Purpose/Objective: Most dose-escalation trials in glioblastoma patients integrate the escalated dose throughout the standard course by targeting a specific subvolume. We hypothesize that anatomical changes during irradiation may affect the dose coverage of this subvolume for both proton- and photon-based radiotherapy. Material and Methods: For 24 glioblastoma patients a photon- and proton-based dose escalation treatment plan (of 75 Gy/30 fr) was simulated on the dedicated radiotherapy planning MRI obtained before treatment. The escalated dose was planned to cover the resection cavity and/or contrast enhancing lesion on the T1w post-gadolinium MRI sequence. To analyze the effect of anatomical changes during treatment, we evaluated on an additional MRI that was obtained during treatment the changes of the dose distribution on this specific high dose region. Results: The median time between the planning MRI and additional MRI was 26 days (range 16–37 days). The median time between the planning MRI and start of radiotherapy was relatively short (7 days, range 3–11 days). In 3 patients (12.5%) changes were observed which resulted in a substantial deterioration of both the photon and proton treatment plans. All these patients underwent a subtotal resection, and a decrease in dose coverage of more than 5% and 10% was observed for the photon- and proton-based treatment plans, respectively. Conclusion: Our study showed that only for a limited number of patients anatomical changes during photon or proton based radiotherapy resulted in a potentially clinically relevant underdosage in the subvolume. Therefore, volume changes during treatment are unlikely to be responsible for the negative outcome of dose-escalation studies.
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Op verzoek van Jelle Scheurleer: Purpose: To investigate the accuracy of dose calculation on cone beam CT (CBCT) data sets after HU-RED calibration and validation in phantom studies and clinical patients. Material and methods: Calibration of HU-RED curves for kV-CBCT were generated for three clinical protocols (H&N, thorax and pelvis) by using a Gammex RMI phantom with human tissue equivalent inserts and additional perspex blocks to account for patient scatter. Two calibration curves per clinical protocol were defined, one for the Varian Truebeam 2.0 and another for the OBI systems (Varian, Palo Ato). Differences in HU values with respect to the CT-calibration curve were evaluated for all the inserts. Four radiotherapy plans (breast, prostate, H&N and lung) were produced on an anthropomorphic phantom (Alderson) to evaluate dose differences on the kV-CBCT with the new calibration curves with respect to the CT based dose calculation. Dose differences were evaluated according to the D2%, D98% and Dmean metrics extracted from the DVHs of the plans and - evaluation (2%, 1mm) on the three planes at the isocenter for all plans. Clinical evaluation was performed on 5 patients and dose differences were evaluated as in the phantom study.
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