Accuracy of Dose Calculation for Hemibody Treatments at Extended Distance Using a Commercial Treatment Planning System

PURPOSE: The objective of this study was to assess the accuracy of monitor units (MUs) calculation for extended distance hemibody (HB) treatments in Pinnacle, a commercial treatment planning system. The agreement between planning and delivery of low-dose radiation therapy (LD-RT) was assessed with direct comparison to expected doses and tabulated total body irradiation (TBI) calculations. Studies over the past decades indicate that LD-RT has strong potential to be an effective treatment modality for cancer patients with minimal toxicities. This physics-based study aims to provide sufficient conclusions required for prospective clinical studies involving HB irradiation regimes. Specifically, this study may provide reassurance of MU calculation in the Pinnacle system for an upcoming trial regarding nontargeted LD-RT for recurrent prostate cancer. METHODS: Water phantom: A plan was created in Pinnacle to deliver 100 cGy to a water phantom with an ion chamber mount. A percent depth dose was obtained. Electrometer readings were recorded with each irradiation of 400 MUs at varying ion chamber depths at extended distance. A percent depth dose was created from tabulated data. Anthropomorphic phantom: A parallel opposed pair plan was created in Pinnacle to deliver 150 cGy over 10 fractions to the umbilicus of the phantom at 4 m extended source-to-surface distance. The MUs required to deliver 150 cGy, as per Pinnacle were delivered to the phantom using 6 MV photons. Thermoluminescent dosimeters (TLD), used to measure exposure using light-emitting crystals, were placed along six reference locations (lung, mid-T-spine, abdomen, mid-pelvis, thigh, and mid-abdomen) on the phantom. TLD measurements were then compared with the Pinnacle-derived ROI mean doses. For experiment 2, TBI calculation factors were used to determine the required MUs to deliver 150 cGy to the prescription (Rx) point. The calculated MUs were delivered, and TLD readings were recorded to compare the level of agreement of using TBI calculations for HB treatments. RESULTS: Water phantom: Pinnacle did not accurately estimate d_max at extended distance; however, it did accurately estimate the dose past d_max. Anthropomorphic phantom: A 10% variation to expected dose was deemed significant. Both Pinnacle and TBI calculations were accurate methods of planning HB LD-RT treatment, with insignificant difference. Pinnacle's overall average variation across ROIs was borderline significant at 12.1%. CONCLUSION: At extended source-to-surface distance, Pinnacle inaccurately estimated the entrance dose and d_max. Anthropomorphic phantom studies indicated borderline significant variation, as per the implemented 10% limit. TBI calculations presented similar conclusions. For purposes of HB LD-RT, a borderline 10% variation will have insignificant impact to the patient's ability to tolerate treatment. Trial-eligible prostate cancer patients are currently being treated for HB LD-RT at the Juravinski Cancer Centre.