Accuracy of Dose Calculation for Hemi-Body Treatment at Extended Distance using a Commercial Treatment Planning System

Abstract

The objective of this study was to assess the accuracy of Monitor Unit (MU) calculation for extended distance hemi-body (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 patients, particularly those with Non-Hodgkin’s Lymphoma (NHL), a cancer of the lymphocytes. Past studies also indicate that a LD-RT regime will present minimal toxicity and limited adverse side effects. LD-RT may directly trigger a systemic immune response. 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 non-targeted LD-RT for recurrent prostate cancer. Water Phantom: A plan was created in Pinnacle to deliver 100 cGy to a water phantom with an ion chamber (IC) mount. A percent depth dose (PDD) was obtained. Electrometer readings were recorded with each irradiation of 400 MU at varying IC depths at extended distance. A PDD was created from tabulated data. Anthropomorphic Phantom: A parallel opposed pair (POP) 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 (SSD). The MU required to deliver 150 cGy, as per Pinnacle were delivered to the phantom using 6 MV photons. Thermoluminescent dosimeters, used to measure exposure using light-emitting crystals, were placed along six reference locations on the phantom (lung, mid-T-spine, abdomen, mid-pelvis, thigh, and mid-abdomen). TLD measurements were then compared to the Pinnacle-derived ROI mean doses. For Experiment 2, TBI calculation factors were used to determine the required MU 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. Water Phantom: Pinnacle did not accurately estimate dmax at extended distance, however, it did accurately estimate the dose past dmax. Anthropomorphic Phantom: A 10% variation to expected dose was deemed significant. Lung dose did not present significant variation. The abdomen presented borderline significance. Remaining ROIs presented significant variation for both Pinnacle and TBI calculations. Both Pinnacle and TBI calculations were accurate methods of planning HB LD-RT treatment, with insignificant difference. Pinnacle accurately estimated dose to the Rx point but overestimated the dose to remaining ROIs. Overall average variation was borderline significant at 12.1%. At extended SSD, Pinnacle inaccurately estimated the entrance dose and dmax. PDD data supported Pinnacle’s ability to accurately estimate dose at depths greater than dmax. 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 recruited for HB LD-RT at the Juravinski Cancer Centre.