PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s): For the management of small brain tumors, cranial stereotactic radiosurgery (SRS) has been used for several years. The administration of high, ablative doses in SRS, requires rigorous quality assurance (QA) procedures to verify the high spatial and dosimetric accuracy of the delivery. Patient specific pre-treatment measurements for plan verification, when performed, use generic non-anatomical water equivalent phantoms. This work aims to evaluate a 3D printed personalized QA phantom as a means to comprehensively perform pre-treatment QA for SRS. In our implementation, the in-phantom dose measurements with a film are evaluated against the corresponding in-patient TPS calculations for a multi-focal SRS case.
Materials/Methods: An anonymized mono-isocentric multi-focal SRS case of 6 brain metastasis was selected for this study. The patient’s planning CT scan was used to produce a 3D model of the bony and skin anatomy of the patient. A 3D printer was used to construct the personalized phantom using bone equivalent 3D printed material. The phantom was machined to accommodate a tissue equivalent insert for film dosimetry on a coronal plane that included three of the six targets. The phantom was then filled with water. Four metal pins were introduced in the film holder to secure film positioning and aid in the spatial registration during film analysis. A radiochromic film from a calibrated batch was used for dose measurements. The phantom was treated as if it was the real patient, including localization with image guidance and plan delivery. To evaluate a ‘phantom-to-patient’ dosimetric equivalency, film measurements were compared against the 3D dose distribution calculated in the real patient CT scan, based on 1D, 2D dose distributions, and gamma index.
Results: In-phantom absolute dose measurements were in very good agreement with corresponding in-patient TPS calculations within the targets we evaluated and the low dose area interspace. After applying a dose threshold of 10%, gamma passing rates exceeded 98% using evaluation criteria of 2mm and 2%. A reduced gamma passing rate of 87% was calculated for 1mm and 2% gamma evaluation. The reduced passing rate is reasonable since it was at the limit of the spatial and dosimetric accuracy of the film dosimetry protocol. Spatial uncertainties related to set-up, image-guidance, dose delivery and registration of film measurements to the 3D calculated data, are also reflected in the gamma passing rates.
Conclusion: A feasibility study was presented for personalized pre-treatment QA. A 3D printed patient-specific QA phantom was constructed from the CT scan of an SRS patient. The patient’s plan was verified both for geometric and dosimetric accuracy with a 98% passing rate. The ‘phantom-to-patient’ dosimetric equivalency was verified within the accuracy of the dosimetric protocol and dose delivery.
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