S645
Clinical – Head & neck
ESTRO 2026
strongly associated with volumetric muscle loss and trismus progression in OSCC patients. Importantly, trismus is a recognized long term side effect that may continue to worsen beyond the early post treatment period. Preliminary thresholds suggest mean doses >6600 cGy to the masseter and pterygoid predict clinically significant trismus within 3 months. These findings highlight the need for contouring and dose constraint protocols targeting the masticatory muscles to mitigate functional morbidity. Long term prospective studies are required to validate predictive thresholds and better characterize chronic trismus trajectories in South Asian populations. Keywords: Muscle Volume Loss, Maximal incisor distance Impact of the 2018 international consensus 5+5 mm target volume delineation on acute nutritional state during radiotherapy in head and neck cancer Taha Amir 1,2 , Katie Wakeham 1,2 , Dorothy M Gujral 1,2 1 Head and Neck Unit, Imperial College Healthcare NHS Trust, London, United Kingdom. 2 Department of Clinical Oncology, Imperial College Healthcare NHS Trust, London, United Kingdom Purpose/Objective: International consensus guidelines recommending the ‘5+5 mm’ technique for contouring primary tumour target volumes for radical radiotherapy (RT) in head and neck squamous cell carcinomas (HNSCC) were published in 20181. These were adopted by our centre in 2024, replacing the previous subsite-based approach. We aimed to evaluate the impact of the ‘5+5 mm’ technique on acute nutritional state during RT. Material/Methods: We retrospectively reviewed consecutive patients with HNSCC treated with primary radical Digital Poster 4019 (chemo)radiotherapy prior to and after adopting the 5+5 outlining consensus. A dose of 65.1Gy in 30 fractions was delivered using intensity-modulated radiotherapy (IMRT). Concurrent cisplatin or carboplatin was administered for locally advanced disease unless frailty or comorbidities precluded this. Weight and serum albumin levels were recorded at baseline and end of treatment. Feeding tube dwell time was also recorded, defined as the interval from the final radiotherapy fraction to the earliest of either tube removal, death, or a fixed analysis date three months after completion of treatment. Results: Twenty five patients were treated between 01/22 and 12/22 (pre-5+5 cohort), and 30 patients between 07/24 and 06/25 (5+5 cohort). Concurrent chemoradiotherapy (CRT) was administered in 18
trismus incidence is well documented, radiation induced dose–volume effects on the masticatory apparatus remain underexplored in South Asian populations. This study aimed to quantify radiation dose and volumetric changes in masticatory muscles and correlate these with trismus severity. Material/Methods: This prospective observational study enrolled thirty patients with OSCC undergoing adjuvant radiotherapy. Baseline maximal incisor distance (MID) and masticatory muscle volumes were recorded at CT simulation. Dose–volume histogram parameters were extracted for the masseter, pterygoids, and temporalis muscles. Trismus was reassessed at 3 month follow up, with repeat CT for volumetric analysis. Results: The cohort comprised 28 males and 2 females, median age 56 years. Buccal mucosa was the most frequent subsite (n=24, 80%), followed by tongue cancers (n=6, 20%). All patients received adjuvant radiotherapy (6600–7000 cGy), and 70% received concurrent chemotherapy. Mean ipsilateral doses were highest for the masseter (6400–7039 cGy) and pterygoid (6540–6999 cGy), with contralateral structures receiving substantially lower doses (1700–4300 cGy). Volumetric analysis demonstrated consistent reductions: ipsilateral pterygoid decreased by 20–35% (baseline 13–20 cc vs. 3 month 8–12 cc), masseter by 15–30% (baseline 12–30 cc vs. 7–24 cc), and temporalis by 10– 25%.
Contralateral muscles showed minimal change. Clinically, trismus worsened in 24/30 patients, with MID declining by 0.2–0.5 cm in most cases. Severe reductions were observed in tongue cancer patients (e.g., MID 0.8 cm to 0.5 cm). Correlation analysis demonstrated that higher mean dose (>6600 cGy) to ipsilateral masseter and pterygoid was significantly associated with greater volumetric loss and worsening trismus (r ≈ 0.6–0.7, p < 0.05). Conclusion: Radiation dose to ipsilateral masticatory apparatus is
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