Abstract
An anthropomorphic head phantom, constructed from a water‐equivalent plastic shell with only a spherical target, was modified to include a nonspherical target (pituitary) and an adjacent organ at risk (OAR) (optic chiasm), within 2 mm, simulating the anatomy encountered when treating acromegaly. The target and OAR spatial proximity provided a more realistic treatment planning and dose delivery exercise. A separate dosimetry insert contained two TLD for absolute dosimetry and radiochromic film, in the sagittal and coronal planes, for relative dosimetry. The prescription was 25 Gy to 90% of the GTV, with ≤10% of the OAR volume receiving ≥8Gy for the phantom trial. The modified phantom was used to test the rigor of the treatment planning process and phantom reproducibility using a Gamma Knife, CyberKnife, and linear accelerator (linac)‐based radiosurgery system. Delivery reproducibility was tested by repeating each irradiation three times. TLD results from three irradiations on a CyberKnife and Gamma Knife agreed with the calculated target dose to within ± 4% with a maximum coefficient of variation of ±2.1%. Gamma analysis in the coronal and sagittal film planes showed an average passing rate of 99.4% and 99.5% using ±5%/3mm criteria, respectively. Results from the linac irradiation were within ±6.2% for TLD with a coefficient of variation of ±0.1%. Distance to agreement was calculated to be 1.2 mm and 1.3 mm along the inferior and superior edges of the target in the sagittal film plane, and 1.2 mm for both superior and inferior edges in the coronal film plane. A modified, anatomically realistic SRS phantom was developed that provided a realistic clinical planning and delivery challenge that can be used to credential institutions wanting to participate in NCI‐funded clinical trials.PACS number: 87.55 ‐v
Highlights
IntroductionIt has been shown that pituitary adenomas, such as acromegaly, can be safely and effectively treated with stereotactic radiosurgery (SRS).(1,2) Frequently used radiosurgery systems for treating such brain lesions include Gamma Knife,(1,3-5) CyberKnife,(1,6,7) and linear accelerator (linac)-based treatments.[1,8,9] Because each system involves higher-than-normal doses per fraction and steep dose gradients,(2,8) there is an increased importance to accurately delivering the treatment to the patient.[2]
It has been shown that pituitary adenomas, such as acromegaly, can be safely and effectively treated with stereotactic radiosurgery (SRS).(1,2) Frequently used radiosurgery systems for treating such brain lesions include Gamma Knife,(1,3-5) CyberKnife,(1,6,7) and linear accelerator-based treatments.[1,8,9] Because each system involves higher-than-normal doses per fraction and steep dose gradients,(2,8) there is an increased importance to accurately delivering the treatment to the patient.[2]. Otherwise, the consequences can be devastating, as recently reported in the press.[10,11,12,13,14,15,16,17] It has been found that the most effective means of reducing the likelihood of a medical event is to attempt to reduce human errors involved in the radiation treatment.[18]
Treatment criteria and SRS guidelines were established by the RPC in conjunction with the Radiation Therapy Oncology Group (RTOG) to assess an institution’s ability to adequately deliver an SRS treatment.[21]. Phantom results are evaluated based on the following four criteria: 1) ratio of measured dose to reported dose at the center of the target to be 1.00 ± 0.05; 2) ratio of measured treated volume to reported treated volume falls within the range of 0.75–1.25; 3) minimum dose to the target is ≥ 90% of the prescription dose; and 4) treated volume to target volume ratio is between 1.00 and 2.00.(21) During phantom evaluation, the reported treated volume is determined from the prescription isodose line reported by the institution’s treatment planning system
Summary
It has been shown that pituitary adenomas, such as acromegaly, can be safely and effectively treated with stereotactic radiosurgery (SRS).(1,2) Frequently used radiosurgery systems for treating such brain lesions include Gamma Knife,(1,3-5) CyberKnife,(1,6,7) and linear accelerator (linac)-based treatments.[1,8,9] Because each system involves higher-than-normal doses per fraction and steep dose gradients,(2,8) there is an increased importance to accurately delivering the treatment to the patient.[2]. Treatment criteria and SRS guidelines were established by the RPC in conjunction with the Radiation Therapy Oncology Group (RTOG) to assess an institution’s ability to adequately deliver an SRS treatment.[21] Phantom results are evaluated based on the following four criteria: 1) ratio of measured dose to reported dose at the center of the target to be 1.00 ± 0.05; 2) ratio of measured treated volume to reported treated volume falls within the range of 0.75–1.25; 3) minimum dose to the target is ≥ 90% of the prescription dose; and 4) treated volume to target volume ratio is between 1.00 and 2.00.(21) During phantom evaluation, the reported treated volume is determined from the prescription isodose line reported by the institution’s treatment planning system. It has been reported that even with a single simple target, the majority of institutions irradiating this phantom fail to meet all four criteria,(21) indicating a need for this QA tool to improve patient treatments.[21]
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