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While the accuracy of patient setup itself has been well documented in stereotactic radiosurgery (SRS), the amount of rotational patient setup errors needed to impact this accuracy is not well defined. The purpose of this study is to explore potential rotational setup error thresholds for SRS treatments of brain metastases. Two groups were analyzed in this study: Group A consisted of twenty cases where a single brain metastasis treatment was planned using a single isocenter located within the tumor; Group B consisted of five cases where two brain metastases were planned using a single isocenter located between the tumors. All twenty-five cases underwent simulations for rotations around the treatment isocenter of ±1°, ±3°, ±5°, and ±7°. In addition, cases in Group B included simulations of ±0.5° and ±2°. The percent of the gross target volume (GTV) and planning target volume (PTV) that received the prescription dose (V p) was compared between the original plans and rotation simulations to determine what degree of rotations will cause V p loss greater than 0% and 1% respectively. In Group A, the average loss in V p for the GTV and PTV was 0% and below 1% respectively for rotations below ±3°. Using the Spearman correlation coefficient, V p loss in Group A showed very weak to weak correlation with physical characteristic of the tumor. In Group B, the average loss in V p for the GTV and PTV was 0% and below 1% respectively for rotations of ±0.5°. V p loss as a result of rotations showed very strong correlation with increasing tumor distance from treatment isocenter. Our study suggests that a patient rotational setup error threshold of ±2° around all axes for isocentric treatments is conservative. For nonisocentric treatments, the rotational setup error threshold may need to be as low as or lower than ±0.5° for tumors that are far from the treatment isocenter.

Purpose. Superposition dose calculation algorithms are known to suffer compromised accuracy in regions of electronic disequilibrium. This has particular relevance for treatment of lung nodules with electronic disequilibrium in their periphery using stereotactic body radiotherapy (SBRT). This study assessed the accuracy of a commercial superposition dose calculation algorithm within the context of SBRT in order to quantify the impact of these potential dosimetric uncertainties particularly in the periphery of the target. Method and materials. Selected lung SBRT 3D-CRT plans with 10–13 beams of 6 MV were originally created on the CMS/XiO treatment planning system with the superposition algorithm. The corresponding Monte Carlo (MC) plans were produced using the same patient geometry and plan parameters and compared with the CMS plans. Electronic disequilibrium was investigated by constructing a 2 mm thick 'peel' of the gross target volume (GTV) and planning target volume (PTV). An 18 MV plan was created corresponding to each 6 MV plan. Beam reduction was studied to investigate the dosimetric effect of beam arrangement. Results. The doses calculated by CMS agreed with MC in a water phantom but overestimated in very low density areas and the buildup and builddown regions. In patient plan comparison, for GTVs and GTV peels, CMS agreed with MC in mean dose within 2.4% for the 6 MV plans, and 3.3% for the 18 MV plans. For PTVs and PTV peels, CMS agreed with MC in mean doses within 2.5% for both 6 and 18 MV beams. The ratio in D 95 between CMS and MC was close to 1.0 for all 6 MV plans. Reducing beams could result in underdoses. Conclusions. The CMS superposition algorithm is able to perform accurate dose calculations for SBRT lung plans and ensure adequate peripheral dose. Dosimetric uncertainties are correlated to many factors such as density, energy, number of beams, tumor size/location, and density variation of the target and surrounding tissues.

Despite radical treatment with surgery, radiotherapy and chemotherapy, advanced gliomas recur within months. Geographic misses in radiotherapy planning may play a role in this seemingly ineluctable recurrence. Planning is typically performed on post-contrast MRIs, which are known to underreport tumour volume relative to FDOPA PET scans. FDOPA PET fused with contrast enhanced MRI has demonstrated greater sensitivity and specificity than MRI alone. One sign of potential misses would be differences between gross target volumes (GTVs) defined using MRI alone and when fused with PET. This work examined whether such a discrepancy may occur. Materials and Methods: For six patients, a 75 minute PET scan using 3,4-dihydroxy-6-18F-fluoro-L-phynel-alanine (18F-FDOPA) was taken within 2 days of gadolinium enhanced MRI scans. In addition to standard radiotherapy planning by an experienced radiotherapy oncologist, a second gross target volume (GTV) was defined by an experienced nuclear medicine specialist for fused PET and MRI, while blinded to the radiotherapy plans. The volumes from standard radiotherapy planning were compared to the PET defined GTV. Results: The comparison indicated radiotherapy planning would change in several cases if FDOPA PET data was available. PET-defined contours were external to 95% prescribed dose for several patients. However, due to the radiotherapy margins, the discrepancies were relatively small in size and all received a dose of 50 Gray or more. Conclusions: Given the limited size of the discrepancies it is uncertain that geographic misses played a major role in patient outcome. Even so, the existence of discrepancies indicates that FDOPA PET could assist in better defining margins when planning radiotherapy for advanced glioma, which could be important for highly conformal radiotherapy plans.