2748 - Spare the Carotid, Spoil the Larynx: Assessment of Dosimetric Coverage of IMRT Target Volumes with Conventional 3D-Conformal Radiotherapy for Early Glottic Larynx Carcinoma

Purpose/Objective(s): Early (stage I-II) glottic larynx cancer (EGLC) has been successfully treated with 3D-conformal radiotherapy (3DCRT) for decades. More recently, IMRT has been increasingly used to limit high doses to surrounding structures such as the carotid arteries or uninvolved larynx. However, the practice of IMRT for EGLC is extremely heterogeneous, and there is a substantial lack of consensus in target delineation. Considering the well-established efficacy and safety of 3DCRT for EGLC, a better understanding of physical doses delivered by 3DCRT to commonly utilized IMRT target volumes may inform target delineation and dose prescription.

Materials/Methods: Patients with EGLC treated with 3DCRT from 2010-2024 were included. DICOM RT plans were exported from the treatment planning system. Glottic IMRT targets selected based on a systematic review were delineated on the planning CT: gross tumor volume (GTV) comprised the involved vocal cord(s), the carotid-sparing clinical target volume (CTVcs) encompassed the larynx with the craniocaudal extent defined using conventional anatomic 3DCRT field boundaries (T1: top of thyroid cartilage to bottom of cricoid; T2: hyoid bone to first tracheal ring). Inspired vocal cord-only IMRT, a glottis-only CTVg was defined as GTV expanded by 3 mm, confined to the larynx. Several planning target volumes were determined based on these targets: CTVcs expanded by 3 mm (PTVcs03), 5 mm (PTVcs05), and by 5 mm in all directions except 3 mm posteriorly (PTVcs0503), CTVg expanded by 3 mm (PTVg1), and CTVg expanded by 3 mm in all directions except 1 cm craniocaudal dimensions (PTVg2). The minimum dose to 95% of each PTV (D95) and the % volume receiving prescription dose (VRx) were recorded and described.

Results: A total of 67 patients were identified; 55 had evaluable RT plans and were included in the analysis. Median age was 65 years, 43 were male, 12 were female, 37 (67%) had T1 and 18 (33%) had T2 disease; local failure occurred in 3 patients. Delivered dose coverage when 3DCRT plans were applied to the standardized IMRT targets are described in Table 1. About 63-70% of carotid-sparing PTVs and 89-99% of glottic-only PTVs were treated with prescription dose. The median values for D95 as a percent of prescription dose (i.e., % of total prescription dose received by 95% of the target volume) were: 86% for PTVcs03, 75% for PTVcs05 and PTVcs0503, 101% for PTVg1 and 98% for PTVg2.