Room: AAPM ePoster Library
Purpose: The strict sensitivities of intensity-modulated proton radiotherapy (IMPT) to changes in tumor size, and patient geometry warrant adaptive radiotherapy (ART). As a result, weekly quality-assurance CT scans (QACTs) are commonly used as indicators for ART. We investigate the feasibility of using as ART indicator the dose calculated on in-situ cone-beam CTs (CBCTs) for patients with bulky head-and-neck (HN), abdomen, and pelvis tumors.
Methods: Same-day QACT and CBCTs of 10 IMPT patients (pelvis, head & neck, abdomen and lung) were randomly selected and imported into a research version of the clinical treatment-planning system (TPS; RayStation 9B, RaySearch). Each patient had 1-2 QACTs and CBCTs (total: 15). Using a vendor developed prototype (RaySearch Laboratories, Sweden), the CBCT was corrected from artifacts. Subsequently, a CBCT-based virtual-CT with correct Hounsfield units was generated by deforming the planning CT. The treatment plan was subsequently recomputed on both the QACT and virtual-CT. Differences between clinical target volume (CTV) dose parameters such as D98%, D95%, D2%, mean dose, and V95% were compared between the two image sets.
Results: For all 10 patients (15 different scenarios), the average difference in CTV mean dose was 0.3% ± 0.5%, D2% was 0.0% ± 0.8%, D95% was 0.8% ± 1.7%, D98% was 0.8% ± 1.6%, and V95% was 0.4% ± 0.9%. Virtual-CTs also accurately indicated scenarios where ART had been required from QA-CTs.
Conclusion: For the ten patients cohort considered, dose distributions computed on virtual-CTs closely matched high fidelity QACT scans. Our results suggest that CBCT-based dosimetry can be used to evaluate treatment plans, and guide the decision for ART over the treatment course.