ESTRO 2020 Abstract book

S473 ESTRO 2020

PO-0790 Single-fraction adjuvant electronic brachytherapy (e-BT) after resection of conjunctival carcinoma S. Serpa 1 , M. Buitrago 1 , D. Ramirez 1 , G.J. Sarria 2 , P. Fuentes Rivera 2 , F.A. Giordano 3 , G. Sarria 2,3 1 Instituto Nacional de Enfermedades Neoplasicas, Department of Ophtalmic Oncology, Lima, Peru ; 2 Instituto Nacional de Enfermedades Neoplasicas, Department of Radiotherapy, Lima, Peru ; 3 University Medical Center Mannheim - University of Heidelberg - Medical Faculty Mannheim, Department of Radiation Oncology, Mannheim, Germany Purpose or Objective To analyze treatment outcome in terms of efficiency and toxicity of single-fraction adjuvant electronic brachytherapy (e-BT) for patients diagnosed with squamous cell conjunctival carcinoma (SCCC). Material and Methods Patients diagnosed with T1-T3 infiltrating SCCC, who were treated between October 2014 and January 2018, were retrospectively assessed. All patients underwent local surgical excision followed by adjuvant surface e-BT with a portable 50-kV miniaturized linear accelerator in a single fraction. Depending on margin involvement, e-BT doses ranged between 18-22 Gy prescribed to 2 mm depth, resembling equivalent doses of 2 Gy per fraction (EQD2) of 46-66 Gy (assuming α/β ratios of 8-10 Gy). Factors of interest included median progression-free survival (PFS), incidence of adverse events (graded by Common Terminology Criteria for Adverse Events v.4.03) and factors determining toxicity.

Results Visualising actual delivered radiotherapy dose assists surgical planning by informing surgeons about the areas of mandible that received high dose, guiding resection design (Figure 2b/c). Displaying areas that received low doses of radiotherapy also helps in guiding the location of fixation devices (Figures 2c). Using this as a decision support tool increases the likelihood that areas of the mandible receiving high dose radiotherapy, and at increased risk of osteoradionecrosis,is included in the resected volume, thereby minimisng the risk of failure.

Results 47 patients were included in the analysis. The median age was 69 [29 – 87] years. The majority of tumors were staged as T1 (40.4%) or T2 (57.5%) with a median tumor size of 7 mm [1.5–20]. Surgical margins were negative in 55.3% and positive in 42.6%. The median time from surgery to e-BT was 9 weeks [0–37]. After a median follow-up of 24 [17 – 40] months recurrence occurred in only 2 patients (6 and 7 months after e-BT), yielding a median PFS of 24 [6 – 40] months and an estimated PFS rate at 2 years of 95.7%. Acute grade 2 conjunctivitis occurred in 25.5% of the patients and was managed with a short-course of topical steroids. No other side effects were reported during follow-up and no toxicity-related factors were identified.

Conclusion This is a new technique and therefore a paucity of data alluding to strict relationships between radiotherapy dose delivered and onset of osteoradionecrosis, however dose distributions in conjunction with diagnostic information assists surgical planning, enabling resection of the mandible that received high dose (high risk) and locating fixation devices in areas that received low dose (low risk). Using this as a decision support tool increases the likelihood that the volume of mandible having receiving high dose and at increased risk of osteoradionecrosis is included in the resected volume.