2473 - CBCT Guided Daily Online Adaptive Treatment with Reduced Margin to Treat Gastric MALT Lymphoma

X. Zhong, M. Rahman, A. Simmons, X. D. Li, M. Kozak, N. B. Desai, R. D. Timmerman, D. D. M. Parsons, B. Cai, K. A. Kumar Jr, and M. H. Lin; Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX

Purpose/Objective(s): Gastric MALT lymphoma is typically treated with a 1-2 cm margin using IGRT due to significant daily variations in target size and location. With the cone-beam computed tomography (CBCT)-guided online adaptive treatment system, it is possible to reduce margins and spare doses to OARs. This study evaluates the feasibility of CBCT-based online adaptive treatment for gastric MALT lymphoma patients. Materials/

Methods: Eight patients with gastric MALT lymphoma receiving a 24 Gy prescription from 2022 to 2024 were retrospectively analyzed with IRB approval. All patients underwent CBCT-guided daily adaptive treatment with breath-hold over ninety fractions. 5mm margin was used to derive PTV from ITV based on previous intra-fraction motion study. Scheduled plans, which were pre-plans calculated on daily CBCT contours, were compared with adapted plans, which were reoptimized based on daily CBCT contours. PTV and ITV coverage, conformity index (VRx/VPTV), heart dose, and monitor unit numbers (MU) were assessed for both the scheduled plans and adapted plans.
Results: Across all fractions, the adapted plan was picked over the scheduled plan. The mean coverage of the PTV and ITV dropped from 95% to 69% and 70% respectively with the scheduled plan, showing the necessity of daily adaptive treatment. The mean heart dose is 1.7 ± 0.8 Gy for adapted plans, which is lower than the 3.2Gy (1.2–7.0) reported in previous large margin study. The preplan monitor units (MU) are 1618 ± 214, and the MU of the adaptive plan has a 4.7 ± 10.5% variation from the preplan. Since the MU is related to the patient’s number of breath hold. A stable and relatively low MU ensures that the patient’s breath hold is more consistent, resulting in a more reliable treatment.
Conclusion: CBCT-guided daily adaptive treatment allows for reduced margins (0.5 cm) for gastric MALT lymphoma, maintaining PTV coverage while reducing heart doses compared to conventional IGRT large margin plans. Adapted plans demonstrate consistent MU and plan quality with low standard deviation. This demonstrates the potential of CBCT-based adaptive treatment in reducing the long-term treatment toxicity. Abstract 2473 – Table 1: Comparison of plan quality metrics

Parameter	Scheduled Plan	Adapted Plan	p-value (t test)
PTV Coverage (%)	59.9±18.7	94.9±0.2	<0.01
Conformity Index	0.7±0.2	1.0±0.0	<0.01
ITV Coverage (%)	69.9±22.0	99.9±0.1	<0.01
Heart D0.03cc (cGy)	2052±499	2258±291	<0.01
Heart Dmean (cGy)	162±96	171±82	0.5