ESTRO 2023 - Abstract Book

S300

Sunday 14 May 2023

ESTRO 2023

7.03 vs 3.92 µIU/L (p=0.307), 1.39 vs 1.11 ng/ml (p=0.422) and 6.65 vs 7.67 mcg/dL (p=0.268). At nine months post- radiotherapy, three patients in the non-thyroid-sparing group and two patients in the thyroid-sparing group developed any degree of hypothyroidism (both clinical and subclinical). At a median follow-up of 20 months, six out of thirteen patients (46.15%) developed any degree of hypothyroidism in the non-thyroid sparing group whereas three out of thirteen patients (23.07%) developed any degree of hypothyroidism in the thyroid-sparing group (p=0.216). The thyroid V50Gy in the Thyroid sparing group was 65.33±6.63 % (volume in %) compared to 80.35 ±13.40 % in non-thyroid sparing radiotherapy (p=0.003). The median PFS of the study group was found to be comparable to that of the control group (8.9 v 10.2 months; p=0.578). Conclusion The usage of thyroid constraint of V50Gy<75% halved the incidence of any degree of hypothyroidism in our study without affecting the survival outcomes. The routine use of thyroid constraints can reduce the risk of radiation-induced hypothyroidism and must be evaluated in a larger study. PD-0399 Timing of Response Assessment PET CT in OPSCC Post Treatment and Salvage Neck Dissection Rates F. Williams 1 , N. Morley 2 , D. Smith 3 , C. Passsant 4 , S. Quine 5 , S. Berry 5 , M. Doddi 3 , D. Owens 5 , A. Harris 6 , T. Rackley 1 , R. Webster 1 , E. Evans 1 , M. Evans 1 , N. Palaniappan 1 1 Velindre Cancer Centre, Clinical Oncology, Cardiff, United Kingdom; 2 Velindre Cancer Centre, Radiology, Cardiff, United Kingdom; 3 Royal Glamorgan Hospital, ENT Surgery, Cardiff, United Kingdom; 4 Royal Gwent Hospital, ENT Surgery, Newport, United Kingdom; 5 University Hospital of Wales, ENT Surgery, Cardiff, United Kingdom; 6 Royal Gwent Hospital, ENT Surgery, Cardiff, United Kingdom Purpose or Objective Response assessment PET CT scans post curative chemoradiotherapy (CRT) or radiotherapy (RT) for oropharyngeal SCC (OPSCC) are routinely performed, 12 weeks after completion of treatment. Patients who are found to have nodal equivocal response (EQR) and incomplete response (ICR) undergo further investigations and are assessed for salvage treatment. Studies have shown delaying the PET CT or repeating the scan 4 weeks later, improves nodal complete response (CR) rates. We analysed the rates of salvage neck dissection for patients who had a response assessment PET CT scan less than (<) 16 weeks (Group A) versus (vs) greater than or equal to ( ≥ ) 16 weeks (Group B) post treatment. Materials and Methods Patients who had PET CT scans following curative treatment for OPSCC from 2016 to 2022 in South East Wales were identified using the PET CT request database. 260 patients fulfilled the criteria. Demographics, treatment details, PET CT outcomes, rates of salvage neck dissection and patient outcomes (overall survival, disease free survival, nodal, primary and distant recurrence rate and second primary) were analysed and compared between groups. Last follow up date was 30/09/2022. PET CT response for nodes were categorised as CR, EQR and ICR. The study population was grouped into Group A and B based on the timing of the PET CT. We then analysed the number of patients who underwent salvage neck dissections in each group. Results Of the 260 patients, 204 were male, 217 were p16 positive and 200 had concurrent CRT. There were 169/260 (65%) patients in Group A and 91/260 (35%) patients in Group B. The median time to PET CT was 13.5 weeks vs 17.5 weeks in Group A and B respectively. The nodal CR rates were 134/169 (79%) vs 77/91 (85%); nodal EQR/ ICR rates were 35/169 (21%) vs 14/91 (15%) in Group A and B respectively. Of the EQR/ICR cohort, 9/35 (26%) vs 8/14 (57%) patients underwent nodal biopsies in Group A and B respectively. None of the biopsies were positive in Group A and one biopsy was positive in Group B. Twenty-three patients underwent a salvage neck dissection, of these 17 (74%) patients who were in Group A, 9/17 (53%) had residual disease and of the 6 (26%) patients who were in Group B, 3/6 (50%) had residual disease in the neck nodes. The overall survival, disease free survival, nodal, primary and distant recurrence rate and second primary rates are 139/169 (82%) vs 78/91 (86%); 123/169 (73%) vs 72/91 (79%); 8/169 (5%) vs 0/91 (0%); 14/169 (9%) vs 4/91 (4%); 16/169 (9%) vs 9/91 (10%); 12/160 (7%) vs 6/91 (7%) in Group A and B respectively. Conclusion Of the patients who underwent salvage neck dissections, a higher proportion were in the group who had a PET CT performed <16 weeks post treatment. The disease outcomes in both groups were similar. More work needs to be done to assess whether delaying PET CT scans could reduce surgical morbidity in the future. PD-0400 External validation of a PET radiomic model predicting outcome after RT for oropharyngeal cancer M. Mori 1 , C. Deantoni 2 , G. Palazzo 1 , M. Olivieri 1 , E. Spezi 3 , A. Chiara 2 , M. Picchio 4 , A. Del Vecchio 1 , N.G. Di Muzio 2 , C.A. Fiorino 1 , I. Dell'Oca 2 1 IRCCS San Raffaele Scientific Institute, Medical Physics, Milan, Italy; 2 IRCCS San Raffaele Scientific Institute, Radiotherapy, Milan, Italy; 3 School of Engineering Cardiff University, Medical Physics, Cardiff, United Kingdom; 4 IRCCS San Raffaele Scientific Institute, Nuclear Medicine, Milan, Italy Purpose or Objective To externally validate the 18Fluorodeoxyglucose Positron Emission Tomography (18F-FDG-PET ) radiomic-based model suggested by Martens et al. (EJNMI Research (2020) 10:102) predicting overall survival (OS) in patients with oropharyngeal cancer treated with chemo-radiotherapy.

Materials and Methods