ESTRO 2024 - Abstract Book

S1277

Clinical - Head & neck

ESTRO 2024

plexus, cochlea, lacrimal glands, eyes,lens, optic nerves and chiasm ,thyroid gland,oral cavity etc were contoured. As mucositis can be one of the main toxicities of head and neck radiotherapy with detrimental effects on the patients quality of life, a composite structure called mucosa ( i.e oral cavity+ PCM +Larynx + Oesophagus+ soft palate) was created, to serve as a surrogate marker for the extent of mucosa irradiated which could reflect the extent of mucositis expected. Constraints based on previously published references (1,2) and EQD2 calculations were used eg: Sp cord PRV: Dmax < 25.3Gy, Br.Stem PRV <31Gy; Carotid artery Dmax <44Gy & V40 <50%, Esophagus Dmax <30Gy ; Br.plexus; Dmax <30.5Gy; Larynx Dmean <27Gy, Skin Dmax < 35Gy, Pharynx constrictors Dmean<27Gy, Parotid Dmean<19Gy and SMG Dmean<19Gy Cochlea Dmean<25Gy, Optics Dmax<22.5Gy ; Brain <24Gy, Lens Dmax <6Gy; Lacrimal Gland Dmean<19Gy, Thyroid Gland Dmean <24Gy, Oral cavity Dmean <19Gy etc. The imaging and treatment planning data from seven head and neck squamous cancer patients who had undergone palliative radiotherapy were used for this study. Five of the the patients selected had low volume disease with primary disease in the oral cavity (1) oropharynx (1), larynx (2) , and hypopharynx (1). One patient had a locally advanced primary with a lymph node ( medium volume disease) and one had a large volume bulky maxillary tumour with multiple lymph nodes ( high vol disease). Contouring the GTV was carried out and showed a GTV with mean volume of 22.9cc ( range 1.9 -116.1cc). The mean PTV_SBRT volume was 42.1cc (range 6.4 - 185.6cc). Expansion of the GTV to CTV by 5mm and extension of the CTV to cover the nodal level, substantially increased the target volume. For the same GTVs: mean CTV_Pall volume was 84.7cc (range 22.8-328.3 cc) and mean PTV Pall volume was 172.2cc (range 66.4 -563.5 cc). This represented a 3 fold to 12 fold increase in the target volume treated. Results:

Case 1 – non lateralised early T2N0M0 left oropharynx squamous carcinoma HPV negative elderly frail patient with poor Performance Status & comorbidity precluding curative RT.

SBRT Plan: 40Gy/5#/10 days; two arc VMAT plan 2219MU (Eclipse Acuros v18.0)

GTV SBRT vol: 9cc ; PTV SBRT vol 22.1cc ;

GTV_SBRT Dmin - 39.9Gy, Dmax-43.6Gy ; PTV SBRT Dmin- 34.0Gy Dmax: 43.6Gy

Sp canal Dmax: 17.8Gy ; Br stem Dmax: 16.2Gy; Lt carotid Dmax- 36.2Gy & V40 0% ; Larynx Dmean: 0.4Gy; Pharyngeal constr Sup Dmean: 17.3Gy; Skin Dmax 12.4Gy, Mandible Dmax - 40.85Gy, Parotid Left Dmean: 8.73Gy ; Mucosa Dmean 11.9Gy

Palliative plan: CTV pall vol 29.5cc ; PTV Pall Vol: 68.3cc 20Gy/5#/7 days

Conclusion:

This planning study has demonstrated that delivery of higher doses of radiotherapy using SBRT to small volume targets in the head and neck with palliative intent is feasible with good target coverage, conformality and without excessive dose to the adjacent organs at risk. Further exploration of this concept in an early phase trial setting is planned.