3761 - Optimal Target-Specific PTV Margins for Nasopharyngeal Carcinoma Radiotherapy with Daily FBCT Guidance

Y. X. Yang¹, G. Q. Zhou¹, L. Lin², L. Jia^3,4, H. Li³, Y. Liu³, X. Jiang⁵, G. Y. Wang¹, 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, ⁴Zhejiang Engineering Research Center for Innovation and Application of Intelligent Radiotherapy Technology, Wenzhou, China, ⁵Department of Radiation Oncology, Sun Yat-sen University Cancer Center, Guangzhou, China

Purpose/Objective(s): To assess daily targets variations using fan beam computed tomography (FBCT) and determine optimal target-specific planning target volume (PTV) margins for intensity-modulated radiation therapy (IMRT) of nasopharyngeal carcinoma (NPC). Materials/

Methods: From March 2022 to July 2023, 13 NPC patients (II:1, III:9, IVA:3) received radical IMRT (6996cGy/33Fr) with daily FBCT guidance. Excluding missing FBCTs due to machine maintenance and those with insufficient scan coverage, 352 FBCTs were retrospectively collected. Organs at risk (OARs) and targets on FBCTs, including gross tumor volumes (GTVp), involved cervical lymph nodes (GTVnd), clinical target volumes (CTV1 and CTV2) were auto-contoured and reviewed by two radiation oncologists. CTV2 was segmented into upper (CTV2_U), middle (CTV2_M), and lower (CTV2_L) parts at the inferior border of mastoid and hyoid. Targets on daily FBCTs were copied onto the planning CT (pCT) based on treatment registration. Volume coverage of daily FBCT targets by 1-3 mm PTV margins was calculated as (Target_FBCT n PTV_pCT) / Target_FBCT. Auto-planning was performed on the pCT with proposed target-specific and 3mm PTV margins, and then the two plans were copied to FBCTs. Dose to 95% of volume (D95) of PGTVp, PGTVn, PCTVs, and OARs doses were calculated based on weekly FBCTs. Dose coverage of targets and sparing of OARs between two plans were compared.
Results: The average volume coverage of targets by 3 mm PTV margins ranged from 98.18% to 100.00% (Table 1). The minimum coverage, 98.18%, was used as the threshold to determine optimized target-specific margins for these six targets. Optimized margins for GTVp, GTVn, CTV1, CTV2_U, CTV2_M, and CTV2_L were 1 mm, 3 mm, 1 mm, 1 mm, 1 mm, and 3 mm, respectively, achieving volume coverage between 98.18% and 99.94% (Table 1). No significant differences were observed in the percentage of fractions where D95 = 100% of prescription dose for all targets between plans with target-specific and 3 mm PTV margins, with the percentages of 100% and 100% for PGTVp, 83% and 84% for PGTVn, 100% and 100% for PCTV1, and 70% and 75% for PCTV2, respectively. OARs received reduced doses with target-specific margins compared to 3 mm margins. Top three OARs with greatest dose sparing were optic nerves, chiasm, and parotids, with average dose reduction of 1275 cGy, 1218 cGy and 518 cGy.
Conclusion: Optimal PTV margins for radiotherapy should be tailored to specific targets. Compared to 3mm PTV margins, target-specific margins ensure dose coverage of targets while reducing OAR doses in NPC patients. Abstract 3761 – Table 1

PTV margins (mm)	Volume coverage, % (Mean ± SD)
	GTVp	GTVn	CTV1	CTV2_U	CTV2_M	CTV2_L
3 mm	100.00 ± 0.01	98.81 ± 2.82	100.00 ± 0.00	99.99 ± 0.13	99.65 ± 0.82	98.18 ± 3.70
Target-specific	99.83 ± 0.64	98.81 ± 2.82	99.94 ± 0.19	99.72 ± 1.10	98.40 ± 2.44	98.18 ± 3.70