Ординатура / Офтальмология / Английские материалы / Orbital Tumors Diagnosis and Treatment_Karcioglu_2005

and do not interfere with lid function. If surgical repair is needed, it is best to wait 4 to 6 months, to allow the reparative process to stabilize.

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As in many areas of medicine, computerized imaging modalities (CT and MRI in particular) have also revolutionized the diagnosis of or-

bital tumors. The therapeutic impact of these imaging techniques has only recently been explored in select cases of orbital tumors. Computer image guidance has seen remarkable development in other specialties, notably in neurosurgery.1,2 It is therefore not surprising that the initial experience with this technology in the surgery of orbital tumors has entailed tumors with intracranial extension, requiring a multidisciplinary approach of ophthalmological and neurological surgeons. In addition to image guidance, a standard craniotomy to expose the orbital roof and areas of intracranial tumor extension can be supplemented by a skull-base approach in select cases with minimal to no additional morbidity. Other treatment modalities include radiosurgery (computer image-guided stereotactic radiation).3 A limiting factor for radiosurgery both for intraorbital and intracranial treatment is the risk of functional damage to the optic pathways. In this chapter, we discuss our multidisciplinary approach to select orbital tumors with special reference to emerging technologies and surgical approaches.

COMPUTER IMAGE GUIDANCE

In recent years, systems that permit computer-guided navigation during surgery using preoperatively acquired computer-based imaging have seen exponential development in neurosurgery and to a lesser degree in the ear, nose, and throat field. The fundamental principle of surgical navigation is to acquire computed tomography (CT) or magnetic resonance (MR) as a volumetric data set, usually implying image acquisition in the form of an elevated number of parallel slices with no intervening gaps between slices. Such a data set can then be reconstructed into a volume, which can be reformatted in any plane. The volumetric study can also be reconstructed to define three-dimensional renderings of the head, the orbit, the eye, the optic nerves, and the brain, as well as of tumors. These

three-dimensional models are thus defined in “image space.”

To use imaging data for intraoperative real-time navigation, the three-dimensional structures in image space must be correlated with the real anatomic structures in the “surgical space” of the actual patient. To accomplish this, surgical space must be defined so that exact coordinates can be determined for areas of interest at surgery. In neurosurgery, the traditional frame of reference has been a stereotactic frame rigidly affixed to the skull prior to imaging and maintained during surgery. Computer image guidance with a frame was pioneered by Kelly et al.4 Over the past 10 years, there has been an accelerated development of “frameless” image guidance systems, in which the rigid frame has been replaced by using the reconstructed three-dimensional rendering of the head. The position of the head, which needs to be rigidly immobilized only at the time of surgery (using a standard three-point headholder), needs to be referenced in space by means of a mechanical, ultrasonic, optical, or magnetic system.5–11 Ultrasonic referencing has fallen from favor for lack of reliability, and mechanical systems are seen less frequently because of their cumbersome nature. The most common systems presently available use infrared light or ambient light to define the position of probes during surgery. There is growing interest in using a magnetic field as a frame of reference for defining space, since this methodology interferes least with surgical procedures.12

Regardless of the methodology used, the principles of frameless image guidance remain the same. Several landmarks must be chosen on the three-dimensional head rendering (in image space) and the corresponding landmarks on the patient (surgical space) are correlated at the time of surgery by detecting and tracking the position of a probe with regard to a reference (e.g., by two cameras for optical tracking, by a magnetic receiver attached to the probe for magnetic tracking). The procedure of correlating selected points in image space with their homologues in surgical space is known as registration and is performed by touching the designated points with a nonsterile tracked probe

and recording them (Figure 32.1). The accuracy of image guidance at surgery (using a sterile probe) depends greatly on the quality of registration. It must also be kept in mind that following registration, the computer usually gives an estimate of calculated accuracy (should be less than 3 mm), but this number may or may not correspond to the true surgical accuracy (also termed application accuracy) during surgery. Overall, it is therefore necessary to know the principles and limitations of this technology and to evaluate the true accuracy intraoperatively against recognizable surgical landmarks. To facilitate registration, many surgeons apply special adhesive skin markers prior to imaging. The presence of these additional landmarks adds accuracy to registration, provided they are applied to areas of skin or scalp that are unlikely to move during imaging or surgical positioning. In selected cases, where high accuracy is paramount, optimal accuracy can be obtained by MRI/CT markers attached to small titanium screws implanted in the skull under local anesthesia prior to imaging and kept in place until surgery to permit registration.12,13

TECHNIQUES AND SETUP

We have been fortunate to have two different image guidance systems at our disposal. The StealthStation (Medtronics Surgical Navigation Technologies, Louisville, CO) is an optical referencing and tracking system. The Cygnus-PFS (Compass International, Rochester, MN) is a magnetic referencing and tracking system. Since we have a research interest in image guidance, we have used both systems simultaneously in over 70 cases, including in orbital tumors.12 This has allowed us to compare systems and evaluate accuracy.

Preoperative images are acquired according to volumetric protocols, so that the data sets can be imported and reconstructed by the image guidance workstations. Transfer is accomplished either by network or via a variety of digital media (optical disk, DAT

tape, recordable CD, etc.). We usually place adherent markers around the head prior to imaging to facilitate point-to-point registration (Figure 32.1). If imaging is done the day before surgery, the markers can be left overnight and a marking pen used to highlight their position, should they move before surgery. If surgery is to occur several days after imaging, we usually try to align the markers with birthmarks, angiomas, or scars on the scalp to find the correct points at surgery. A small cruciate scratch will also last several days. When accuracy is paramount, we implant small titanium screws (Stryker-Leibinger, Freiburg, Germany) under local anesthesia and attach MRI/CT markers to these (Figure 32.1). The screws can remain in place for days (or weeks) after the markers have been removed.

The optical system employs a large workstation and a dual infrared camera mounted on a pole. To define the position of the head in space during registration, an optical reference arc is attached to the headholder when the head has been immobilized at surgery (Figure 32.1). This nonsterile arc is then replaced by a sterile one after draping (Figure 32.2). The magnetic system runs on a laptop, and there are no cameras. The system is therefore minimally obtrusive. A magnet is attached to the headholder at the time of surgery, and this remains under the drapes. The nonsterile probe is exchanged for a sterile probe after draping (Figure 32.2).

WHY USE IMAGE GUIDANCE

IN ORBITAL TUMORS?

Computer image guidance systems have found applications in neurosurgery for brain tumors and other procedures requiring surgical navigation inside the brain. What can a surgical navigation system contribute to surgery of orbital tumors? Unlike surgery of the brain, finding a tumor within the relatively narrow confines of the orbit and its surroundings is not a great challenge. For this reason, the application of image guidance for the orbit is not frequently reported.14,15

On the other hand, some of the more sophisticated offshoots of image guidance can be quite invaluable. All current image guidance systems are capable of “fusing” or “correlating” a second data set with complementary information. It is thus possible to “fuse” a volumetric CT study with a contrast-enhanced MRI, which permits optimal visualization of bone erosion on CT and soft tissue tumor invasion on MRI. With intraoperative guidance, it is then possible to locate these specific areas at surgery, even if tumor infiltration is not clearly seen under the surgical microscope.

IMAGE FUSION FOR ORBITAL TUMORS

CT/MRI SPGR (T1-Type Weighting)

We have found a number of image fusion strategies to be particularly useful in skull-base surgery including that of orbital tumors with extraorbital invasion. The most common fusion we employed was that described earlier: merged CT and MRI with contrast. Figure 32.3 illustrates surgical image guidance using CT/MRI fusion. The patient had a middle fossa meningioma invading the orbit on the left. The skull renderings show

a previous craniotomy site in which the tumor had been subtotally excised. Tumor is best seen on MRI, yet bone erosion is well visualized only on CT.

CT/MRI FLAIR, MRI SPGR/MRI FLAIR

Fluid-attenuated inversion recovery (FLAIR) MR imaging, introduced by Hajnal et al. in 1992, is an imaging sequence based on the suppression or reduction of the water signal with the use of an inversion recovery pulse sequence employing a long T1 (inversion time).16 This sequence has the potential to highlight pathologic changes including tumor infiltration with great sensitivity. We have fused FLAIR sequences, acquired in 3 mm slices (no gaps) with either T1-type MRI (SPGR: radiofrequency-spoiled gradient-Recalled) or CT volumetric studies and have used the FLAIRbright imaging to determine tumor margins or presence of tumor infiltration of soft tissues.17 A good illustration of the benefits of importing FLAIR images can be seen in Figures 32.4 and 32.5. The patient was a 14-year-old black female with a large optic nerve glioma of the right orbit that had been followed in clinic over many years. The optic nerve glioma was diagnosed at the age of 3 and was treated with exter-

nal beam radiation therapy at the age of 6. Subsequently, the tumor continued to grow within the orbit and extended into the chiasm. Hertel exophthalmometry of the right eye revealed a 2 mm proptosis with a 2 afferent pupillary defect (APD). Extraocular motility was full, and visual acuity in the right eye was sufficient for the patient to count fingers at 1 foot and 20/20 in the left eye. The right optic disk was pale with total atrophy. The left optic disk was within normal limits. The MRI study with oblique images revealed that the tumor extended into the nerve posteriorly, approaching the chiasm, with increased tumor size in comparison to an MRI performed approximately a year earlier. At this point, the Goldmann visual field of the left eye was performed and found to be normal, and it was decided that the tumor should be excised surgically because of the possibility of extension into the chiasm and the increased risk of visual loss in the good eye.

The patient underwent a craniotomy and superior orbitotomy through the roof of the orbit, and the optic nerve glioma was excised. Both anterior and posterior margins of the tumor were confirmed with im-

age guidance. The anterior margin of the tumor was easy to resect. Posteriorly, however, the tumor was extensively adherent to the surrounding tissues within the optic canal that had to be dissected and cauterized following the unroofing of the canal. The image-guided system was particularly useful to determine the posterior extent of the tumor into the chiasm, especially on FLAIR sequences (Figures 32.4 and 32.5). By importing images from the MRI FLAIR sequence into the image guidance systems and fusing these with the MR SPGR sequence (Figure 32.4), or with volumetric CT (Figure 32.5), it was possible to determine the posterior margin of the tumor and to section the nerve just beyond the tumor without undue endangerment of the chiasm. Histopathologic examination of the tumor specimen confirmed tumorfree anterior and posterior resection margins.

Both CT/SPGR and SPGR/ FLAIR fusion were employed in a 43-year-old man presenting with a recurrent benign mixed lacrimal gland tumor. The initial diagnosis of the tumor was made at the age of 14 when the patient was undergoing a ptosis procedure. The lesion was partially excised and histopathologically di-

agnosed to be a pleomorphic adenoma (benign mixed tumor). The patient subsequently underwent three other surgical procedures for tumor recurrence. At the last recurrence, a superior medial mass was identified; the lesion was firm with an irregular surface and extended into the superior orbit. The patient’s best corrected visual acuities were 20/20 and 20/40 2 for the right and left eye, respectively. The left superior eyelid had mechanical ptosis, and the extraocular motility was severely limited at all gazes. Pupils were equal, round, and equally reactive to light; color vision testing was within normal limits. On the CT and MRI examinations, a large superior orbital mass was identified extending into the roof of the orbit and the frontal sinus (Figure 32.6). CT showed irregular bony invasion, and on MRI the mass revealed increased intensity in T2-weighted images and presented a rather homogeneous appearance.

On several cuts of the coronal CT scans, the tumor was seen to involve the bone with questionable extension into the cranial cavity.

At surgery, the tumor was very difficult to distinguish owing to an abundance of scar tissue from the prior operations. Image guidance was particularly useful in this context for indicating areas of contrast enhancement or FLAIR hyperintensity that correlated well with the presence of the tumor within the scar tissue.

On histopathologic examination, the tumor was diagnosed as a recurrent pleomorphic adenoma with focal intraepithelial carcinoma; no invasive malignancy was seen. Two years after surgery, no recurrent disease was present.

EXTENDED CRANIOTOMY APPROACHES AND THE ORBIT

Of extended craniotomy approaches in neurosurgery, the one that most obviously applies to orbital tumors is the orbitofrontozygomatic craniotomy. This skullbase approach does add time to surgery but adds very little in terms of morbidity. This technique has become more routine in the neurosurgical repertoire over recent years.18–20 It consists of an extension of a frontotemporal craniotomy by removing the zygomatic arch along with the roof of the orbit. This is accomplished by performing osteotomies from the inferior orbital fissure through the orbitozygomatic process anteriorly, transecting the root of the zygoma posteriorly, and cutting and mobilizing the orbital rim along with the orbital roof. This craniotomy can be performed in one piece or in two pieces, as illustrated in Figure 32.7.

This approach not only provides wide exposure to the contents of the orbit but also provides extraordinary exposure to the lateral orbital and periorbital structures by the complete mobilization of the temporalis muscle (Figure 32.8). The approach is illustrated by the case of a 75-year-old man with a residual left middle fossa atypical meningioma invading the orbit (Figure 32.3). Surgery had been performed 3 months earlier by a neurosurgeon at another institution. Initial surgery had not been radical and had not extended to the orbit. At the time of our initial assessment, the patient’s vision was 20/30 and 20/60 in the right and left eye, respectively. A firm, irregular

mass was palpable in the left superior lateral orbit, causing a 3 mm proptosis and restriction of horizontal and vertical eye movements. The patient complained of diplopia at all gazes except the primary gaze. The left upper eyelid had ptosis. The left pupil revealed a 2 APD, but no disk edema or atrophy was detected. Goldmann perimetry revealed a large blind spot and arcuate field defect superiorly in the left eye. The ocular and systemic examinations were otherwise unremarkable.

The patient underwent an orbitofrontozygomatic craniotomy and superiolateral orbitotomy under the computer-assisted image guidance (Figure 32.2). The orbitofrontozygomatic osteotomy was mobilized, and the lateral roof of the orbit and the zygomatic process

were removed in one piece. The area just under the lacrimal “keyhole” was suspect for tumor infiltration. The extent of tumor into this area was identified with the help of image guidance. Meningioma was identified in the dura of the middle fossa, the skull base, lateral orbital wall, and extending into the lateral periorbita. Tumor margins of the orbital wall and periorbita were also monitored by image guidance and were confirmed by frozen sections during resection. Although the periorbita was involved, the orbital soft tissues were free of tumor. Ten months after surgery, the patient’s visual acuity in the left eye was 20/25 1 and he was free of diplopia at all gazes with 1 mm proptosis.

FIGURE 32.8. Exposure of orbital structures by an orbitofrontozygomatic craniotomy.

RADIOSURGERY

Stereotactic delivery of single fraction radiation has been available for many years and has on some occasions been employed in orbital tumors, or more commonly, in tumors extending into extraorbital areas such as the cavernous sinus. Radiosurgery requires the precise targeting of a tumor by many convergent linear radiation sources (gamma knife) or a single moving linear radiation source (LINAC: linear accelerator).21–23 At present, most of these systems still require the application of a rigid stereotactic frame, but more systems could develop “frameless” options in the near future. The major limitation in using this form of tumor treatment in or near the orbit is the known sensitivity of the visual pathways to radiation damage. Although we have radiosurgery systems at our disposal, we have little personal experience with radiosurgery of orbital tumors.

BIOPSIES AND MINIMALLY

INVASIVE APPROACHES

There are many advantages of image-guided surgery in the orbit. It can be particularly useful during minimally invasive approaches to pathologically altered orbital anatomy, especially deep in the orbit (i.e., small lesions in the posterior orbit). Even with larger, direct exposure, distorted anatomy from tumors, congenital anomalies and trauma can be confusing. We have already illustrated that image-guidance is also helpful in defining the extent of tumor infiltration in order to obtain tumor-free margins in combination with serial biopsies and pathological analysis. A further advantage of the technology in tumor surgery is the superimposition of CT and MRI images during the operation, since MRI offers greater soft tissue detail, but CT has better bone delineation. One of the potential uses of frameless stereotaxis for the orbital surgeon is to obtain biopsy material from the depth of the orbit without major surgery. Probes can easily reach the orbital apex and be guided to any lesion with accuracy. With a filter-type catching device at the end of a stereotactic suction, sufficient histological/cytological can be obtained. With the availability of Cytospin and/or frozen section capability, the material can be evaluated in 15 to 20 minutes during surgery. This technique can save a considerable amount of time over orbitotomy and is far less invasive. This is particularly useful in posteriorly located small lesions. Although Cygnus has a biopsy kit, we are developing one intended to better preserve orbital biopsy material. Image-guidance has also been reported as an adjunct to posterior orbital foreign body removal.14

CONCLUSIONS

We have presented some of the advantages of computer image guidance in the orbit and its surrounding structures. As with any technology, it is important to be familiar with the basic principles and limitations involved. We have also discussed both extended and minimally invasive approaches to tumors of the orbit.

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