ESTRO 2025 - Abstract Book

S4366

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Purpose/Objective: Parotid gland sparing in nasopharyngeal carcinoma(NPC) is challenging. Huang et al. 1 demonstrated in an RCT that superficial parotid lobe sparing(SPLS) IMRT reduces xerostomia without increasing doses to organs-at-risk (OAR). However, their study did not address 1. individual dosimetric trade-off for SPLS-IMRT, 2. feasibility of achieving the SP constraint, and 3. optimal planning technique(e.g. IMRT with 7 or 9 fields(F) or VMAT). This study aimed to evaluate these 3 aspects, and provide insights into the practicality of SPLS-IMRT. Material/Methods: We retrospectively analyzed 10 NPC cases. Four IMRT plans were generated for each case: 1. without SPLS using 7F IMRT, 2. With SPLS using 7F, 3. With SPLS using 9F, 4. With SPLS using VMAT. Parotids constraints were considered priority 4 OAR, and as follows 1.whole parotid (WP) V30≤50%, WP V36<40%, WP mean≤26Gy. For SPLS plans, SP V26<30% 1 was also constrained. Paired sample T-tests were used to compare dosimetric parameters across plans. Results: Ten cases (20 Parotid glands) were included. Dose prescriptions were 70/(63)/59.4/(54)Gy in 33 fractions. SPLS significantly reduced left SP V26 with all 7F,9F and VMAT (Table1,2). In addition, WP mean, V30 and V36 significantly reduced in most plans after SPLS (all p<0.05; Table2). Among the 4 parotid dose constraints evaluated bilaterally (i.e 8 parameters), significant dose reductions were observed more frequently with 7F and 9F techniques(6/8 and 7/8 parameters, respectively), compared to VMAT(2/8). Compared with 7F-SPLS, 9F-SPLS demonstrated significantly lower left SP V26, WP mean, V30 and V36( all p<0.05). However, the SP V26<30% constraint was achieved in only 3 parotid glands (15%; 2 with 9F, 1 with VMAT). Compared with 7F-IMRT without SPLS, 9F-SPLS-IMRT increased right temporal lobe (TL) Dmax (meanΔ 0.4Gy), left TL D1% (meanΔ 0.9Gy), but reduced left eye Dmax (meanΔ -5.8Gy). VMAT-SPLS increased R TL Dmax (meanΔ 1.8Gy), oral cavity Dmean (meanΔ 2.7Gy ), pharyngeal constrictor Dmean (meanΔ1.0Gy), and left submandibular gland (SG) Dmean (meanΔ 1.0Gy), but reduced optic chiasm (OC) PRV Dmax (meanΔ-6.0Gy), and left eye Dmax (meanΔ-7.4Gy). Compared with 7F-SPLS-IMRT, 9F-SPLS-IMRT reduced left eye Dmax (meanΔ- 6.1Gy); VMAT-SPLS reduced left eye Dmax(meanΔ -7.7Gy) and OC PRV Dmax (meanΔ-5.4Gy), but increased oral cavity Dmean (meanΔ1.4cGy), pharyngeal constrictor Dmean (meanΔ0.7Gy), and right SG Dmean(meanΔ 0.7Gy).