3772 - Dosimetric Benefit of Online Adaptive Radiotherapy for Nasopharyngeal Carcinoma

G. Zhou¹, Y. X. Yang¹, L. Lin², L. Jia³, H. Li³, Y. Liu³, W. Zhang⁴, J. Zhou⁴, and Y. Sun²; ¹Sun Yat-sen University Cancer Center; State Key Laboratory of Oncology in South China; Collaborative Innovation Center for Cancer Medicine; Guangdong Key Laboratory of Nasopharyngeal Carcinoma Diagnosis and Therapy, Guangzhou, China, ²Department of Radiation Oncology, Sun Yat-sen University Cancer Center, State Key Laboratory of Oncology in South China, Collaborative Innovation Center for Cancer Medicine, Guangzhou, China, ³Shenzhen United Imaging Research Institute of Innovative Medical Equipment, Shenzhen, China, ⁴Shanghai United Imaging Healthcare Co., Ltd., Shanghai, China

Purpose/Objective(s): Artificial intelligence-powered online adaptive radiation radiotherapy (ART) workflow is feasible for nasopharyngeal carcinoma (NPC). In this study, we aimed to evaluate the dosimetric benefits of online ART for NPC patients. Materials/

Methods: From April 2022 to November 2023, 120 patients with newly-diagnosed, non-metastatic NPC received a single session of online ART during definitive radiotherapy were included in this study. The cohort predominantly presented with advanced disease (85.8% stage III-IVA), with a median age of 46.5 years. Online ART was implemented using a CT-linear accelerator and integrated multiple radiotherapy procedures, including re-simulation, rigid registration, copying of GTVp, auto-contouring, contours modification, auto-planning, plan evaluation, image-guided beam delivery and in vivo quality assurance. This entire process was performed with the patient positioned on the treatment couch. The median timefor online ART was 21^st fraction (range, 15^th -30^th fraction). The original plan was copied to the re-simulation CT to create the scheduled plan. Difference in dosimetric parameters for targets and organs at risk (OARs) between the adapted plan/scheduled plan and original plan were quantified as percentage deviation. The percentage deviations of the adapted and scheduled plans were analyzed using paired t tests or Wilcoxon signed rank tests. A two-sided P-value < 0.05 was considered statistically significant.
Results: The percentage deviation of the adapted plan from the original plan was significantly reduced for all targets and all OARs except submandibulars compared to that of the scheduled plan. The planning target volume (PTV) of involved cervical lymph nodes (GTVn) and clinical target volumes (CTV2) showed greatest improvement in target coverage after online ART. The mean percentage deviations in the volume of PTV receiving 100% of the prescription dose (V100%) for PGTVn and PCTV2 were -8.91% and -4.14% in the scheduled plan, compared to 0% and -0.20% in the adapted plan (P < 0.001) (Table 1). The parotids and the spinal cord showed the most significant dosimetric reductions after online ART, with deviations of 32.84% and 5.70% in the scheduled plan, compared to 4.71% and -0.33% in the adapted plan, respectively (P < 0.001) (Table 1).
Conclusion: The online ART substantially enhanced target coverage while significantly reducing the dose to most OARs in patients with NPC. Abstract 3772 – Table 1: The percentage deviation of the adapted plan and schedule plan from the original plan

Targets	Parameters	Percentage deviation, %				P
		Adapted plan		Scheduled plan
		Mean	95% CI	Mean	95% CI
PGTVp	V100%	-0.01	-0.11, 0.10	-0.01	-0.27, 0.25	< .05
PGTVn	V100%	0	-0.09, 0.09	-8.91	-10.79, -7.04	< .001
PCTV1	V100%	0.03	-0.06, 0.13	-0.11	-0.30, 0.08	< .001
PCTV2	V100%	-0.20	-0.30, -0.10	-4.14	-4.80, -3.48	< .001
Parotids	Dmean	4.71	3.45, 5.97	32.84	25.36, 40.33	< .001
Spinal cord	D0.03cc	-0.33	-1.12, 0.46	5.70	4.65, 6.74	< .001