Room: AAPM ePoster Library
Purpose: Radiotherapy for treatment of gastric lymphoma should be delivered to the whole stomach with optimal planning target volume (PTV) margins, with consideration of variations in stomach size and respiratory movement. This study retrospectively analyzed daily four-dimensional cone-beam computed tomography (4D-CBCT) images to determine the optimal PTV margin in three target localization methods.
Methods: Eight patients underwent radiotherapy with 15 fractions of gastric mucosa-associated lymphoid tissue lymphoma using 4D-CBCT. The internal target volume of the stomach was delineated based on the 4D-CT images. For the analysis of target localization, we performed matching based on skin marks (skin matching), bone anatomy matching (bone matching), and stomach anatomy matching based on registration between planning CT and 4D-CBCT images (4D matching). For each patient, we calculated the covering ratio (CR) with PTV margins of 5 to 30 mm (5 mm intervals), based on the images of all phases [CR (%) = the number of covering phases / all 150 phases×100], in three target localization methods. We compared the CR between the three target localization methods and defined a PTV margin with an average CR of = 95% for all patients as optimal.
Results: The CR by 4D matching was superior to that by skin and bone matching for PTV margins of 5 to 25 mm. The optimal PTV margins (average CR = 95%) were 20 mm (average CR 95.2%), 25 mm (average CR 99.1%), and 15 mm (average CR 95.3%) for skin, bone, and 4D matching, respectively.
Conclusion: The results of this study revealed the optimal PTV margin required for the three target localization methods of gastric lymphoma radiotherapy. 4D matching using 4D-CBCT provides a smaller PTV margin than skin and bone matching, while assessing variations in stomach size and respiratory movement on a daily treatment.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP17K16459. The authors declare that they have no competing interests.