(Tuesday, 7/31/2018) 10:00 AM - 10:30 AM
Room: Exhibit Hall | Forum 4
Purpose: To perform a dosimetric study that incorporates variable RBE to better assess the treatment of anal squamous cell carcinoma (SCCa) with IMPT.
Methods: We analyzed nine patients who had been treated for SCCa with either VMAT (n=8) or IMPT (n=1) (50 Gy or 54 Gy in 25 or 27 fractions). VMAT or IMPT were created retrospectively such that each patient had both plans. Dose was calculated using an analytical dose calculation algorithm from a commercial treatment planning system (TPS). IMPT plans utilized two posterior oblique beams (40-70 degree separation) and an anterior beam for inguinal nodes. Field specific targets were created for each beam, and the plan was robustly optimized with 0.5 cm isocenter-shift and 3.5% range uncertainty in combination (12 perturbations). Plans’ DICOM files were then exported to the Fast Dose Calculator (FDC), which uses a track repeating algorithm to more accurately calculate dose and account for density heterogeneities than the TPS. FDC was used to re-calculate doses with a 1.1 constant RBE and with two variable RBE models (RBE1 and RBE2).
Results: IMPT plans were able to achieve similar target coverage compared to VMAT with improved OAR avoidance. The largest improvement was for bone marrow (mean: 20.7Gy [ranging 18.5-23.8Gy] vs. 28.2 [23.0-31.2Gy], V10Gy: 60.2% [55.9-66.1%] vs 92.1% [84.4-100.0%]). The FDC showed a maintenance of target coverage with some increases in OAR doses, most notably an increase in max bowel dose with variable RBE models. Bone marrow sparing over VMAT remained with FDC.
Conclusion: IMPT offers a means of lowering dose to OARs, particularly the bone marrow, when treating SCCa. Reduction in hematologic toxicities such as lymphopenia may therefore be possible with IMPT without sacrificing target coverage.
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