ESTRO 2025 - Abstract Book

Brachytherapy - Gastro-intestinal, paediatric brachytherapy, miscellaneous

S181

ESTRO 2025

become increasingly established for very young children after a good response to systemic therapy. We analyzed dosimetric data of the clinical target volume (CTV) and the organs at risk (OAR) to verify the dose coverage and establish radiotherapeutic OAR dose constraints in the field of pediatric iBT. Material/Methods: After tumor resection, 8-10 iBT catheters were placed intraoperatively to cover the tumor region. The subsequent 3D radiation planning was carried out using CT, MRI (partly intraoperatively) and planar X-ray images. A radiation dose of 36 Gy (encompassing the CTV) was prescribed in 12 fractions on six treatment days (twice daily). For the duration of the treatment, the children were under intensive medical supervision in general anesthesia. For the present retrospective analysis, OAR (bladder, rectum, urethra and testes) of a total of 20 patients were uniformly recontoured and examined for their applied dose values. Results: The median patient age was 2.6±1,3 years. The median Dmean of the CTV and the D90% of the dose per fraction were 4.4±0.7Gy and 3.3±0.2Gy. Converted to EQD2, this results in a total dose of 79.2Gy and 50.3Gy (α/β = 2.8Gy (1), no correction for hyperfractionation). The median D0.1cc and D1cc (cumulative EQD2 dose, α/β = 3.0Gy) of the OAR bladder, rectum, urethra and testes in the individual plan was 6.6±1.2Gy (152.1Gy); 3.0±0.3Gy (43.2Gy); 4.2±0.5Gy (72.6Gy); 0.2±0.2Gy (1,5Gy) and 3.9±0.7Gy (64.6Gy); 2.1±0.3Gy (25.7Gy); 0.6±0.5Gy (5.2Gy); 0.1±0.1Gy (0.7Gy). No higher grade radiogenic acute and subacute side (RTOG Grade ≥ 2) until three months after iBT effects were observed.

Conclusion: The CTV EQD2 Dmean dose values exceed the suggested total dose of the CWS- and FaR-RMS protocols (EQD2: max. 53.5Gy and max. 56.9Gy). No higher-grade acute and subacute side effects were observed. The aim is to develop radiotherapeutic dose constraints in the use of iBT in pediatric diseases. For this purpose, the remaining patients will also be analyzed and the values will be correlated with potential late side effects of the follow ups.

Keywords: pediatric radiotherapy, rhabdomyosarcoma References:

1) Timmerman RD, Mendonca M. In regard to Donaldson et al: results from the IRS-IV randomized trial of hyperfractionated radiotherapy in children with rhabdomyosarcoma-a report from the IRSG. IJROBP 2001;51:718 728. Int J Radiat Oncol Biol Phys. 2002;54:1579-80; author reply 1580. doi: 10.1016/s0360-3016(02)03015-8. PMID: 12459396.

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Digital Poster Rectal cancer redefined: enhancing organ preservation with Papillon X-ray therapy in Switzerland Cristina Picardi 1,2 , Francesca Caparrotti 3 , Michael Montemurro 4 , Daniel Christen 5 , Nora Brunner-Schaub 6 , Marie Fargier-Voiron 7 , Laetitia Lestrade 7 , Daniel Helbling 8 , Marco Bernardi 9 , Jan Schmidt 10 , Jean-Pierre Gutzwiller 9 , Frederic Ris 11 , Oscar Matzinger 12 1 Radiation Oncology, Swiss Medical Network, Zürich, Switzerland. 2 Radiation Oncology, Hirslanden Klinik, Zürich, Switzerland. 3 Radiation Oncology, Swiss Medical Network, Geneva, Switzerland. 4 Medical Oncology, Swiss Medical Network, Genolier, Switzerland. 5 Surgery, Swiss Medical Network, Zürich, Switzerland. 6 Gastro -enterology, Swiss Medical Network, Zürich, Switzerland. 7 Radiation Oncology, Swiss Medical Network, Genolier, Switzerland. 8 Medical Oncology, Gastrointestinales Tumorzentrum Zürich, Zürich, Switzerland. 9 Gastro -enterology, Gastrointestinales Tumorzentrum Zürich, Zürich, Switzerland. 10 Surgery, Gastrointestinales Tumorzentrum Zürich, Zürich, Switzerland. 11 Surgery, Geneva University Hospital and Medical School, Geneva, Switzerland. 12 Radiation Oncology, Swiss Medical Network,, Zürich, Switzerland