ESTRO 2024 - Abstract Book

S1213

Clinical - Head & neck

ESTRO 2024

Cambridge, United Kingdom. 5 Cambridge University Hospitals NHS Trust, School of Clinical Medicine, Cambridge, United Kingdom. 6 North West Anglia Foundation Trust, Department of Oncology, Peterborough, United Kingdom. 7 Cambridge University Hospitals NHS Trust, Department of Radiology, Cambridge, United Kingdom

Purpose/Objective:

Locoregional recurrence (LRR) represents a significant burden in cancer-related deaths, impacting 15-50% of patients treated for head-and-neck squamous cell carcinomas (HNSCC) [1]. Accurately classifying the location of, and the dose delivered to, the failure site is a crucial step towards understanding the causes of LRR. The aim of this study is to classify and analyse patterns of LRR after radiotherapy for HNSCC.

Material/Methods:

The medical records of HNC patients receiving radical radiotherapy (≥20 fractions) between December 2016 and June 2019 in a single centre dataset (Cambridge University Hospital) were reviewed. A total of 325 patients underwent radical radiotherapy for HNSCC; 52 (16%) developed LRR; 27/52 (52%) consented to data use in research, 22/52 (42%) had complete datasets for analysis. For each of the 22 patients, the relapse gross tumour volume(s) (rGTV) and clinical target volumes (CTVs) were manually delineated on each patient’s relapse diagnostic CT (rCT) by radiation oncologists. The thyroid cartilage (TC) was delineated by experienced HNC clinicians as a comparative organ at risk (OAR) on both the planning CT (pCT) and relapse diagnostic CT (rCT). The pCT was co-registered to the rCT using deformable image registration (DIR) to obtain the transform for spatial mapping between the two CT scans. The accuracy of the DIR for each patient was quantified using the target registration error (TRE) of the centroid of the thyroid cartilage (TC). The final cohort TRE was calculated as the absolute distance between the centroid of TC on pCT and the centroid of TC mapped from rCT to pCT, averaged across all LRR patients in the analysis. The clinically prescribed dose to the high-risk region (CTV1), intermediate-risk region (CTV2) and low-risk (CTV3) were obtained from the patient’s radiotherapy plan. A dosimetric structure set was then created by delineating structures receiving 95% of the dose clinically prescribed to these risk regions. The rGTV was then spatially mapped onto the pCT. Centroid and volume-based criteria were used to compare the mapped rGTV with each 95% dose structure to classify the failure into one of the five categories: A (central high dose), B (peripheral high dose), C (central elective dose), D (peripheral elective dose), and E (extraneous dose) [2]. Clinical patient variables were used to search for associations between treatment type and LRR classifications. In total, 23 recurrence tumour volumes were identified and classified using DIR methods. The cohort DIR TRE was evaluated as 5.7 mm. Of the 23 LRRs, 16 were identified as Type A (central high-dose), 1 as Type B (peripheral high dose), 3 as Type C (central elective dose), and 3 as Type E (extraneous dose). Among the 17 high-dose failures (Type A and B), 10 received 65 Gy in 30 fractions with bilateral neck irradiation; 4 received post-operative dose of 60 Gy in 30, and 3 received small-volume larynx protocol of 55 Gy in 20. Type A primary sites included oropharynx, larynx, hypopharynx and oral cavity. All Type C patients received 65 Gy in 30 fractions with bilateral neck irradiation to larynx and oropharynx primary sites. Of the three Type E patients, two were oral cavity patients who received post operative dose of 60 Gy without bilateral neck irradiation, and one oropharynx patient received 65 Gy in 30 with bilateral neck irradiation. Using Fisher’s exact test, no clinical covariates had statistically significant associations with classification types. Results: