Evaluating the Accuracy of Different Methods for Estimating the Dose to An IMD (implanted Medical Device) of a TBI Patient, Treated at An Extended Distance with External Blocks
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PO-GePV-T-197 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: We treat TBI patients at an extended distance (440 SAD), with Cerrobend blocks approx. 5 cm from the patient. A CT scan in the treatment planning position is not possible (the MUs are determined by a hand calculation), making estimating the dose to an IMD challenging. We compare the accuracy of different methods for estimating the dose to the IMD.
Methods: We estimated the dose using three different
methods: Eclipse calculations using AAA and Acuros algorithms, and ion chamber and OSLD measurements in solid water. The results were analyzed and compared to OSLD measurements from treatment. Four-7.5 cm thick blocks of different sizes were fabricated. Patient separation (16 cm) and depth of the IMD from the anterior surface (2.5 cm) were estimated from a CT scan. A 16*25*25 cm solid water phantom using an A12 ion chamber was used to perform measurements at IMD depth. NanoDot OSLDs, under 0.5 cm bolus, were used for measurements on the surface. A virtual water phantom of 16*25*25 cm, with a block 5 cm above the phantom, was used for the calculations. Measurements and calculations were performed to estimate the dose to the IMD, under the center of the block, for a single fraction in a 6 MV AP/PA treatment. A patient was treated using one of the blocks, and OSLD measurements were performed on the patient’s skin during the treatment.
Results: The OSLD and ion chamber measurements in solid water agree well: ion chamber 34 cGy, OSLD 35 cGy, Acuros 38 cGy, AAA 65 cGy. They also agree with OSLD measurement during treatment (34 cGy).
Conclusion: Ion chamber and OSLD measurements in solid water predicted the results of the patient measurements well. Acuros was accurate enough to provide a rough estimate, while AAA overestimated the dose to IMD significantly.
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