ESTRO 2025 - Abstract Book

S1676

Clinical - Sarcoma & skin cancer & malignant melanoma

ESTRO 2025

MCC presenting with skin primary only without nodes

No. of patients (N) Definitive dose median (range) Gy2

No. of patients (N) Adjuvant dose median (range) Gy2

Margin negative: N=81 (6.3-66.0) Margin positive: N=23

50.0

N=23 58.5 (37.5-72.0)

Primary tumor

58.5

(37.5-72.0)

Conclusion: Conclusions: For definitive radiotherapy, EQD2<50Gy2 demonstrates significant higher LR than >50Gy2. For adjuvant radiotherapy, a trend of lower LR is observed with higher dose. The optimum dose for lowest LR will require a large multicenter study confirmation.

Keywords: radiotherapy, dose, Merkel cell carcinoma

1554

Digital Poster Controversies of Merkel cell carcinoma: pattern of spread

Aoife Jones Thachuthara 1 , Patricia Tai 2 , Edward Yu 3 , Vimal H Prajapati 4 , Michael Veness 5 , Kurian Joseph 6,7 1 Medical Oncology, Cork Univ Hosp, Cork, Ireland. 2 Radiation oncology, U Saskatchewan, Saskatoon, Canada. 3 Oncology, Western U, London, Canada. 4 Dermatology, U Calgary, Calgary, Canada. 5 Radiation oncology, U Syndney, Syndney, Australia. 6 Oncology, U Alberta, Edmonton, Canada. 7 Radiation oncology, Cross Cancer Institute, Edmonton, Canada Purpose/Objective: It is difficult to study the pattern of spread for rare skin cancers using data from a single institute. We developed an aggregated database for Merkel cell carcinoma (MCC) to achieve an adequate patient number for analysis. Despite this being a neuroendocrine carcinoma that generally spreads by the blood stream, many literature series showed that lymph node metastases (LNM) are more common clinically, although a minority of the published reports showed that DM is the first site of spread. Hence, we sought to deduce the patterns of spread for MCC. Material/Methods: Data from 949 MCC patients from six institutions and the literature were entered into an aggregated database with the following data extracted: baseline patient characteristics, treatment details, and outcomes. Pattern of spread was analyzed as the primary outcome. Results: From March 1982 to Feb 2015, 310/949 MCC patients in the database had lifetime DM. 303/949 patients were treated at our institutions. It demonstrates three interesting aspects with regards to deducing the pattern of spread. (1) A higher proportion of patients presented with LNM than DM. In total 8/303 (2.6%) presented with DM and 59/303 (19.5%) presented with LNM at diagnosis. (2) 79/303 (26.1%) had DM in their lifetime. Among them, 47/79 also developed LNM at different times: 31/47 (66%) prior to and 10/47 (21%) within a month of the clinical diagnosis of DM. (3) The median time interval from initial diagnosis to LNM was 1.5 (range: 0-47.0) months and time interval to DM was 8 (range: 0-107.8) months. Conclusion: The three basic observations favor prior LNM giving rise to the subsequent development of DM as the main pathway of MCC dissemination. Similar deductions may be applicable to other rare cancers or rare variants of more