BackgroundRadiation retinopathy is a possible post-treatment complication of radiation therapy. The pathophysiologic mechanism is hypothesized to be microvascular in origin, but evidence is limited. In an effort to study retinal oxygenation in these patients, we herein evaluate the repeatability and variability of retinal oximetry measurements in subjects who had previously received radiation and make comparisons to a cohort of unirradiated subjects.MethodsUsing retinal oximetry, a non-invasive imaging modality, we performed in vivo measurements of arteriole (SaO2) and venule SO2 (SvO2) in subjects (n = 9, 18 retinas) who had received incidental radiation to their retinas (≥ 45 Gy to one retina) and in healthy subjects (n = 20, 40 retinas). A total of 1367 SO2 observations on 593 vessels in 29 persons were analyzed to assess three sources of variance in vessel SO2: 1) variance in repeated measurements of the same vessel (“repeatability”), 2) variance in different vessels within the same subject (“within-subject variability”), and 3) variance between subjects (“between-subject variability”).ResultsRetinal oximetry measurements were highly repeatable in both irradiated patients and unirradiated subjects. The within-subject variability of SvO2 and SaO2 measurements constituted the highest component of variance in both groups and was significantly higher in venules vs. arterioles (relative effect size 1.8, p<0.001) and in irradiated subjects vs. unirradiated subjects (relative effect size 1.6, p<0.001).ConclusionsRetinal oximetry is a highly repeatable technology and can be reliably used to study vascular oxygenation in irradiated subjects. Different vessels within the same subject exhibit a high degree of variability, suggesting that pooled analyses of multiple vessels are most likely to be informative of regional retinal oxygenation. Finally, irradiated subjects exhibited significantly higher within-subject variability in SO2 measurements, suggesting that radiation may cause regional alterations in retinal oxygen delivery and/or metabolism.
Purpose: We describe a whole spinal cord, cauda equine, and brainstem radiation treatment using an in‐house developed tomotherapy approach for a unique patient diagnosed with an extramedullary spinal melanocytoma with leptomeningeal seeding, treated with 48.6 Gy in 28 fractions. Methods and Materials: Given that the prescribed dose is within the range of tolerance to the spinal cord, tomotherapy was chosen to take advantage of the superior dose uniformity achievable with this technology and ability to deliver modulated radiotherapy in a single treatment to a long volume. The patient was treated supine and immobilized with a thermoplastic mask for the head and shoulders and a long Vac‐Lok bag for the body. The CTV consisted of the entire spinal cord, thecal sac to the level of S2 and brainstem. A 1 cm margin was applied to create the PTV. Jaws, pitch and the modulation factor were set to 5 cm, 0.43 and 2.5, respectively. Before each treatment, the treated volume was imaged for setup verification using the integrated megavoltage CT (MVCT). Weekly the patient was also imaged post‐ treatment to confirm setup stability. Results: The patient is finishing treatment at the time of abstract submission. A highly conformal dose distribution was created with doses to the organs at risk within their tolerance limits. The beam on time was 17 minutes. Patient setup proved to be trouble‐free and reproducible. The total patient‐on‐the‐bed time was approximately one hour. The 1 cm PTV margin was adequate according to pre‐ and post‐treatment MVCT image analysis. Conclusions: Tomotherapy is a safe and effective tool for treating long CNS volumes to high dose. It allows avoiding junctions and sparing healthy CNS tissue and other organs at risk. A relatively simple immobilization technique used for this patient proved to be stable and reproducible with a 1 cm PTV margin.
Purpose: To investigate a new modulated beam orientation optimization (MBOO) approach maximizing treatment planning quality for the state‐of‐the‐art flattening filter free (FFF) beam that has enabled rapid treatments of multiple brain targets. Methods: MBOO selects and optimizes a large number of intensity‐modulated beams (400 or more) from all accessible beam angles surrounding a patient's skull. The optimization algorithm was implemented on a standalone system that interfaced with the 3D Dicom images and structure sets. A standard published data set that consisted of 1 to 12 metastatic brain tumor combinations was selected for MBOO planning. The planning results from various coplanar and non‐coplanar configurations via MBOO were then compared with the results obtained from a clinical volume modulated arc therapy (VMAT) delivery system (Truebeam RapidArc, Varian Oncology). Results: When planning a few number of targets (n<4), MBOO produced results equivalent to non‐coplanar multi‐arc VMAT planning in terms of target volume coverage and normal tissue sparing. For example, the 12‐Gy and 4‐Gy normal brain volumes for the 3‐target plans differed by less than 1 mL ( 3.0 mLvs 3.8 mL; and 35.2 mL vs 36.3 mL, respectively) for MBOO versus VMAT. However, when planning a larger number of targets (n≥4), MBOO significantly reduced the dose to the normal brain as compared to VMAT, though the target volume coverage was equivalent. For example, the 12‐Gy and 4‐Gy normal brain volumes for the 12‐target plans were 10.8 mL vs. 18.0 mL and 217.9 mL vs. 390.0 mL, respectively for the non‐coplanar MBOO versus the non‐coplanar VMAT treatment plans, yielding a reduction in volume of more than 60% for the case. Conclusion: MBOO is a unique approach for maximizing normal tissue sparing when treating a large number (n≥4) of brain tumors with FFF linear accelerators. Dr Ma and Dr Sahgal are currently on the board of international society of stereotactic radiosurgery. Dr Sahgal has received support for educational presentations from Elekta company
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