Room: AAPM ePoster Library
Purpose: The stomach displays large changes in size and shape, which implies the need for adaptive radiotherapy in pre-operative gastric cancer. We evaluated the feasibility and necessity of a CBCT-guided library of plans (LoP) strategy to increase the accuracy of gastric cancer radiotherapy.
Methods: Eight patients treated with 24-25 fractions of single-plan pre-operative radiotherapy with daily CBCT-guidance within the CRITICS-II trial (NCT02931890) were included. The target (i.e., tumor, entire stomach and regional lymph nodes) was delineated retrospectively by an expert radiation oncologist on planning CT (pCT) and the first 5 CBCTs. These delineations were used to evaluate single-plan radiotherapy and to create a patient-specific LoP. Plan selections were performed on the remaining CBCTs by 12 observers (6 expert radiation therapy technologists (RTTs), 2 RTTs, 3 radiation oncologists, 1 physicist) in a training stage (2-3 CBCTs per patient; 23 total) and an assessment stage (17 CBCTs per patient; 136 total). Observers were asked to select the smallest plan encompassing the target on CBCT.
Results: Of the 40 delineated targets on CBCTs, 90% were partially outside the PTV of the pCT (PTV, planned high dose region, is target + 10 mm), with on average 2.9% of the volume (range: 0.0%-12.5%). At least two-thirds of the observers agreed on a selected plan in 65.2% and 70% in the training and assessment stage, respectively. For each patient, at least two plans from the library were the most selected plan in the 17 fractions per patient.
Conclusion: A CBCT-guided patient-specific LoP strategy is feasible for gastric cancer patients, yielding reasonable agreement in plan selections. Likely, part of the target is missed with single-plan radiotherapy, signifying the urgent need for gastric adaptive radiotherapy. We clinically implemented this strategy thus far in 2 patients for whom all plans in the library were selected at least once.
Funding Support, Disclosures, and Conflict of Interest: This work was funded by the Dutch Cancer Society (KWF Kankerbestrijding; Grant No. KWF 10882). The department of radiation oncology of the Netherlands Cancer Institute receives license fees from Elekta Systems AB, Sweden for cone beam CT-guided software.