high glucose consumption. This
allows for the precise detection and monitoring of various diseases, particularly in
oncology, by providing detailed images of how tissues and organs function at a
molecular level. As a result, PET imaging has become an indispensable tool in
diagnosis, treatment planning, and the evaluation of therapeutic responses, paving
the way for personalized medicine through its ability to reveal the unique functional
characteristics of diseases within the body.
17.1.1.1 Principles of PET imaging
The process of generating PET images is a fascinating intersection of physics,
chemistry, and medical science, providing a window into the body’s functional
processes. This journey begins with the careful administration of a radiotracer, a
specially designed molecule tagged with a radioactive isotope. These radiotracers are
ingeniously crafted to seek out specific biological activities or cell types, with
fluorodeoxyglucose (FDG), a glucose analog, being among the most commonly
used. Once injected, this compound circulates through the bloodstream, distributing
itself across various tissues but preferentially accumulating in areas with high
metabolic demand, such as rapidly growing tumors.
17-2

Artificial Intelligence in Adaptive Radiation Therapy
Upon reaching its target, the radiotracer undergoes radioactive decay, emitting
positrons. These subatomic particles travel a short distance within the tissue
before encountering electrons, their negatively charged counterparts. This meeting results in a phenomenon known as annihilation, where the mass of the
positron and electron is converted into energy in t he form of two gamma rays,
ejected in nearly opposite directions. This release of energy is a critical moment,
marking the point where invisible biological activities start to translate into
visible signals.
The PET scanner plays a crucial role in capturing these signals. Encircling the
patient, its ring of detectors is finely tuned to detect the high-energy gamma rays
emerging from the annihilation events. By registering the precise timing and location
of these rays, the scanner reconstructs a detailed 3D-map of where the radiotracer has
accumulated. Advanced algorithms then process these raw data, piecing together a
comprehensive three-dimensional image. This image not only reveals the physical
structure of the scanned area but, more importantly, highlights the variations in
biological activities across different tissues. Bright spots in the image indicate regions
of high radiotracer concentration, often correlating with areas of disease, such as
cancerous growths. Through this detailed visual representation, PET imaging offers an
unparalleled view into the body’s inner workings, providing crucial information for the
diagnosis, treatment planning, and monitoring of various diseases, thereby embodying
a remarkable blend of scientific innovation and clinical utility.
17.1.1.2 Radiotracers used in PET
Common radiotracers used in PET imaging play a crucial role in visualizing
different physiological and biochemical processes within the body. Each radiotracer
is designed to target specific functions or tissues, enabling the detailed study of
various diseases and conditions. The following describes some of the most frequently
used radiotracers in PET.
18F-fluorodeoxyglucose (FDG) [6] is the most widely used radiotracer in PET
imaging. FDG is a glucose analog that is taken up by cells with high glucose
metabolism, making it particularly useful for identifying cancerous tumors, as
cancer cells often have higher rates of glucose uptake than normal cells. FDG-PET
is also used in the evaluation of brain disorders such as Alzheimer’s disease [7] and in
cardiology to assess myocardial viability [8].
11C-choline is used primarily for imaging prostate cancer [9] and brain tumors
[10]. Choline is a nutrient that is involved in building cell membranes, and prostate
cancer cells tend to take up more choline than normal cells due to their increased
need for membrane synthesis as they grow and multiply. By labeling choline with
carbon-11, doctors can use PET scans to detect areas of increased choline uptake in
the body, which may indicate the presence of tumors.
Prostate-specific membrane antigen (PSMA) tracers are used in PET imaging for
prostate cancer [11]. They target the PSMA, a protein abundantly expressed on
prostate cancer cells. 68Ga-PSMA-11 and 18F-DCFPyL are common examples.
68Ga-DOTATATE (or DOTATOC/DOTANOC) targets somatostatin receptors,
which are often overexpressed in neuroendocrine tumors. The use of 68Ga-labeled
17-3

Artificial Intelligence in Adaptive Radiation Therapy
peptides allows for the precise detection and localization of neuroendocrine tumors.
18F-DCFPyL is marked with the radioactive isotope fluorine-18, which allows for
detection of prostate cancer cells due to their expression of PSMA. This tracer has
been particularly useful for identifying prostate cancer metastases and is valuable in
both initial staging and the detection of recurrence. The high affinity of 18FDCFPyL for PSMA-expressing cells leads to more accurate imaging results, which
can significantly impact the treatment decisions and management of prostate cancer.
PSMA tracers have significantly improved the detection of prostate cancer metastases and recurrence, offering high sensitivity and specificity. This advancement
aids in accurate staging and treatment planning, enhancing personalized therapy
approaches for prostate cancer patients.
18F-sodium fluoride (NaF) is used for bone scanning [12]. When injected into the
body, 18F-sodium fluoride binds to areas of bone remodeling, which is indicative of
bone growth or repair. Due to its high affinity for areas of calci fi cation, it is
particularly effective for detecting bone metastases in cancer patients. 18F-NaF PET
scans offer higher sensitivity and resolution compared to traditional bone scintigraphy, providing valuable information for the diagnosis and management of skeletal
diseases.
18F-fluciclovine (FACBC) is an amino acid analog used primarily for imaging
prostate cancer [13]. It is useful for detecting recurrent prostate cancer, particularly
in cases where standard imaging has been inconclusive.
These radiotracers, each with their specific targeting mechanisms, underscore the
versatility of PET imaging in diagnosing and monitoring a wide range of conditions.
The development and application of new radiotracers continue to expand the
capabilities of PET, offering more detailed insights into disease processes and
enhancing personalized treatment strategies.
17.1.1.3 Advantages of PET as a functional imaging modality
PET offers unparalleled insight into cellular activity and metabolic processes. Its
ability to differentiate between active and dormant cells enhances radiation
therapy’s precision and effectiveness. PET distinguishes itself in the landscape
of functional imaging by its exceptional sensitivity to minute changes in metabolic
processes, a trait not as pronounced in other functional imaging techniques such
as SPECT [14], which, while functional, cannot match the resolution or quantitative precision of PET. Unlike fMRI, which tracks blood flow as a surrogate for
neural activity, PET measures cellular metabolism directly, offering insights into
a broader spectrum of diseases, including cancer, beyond the scope of fMRI’s
primarily neurological applications. MR spectroscopy [15], which o ffers chemical
composition data of tissue, provides a localized spectrum but lacks the wholebody metabolic mapping that PET delivers. Thus, in the functional imaging
sphere, PET stands out for its comprehensive metabolic profiling, which, when
integrated with anatomical imaging from CT or MRI, provides a holistic view of
a patient’s condition, crucial for personalized medicine and targeted treatment
strategies.
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Artificial Intelligence in Adaptive Radiation Therapy
17.1.2 From anatomy to function: the power of PET in radiation therapy
17.1.2.1 Differentiating between anatomical and functional imaging
Anatomical imaging, such as regular MRI and CT scans, excels in providing
detailed visualizations of the body’s structures, showcasing the physical form, size,
shape, and position of organs and tissues. These modalities are particularly adept at
detecting structural abnormalities, such as tumors, fractures, or anatomical malformations, by producing high-resolution images that delineate the intricate details of
the body’s anatomy.
In contrast, functional imaging techniques, such as PET, delve into the biochemical and physiological processes occurring within tissues and organs. PET
scans, for instance, track the distribution of radiotracers to reveal metabolic activity,
offering insights into cellular function that can indicate the presence of disease even
before structural changes become apparent. This differentiation between anatomical
and functional imaging underscores their complementary roles in medical diagnosis
and treatment planning. While anatomical imaging offers a static picture of what is
present, functional imaging provides a dynamic view of how the body operates,
enabling early detection of diseases based on metabolic changes and aiding in the
assessment of treatment efficacy. Together, these imaging modalities furnish a
comprehensive understanding of both the form and function of the human body,
facilitating precise diagnoses and tailored therapeutic approaches.
17.1.2.2 Role of PET imaging in detecting and targeting tumor heterogeneity and
treatment response
Tumors are not uniform; they possess areas of varied metabolic activity. PET
imaging can identify these active zones [16], enabling precise targeting during
radiation treatment. PET imaging significantly influences the planning and execution of radiation therapy by illuminating the metabolic heterogeneity within tumors.
This advanced imaging technique identifies areas within the tumor that exhibit
higher metabolic activity, indicative of aggressive cancer cells. By precisely targeting
these hotspots with tailored radiation doses, clinicians can optimize treatment
efficacy, sparing surrounding healthy tissues. Furthermore, PET’s capability to
monitor the tumor’s metabolic response to treatment over time provides invaluable
feedback, allowing for the dynamic adjustment of radiation plans [17]. This
adaptability ensures that therapy remains aligned with the evolving nature of the
tumor, enhancing the potential for personalized treatment strategies that directly
address the unique characteristics of the cancer. Such a focused approach not only
aims to improve therapeutic outcomes but also minimizes the risk of side effects,
contributing to an overall enhancement in patient care and prognosis in the battle
against cancer.
Regular PET scans during treatment allow clinicians to monitor tumor response,
adjusting radiation plans as necessary. This adaptive approach ensures optimal
radiation delivery while preserving healthy tissues. PET imaging is pivotal in
monitoring the evolution of tumors over the course of treatment by meticulously
assessing the metabolic activity within the tumor at various stages prior to the
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initiation of therapy, during the treatment period, and following the conclusion of
therapy. This is achieved through the measurement of fluctuations in the uptake of
specific radiotracers, such as FDG, which are indicative of the tumor’s metabolic
rate. A discernible reduction in the uptake of these radiotracers over successive scans
generally signals a favorable response to the treatment, manifesting the therapy’s
effectiveness in curtailing the tumor’s metabolic activity. Conversely, a consistent or
escalating uptake could denote the tumor’s resistance to the current treatment
regimen or the advancement of the disease. This capability of PET imaging to noninvasively track these metabolic changes offers a dynamic insight into the tumor’s
response, enabling clinicians to tailor treatment plans more accurately and make
informed decisions regarding the patient’s therapeutic strategy, thereby significantly
impacting the overall management and prognosis of the disease.
17.1.3 Practicalities and clinical implications of PET-guided adaptive radiation
therapy
17.1.3.1 Treatment planning: incorporating PET information
In the treatment planning phase, PET data can be invaluable. It aids in delineating
tumor boundaries, understanding metabolic hotspots, and designing precise radiation beams. Incorporating PET information into daily radiation treatments significantly enhances the personalization and precision of therapy strategies. By utilizing
the detailed metabolic activity and tumor extent data provided by PET scans,
clinicians can more accurately define the target areas for radiation, differentiating
between cancerous and healthy tissues with greater precision than conventional
imaging allows. This detailed insight enables the meticulous adjustment of radiation
dose distributions, focusing on eradicating cancer cells while minimizing exposure to
surrounding healthy tissues. Furthermore, the concept of ART comes into play,
wherein PET imaging is repeatedly used to monitor the tumor’s response to
treatment over time. Adjustments to the radiation plan are made dynamically,
based on these sequential PET scans, to address changes in tumor size, shape, or
metabolic activity. Such an adaptive approach not only aims to improve the
accuracy of radiation delivery but also holds the promise of enhancing patient
outcomes by reducing treatment-related side effects. Through this integration of
PET data, radiation therapy is tailored to the unique characteristics of each patient’s
tumor, ensuring a highly individualized treatment process that adapts to the
evolving nature of the disease.
17.1.3.2 PET-based biology-guided adaptive radiation therapy
The SCINTIX
®
biology-guided radiotherapy (BgRT) represents a cutting-edge
advancement that merges real-time PET imaging with radiotherapy, enhancing the
precision of tumor targeting and the efficacy of treatments. The RefleXion X1 system
(RefleXion Medical, Inc., Hayward, CA) is a novel PET-guided radiation therapy
machine [18, 19] featuring an 85 cm O-ring gantry linear accelerator (linac) capable of
rotating at 60 revolutions per minute (rpm). It incorporates fan-beam kilovoltage
computed tomography (kVCT) for precise image guidance in intensity-modulated
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radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT).
Additionally, it utilizes PET imaging for real-time tumor tracking in BgRT. The
design of the X1 system is highlighted by its two symmetrically opposing 90-degree
arcs of PET detectors seamlessly integrated into the ring gantry’s architecture, to
direct therapeutic radiation beams in real time, utilizing the tumor as a natural marker
to guide and adapt the radiation dose dynamically during treatment. For real-time
guidance, the system performs high-speed computations to generate limited timesample PET images at 100-millisecond intervals, drawing on 500 milliseconds worth
of accumulated line of response data.
This sophisticated system is specially tailored for providing comprehensive, realtime, BgRT for the treatment of bone and lung tumors, ensuring precise targeting
and treatment delivery. The BIOGUIDE-X study [20] conducted sequential cohorts
of participants to ascertain the optimal FDG dosage for SCINTIX therapy
application, and to validate that the emulated radiation dose distribution corresponds with the physician-endorsed radiotherapy scheme. This forward-looking
study enrolled individuals who presented with at least one FDG-avid tumor that was
primary or metastatic, targetable, and measured between 2 and 5 cm, located in the
lung or bone. Cohort I employed a modified 3 + 3 scheme to identify the FDG dose
necessary for SCINTIX therapy to produce an adequate signal. In cohort II, PET
imaging was utilized on the X1 system to acquire data before the commencement
and after the conclusion of the initial and final sessions of conventional stereotactic
body radiotherapy. The SCINTIX therapy dose distributions were emulated using
patient-specific CT anatomy and the acquired PET data for each treatment fraction.
These were then compared against the physician-sanctioned plan.
The findings from cohort I showing sufficient FDG activity in all six evaluable
participants following the administration of an initial dose level of 15 mCi FDG. In
cohort II, the study saw the enrollment of four patients with lung tumors and five
with bone tumors, from which data points for 17 treatment fractions were collected
and evaluated. Out of these, 16 emulated deliveries yielded SCINTIX dose
distributions that aligned accurately with the authorized SCINTIX therapy plan.
Notably, all emulated fluences were found to be feasible for delivery. Furthermore,
no adverse effects were ascribed to the repeated administrations of FDG. In essence,
SCINTIX therapy presents a pioneering approach in radiotherapy where the
radiolabeled tumor inherently serves as a fiducial marker for targeting.
As it stands, FDG is the sole radionuclide that has received FDA approval for use
in BgRT. However, the scope of BgRT is set to broaden with the exploration of
additional promising radionuclides such as PSMA and
89
Zr-labeled Panitumumab
[21]. The incorporation of these new radionuclides into BgRT practices aims to
enhance the specificity with which radiation targets tumor biology. This advancement has the potential to extend BgRT applicability across a wider spectrum of
diseases and clinical contexts, particularly in improving the targeting of challenging
tumors with more precise radionuclide-tumor binding. The SCINTIX radiotherapy
system stands out due to its consistent accuracy and replicability in dose delivery.
Such reliability, particularly under static conditions, marks SCINTIX as a
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significant technological development poised to push the boundaries of current
radiotherapy modalities.
17.1.4 Artificial intelligence in PET-guided adaptive radiation therapy
17.1.4.1 Role of AI in enhancing image interpretation
Artificial intelligence can significantly enhance the analysis and interpretation of
PET images [22, 23]. Machine learning algorithms can detect patterns or anomalies
that might be overlooked by human observers, ensuring a more accurate diagnosis
and treatment planning. AI algorithms can analyse complex imaging data with high
precision and speed, identifying patterns and anomalies that may be subtle or
invisible to the human eye. This capability significantly improves diagnostic
accuracy and efficiency, enabling earlier and more accurate detection of diseases.
AI can also learn from vast datasets to continuously improve its diagnostic
capabilities, supporting radiologists in making more informed decisions. By reducing the potential for human error and increasing the consistency of image
interpretations, AI ultimately contributes to better patient outcomes and streamlined workflows in healthcare settings.
17.1.4.2 Predictive modeling for treatment outcomes
AI, combined with PET imaging, can predict patient-specific responses to radiation
therapy. These predictive models utilize vast amounts of data to estimate how a
tumor might react, allowing for more personalized treatments [24]. It enhances the
ability to predict patient-specific responses to radiation therapy by analysing
metabolic and physiological data from PET scans. AI algorithms can identify
patterns in the data that correlate with treatment outcomes, enabling personalized
treatment plans that are more likely to be effective for individual patients. This
predictive capability allows for the optimization of radiation doses, minimizing
exposure to healthy tissues while targeting tumors more precisely. AI-driven analysis
of PET imaging data can lead to better treatment decisions, reduced side effects, and
potentially improved survival rates for patients undergoing radiation therapy.
17.1.4.3 AI-driven real-time treatment adjustments
With the assistance of AI, real-time adjustments during radiation sessions become
feasible [25]. By analysing the ongoing PET data, AI systems can suggest immediate
modifications to the treatment plan if deviations or unexpected responses are
detected. It is a pivotal advancement in adaptive radiation therapy by enabling
highly personalized and dynamic treatment protocols. By continuously analysing
data from diagnostic images, patient responses, and other relevant clinical information, AI algorithms can detect subtle changes in tumor size, shape, and metabolic
activity. This capability allows for the immediate adjustment of radiation doses and
targeting strategies to reflect the tumor’s current state and the patient’s unique
response to treatment. Consequently, treatments can be optimized on-the-fly,
enhancing efficacy while minimizing damage to surrounding healthy tissues. This
approach not only improves the precision and adaptability of radiation therapy but
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Artificial Intelligence in Adaptive Radiation Therapy
also opens up new possibilities for individualized patient care, potentially leading to
better outcomes and reduced side effects.
17.1.5 Conclusions and future prospects
After exploring PET’s role in adaptive radiation therapy and AI’s promising
potential, it is evident that these technologies are revolutionizing oncology treatments. Their convergence offers better patient outcomes with increased precision.
The use of PET in oncology treatments is revolutionizing the field due to its ability to
provide detailed metabolic insights into tumors, beyond what conventional imaging
offers. PET’s capacity to visualize the biological activity of tumors allows for earlier
detection, precise staging, and monitoring of treatment responses, leading to more
personalized and effective therapy strategies. This improved diagnostic accuracy and
treatment monitoring enhance patient outcomes, making PET a cornerstone in the
advancement of oncology care.
The synergistic combination of PET and adaptive radiation therapy can lead to
innovations such as time-of-flight PET. Using PET functional images in radiation
therapy offers potential benefits such as precise tumor targeting, enhanced treatment
personalization, and the ability to adapt therapy based on real-time tumor
responses. Innovations include more accurate dose distribution, minimizing exposure to healthy tissues, and improved detection of treatment-resistant tumor areas.
These advancements lead to better patient outcomes and reduced side effects,
marking a significant leap forward in the precision and effectiveness of cancer
treatment. Such advancements, along with integration possibilities with other
modalities, promise even more accurate and effective treatments in the future.
As technology evolves, so will the landscape of adaptive radiation therapy.
Future research should delve into refining techniques, improving patient comfort,
and reducing costs while maximizing outcomes. The use of PET in adaptive
radiation therapy is poised for significant growth, with future research focusing on
enhancing tumor characterization, treatment personalization, and real-time monitoring of therapy effectiveness. Advancements in PET technology and AI integration are expected to improve the precision of radiation dose delivery, reduce side
effects, and facilitate the development of novel therapeutic strategies. These trends
underscore PET’s pivotal role in evolving adaptive radiation therapy towards more
targeted, efficient, and patient-specific approaches, promising substantial improvements in cancer care and outcomes.
17.2 Functional MRI-guided ART
MRI has been routinely used in the clinic to facilitate target and OAR delineation
owing to its superior soft-tissue contrast. Recent advancements in engineering have led
to the successful integration of MR scanners with linear accelerators (linacs) for
radiation treatment [26–28]. This integration opens up exciting possibilities for online
MRI-based treatment adaptation to account for inter-fraction anatomy variations as
well as real-time MRI-based gating to minimize the effect of intra-fraction motion on
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