Technological advances in CT have been applied to radiation oncology. Coolens et al implemented a 320-slice volumetric scanner for CT simulation [7]. By using volume scanning instead of helical scanning, the scanner can capture the entire treatment site within a single gantry rotation. The scan time is thus reduced, and motion artifacts are minimized. Most centers, however, still use the larger bore helical scanners. Dual-energy CT (DECT) was investigated as a candidate for simulation in proton therapy. By capturing CT images under two energies, DECT can differentiate x-ray attenuation change in density or chemical composition [8]. It can then calculate stopping power ratio and improve proton radiotherapy planning by reducing range uncertainties. Currently, the high level of noise still hinders the wider adoption of DECT in proton therapy simulation [9]. More recently, clinical deployment of photon counting CT (PCCT) also provides opportunities in the improvement of radiation simulation. The photon counting detector can resolve energy by counting photons in selected energy bins. It has the potential to directly calculate the stopping power ratio using the Bethe–Bloch equation. The scanner achieved stopping power ratio estimates that are comparable to DECT scanners [10]. PCCT is also capable of generating quantitative contrast enhanced CT scans. It can leverage the photon counting detector’s energy resolution to provide increased tissue and contrast agent discrimination. This capability helps PCCT generate virtual non-contrast electron density and stopping power [11]. Patients can poten­tially avoid a non-contrast scan as well as its dose during simulation.

Studies have underscored the efficacy of PET/CT in tumor delineation for various cancers, including lung, head and neck, esophageal, and cervical malignancies [15]. In lung cancer, PET/CT facilitates more accurate lymph node staging [16], reduces inter-observer variability during tumor delineation [17], and refines the delineation of tumor borders in conjunction with atelectasis [18]. Notably, PET/CT-derived gross tumor volumes (GTVs) have demonstrated superior accuracy when comparing with surgical specimens in a head and neck cancer study [19]. Prospective investigations in esophageal and cervical cancers have corroborated the positive impact of PET/CT on tumor delineation and the significant reduction in late radiation toxicity [20–22]. The evolution of PET/CT-guided radiotherapy has gained interest in dose painting strategies targeting specific tumor sub-volumes based on biological imaging [23]. Escalated doses can be prescribed to radio-resistant tumor regions to enhance local tumor control [24].

Despite the widespread use of

F-FDG, other PET tracers such as fluorine-18 fluoromisonidazole (FMISO) and fluorine-18 fluoroazomycin arabinoside (FAZA) provide non-invasive and quantitative assessments of tumor hypoxia—acritical determinant of radiation treatment resistance. Hypoxia volumes measured via FMISO-PET serve as predictive factors for patient survival in head and neck cancer [25]. Targeting hypoxic tumor volumes with escalated doses represents a promising strategy to improve tumor control. Additionally, the active cellular proliferation characteristic of tumors can be assessed using fluorine-18 fluorothymidine ( a thymidine analogue that monitors thymidine kinase activity—a surrogate marker for cell proliferation [26]. Given its low uptake in inflammatory tissues,