ESTRO 2021 Abstract Book

S274

ESTRO 2021

Conclusion We have demonstrated the feasibility of BM sparing irradiation in the pelvis with a statistically significant reduction of the bone marrow volume irradiated, while maintaining target coverage and respecting OARs constraints. These results prompt to pursue with a randomized phase II trial in which we will assess the reduction in hematotoxicity of LS-RT versus SOC for PCa patients. PH-0376 Dose escalation to lymph nodes in cervical cancer using a sequential boost on the MR-Linac (MRL) J. Lodeweges 1 , F. van der Leij 1 , I. Jürgenliemk Schulz 1 , A. de Leeuw 1 , J. Hes 2 , A. van Lier 1 1 University Medical Center Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands; 2 University Medical Center Utrecht, Department of Radiation Onocology, Utrecht, The Netherlands Purpose or Objective Nodal failure in locally advanced cervical cancer (LACC) treatment is still a concern. External beam radiotherapy (EBRT) with a simultaneous integrated boost (SIB) to the lymph nodes (LN) combined with brachytherapy (BT) does not always ensure sufficient dose, mostly in LNs in the higher pelvic or para-aortic regions or in bulky LNs, the proximity of the small bowel is limiting the dose that can be given. The aim of this planning study was to evaluate if, instead of a SIB, a stereotactic sequential boost (SSB) on the MRL is a viable option for dose escalation in the LNs. We hypothesize that smaller PTVs due to 1) smaller CTV-to-PTV margins, feasible by daily adaptation, and 2) LN regression due to prior EBRT, leads to the possibility of dose escalation without compromising the dose to the OAR.