3390 - Treatment of Patients with Gastric Extranodal Marginal Zone Lymphomas (MZL) Using CBCT-based Adaptive Radiotherapy (ART) to Reduce Margins

K. A. Kumar Jr¹, M. Kozak¹, X. Zhong¹, M. Rahman¹, A. Simmons¹, X. D. Li², N. B. Desai¹, D. D. M. Parsons¹, B. Cai¹, M. H. Lin¹, and R. D. Timmerman³; ¹Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, ²Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, ³Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, TX

Purpose/Objective(s): Patients with gastric extranodal MZL of mucosa associated lymphoid tissue i.e. gastric MALT lymphoma have excellent outcomes after involved site radiation treatment (ISRT), but treatment with IGRT requires large (1-2 cm) margins due to significant daily variations in target size, shape, and location, resulting in extra dose to nearby OARs such as the heart, liver, kidney, and bowel. Therefore, we implemented daily CBCT-based ART in the treatment of gastric MALT lymphoma to reduce margins. We hypothesize that this novel technique will be feasible in reducing dose to nearby OARs while resulting in similar excellent clinical outcomes to conventional IGRT with larger margins. Materials/

Methods: An IRB-approved prospective database was reviewed identifying patients with gastric MALT lymphoma who were treated with CBCT-guided daily ART using breath hold with a planning treatment volume (PTV) margin of 0.5cm. As a comparator, new plans were created for the same patients using a 1.5cm margin per current International Lymphoma Radiation Oncology Group guidelines, and dosimetric parameters to OARs were compared. Additionally, for each fraction of ART, PTV coverage was compared between adapted plans vs. scheduled plans (original 0.5 cm PTV plan without daily adaptation). Demographic, disease- and treatment-related variables were collected. Primary outcomes include objective response rate (ORR), defined as complete (CR) or partial response (PR) based on follow up EGDs, local relapse, and radiation-related toxicity per CTCAE v5. Secondary outcomes include overall disease control and survival.
Results: 10 patients were identified with median age of 62 and 7 (70%) women. 8 (80%) had stage IE, 1 (10%) stage IIE, and 1 (10%) stage IV gastric MALT lymphoma. 9 (90%) were H. pylori-, with the 1 H. pylori+ having not responded to triple therapy initially. One patient had rituximab monotherapy x 4 prior to ISRT. 8 (80%) were treated to 24 Gy in 12 fractions, while the other 2 (20%) received 4 Gy in 2 fractions (1 was stage IV and treated with concurrent rituximab, the other stopped treatment early due to acute toxicity/patient preference). The median follow up is 14 months (range 6-24). ORR was 100%, with CR in 90% and PR in 10%. No relapses occurred, and 1-year PFS and OS is 100%. Compared to standard margin (1.5 cm) plans, reduced margin (0.5 cm) daily ART plans have a significant reduction in all OAR doses, with mean heart dose reduced by more than 60% from an average of 3.1Gy to 1.2Gy. With reduced margin (0.5 cm), scheduled plans (CBCT IGRT only) had a mean coverage to PTV and ITV of 69% and 70% respectively, compared to 95% with daily adapted plans, showing the necessity of daily adaptive treatment with reduced margins.
Conclusion: CBCT-based daily ART in gastric MALT lymphomas allows margin reduction to 0.5 cm while still maintain excellent clinical outcomes. This substantially reduces dose to nearby OARs and thus has become the standard treatment of gastric MALT lymphomas at our institution.