ESTRO 2021 Abstract Book

S1297

ESTRO 2021

Conclusion A novel method of deriving bladder planning margin from the displacement vector in the DIR between planning CT and CBCT is presented. The DSM is a useful tool for visualizing the entire displacement across the bladder surface in each fraction. The bladder surface displacement is found to be the greatest in the superior and anterior positions. PO-1572 Lessons learnt from the first radical cervix treatment on the MR-Linac L. Freear 1 , J. Berresford 1 , R. Chuter 1,2 , G. Budgell 1 , P. Whitehurst 1 1 The Christie NHS Foundation Trust, Radiotherapy Physics, Manchester, United Kingdom; 2 The University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom Purpose or Objective The MR-linac is ideal for cervical cancer radiotherapy due to excellent soft tissue visualisation and the ability to do daily adaptation of the treatment plan, as target position can vary due to bladder, rectal and bowel filling. Occasionally the target can be ante- or retroverted; in these instances when treating on conventional linacs with CBCT, high-risk disease is prioritised for matching, resulting in sub-optimal coverage of the uterus and nodal chains. On the MR-Linac however, targets and OARs are recontoured daily to create a new treatment plan that meets planning objectives. Up to now, only palliative cervical cancers have been treated on the MR-Linac. Here we present the lessons learnt from the first radical cervix treatment. Materials and Methods Patient selection was restricted due to the limited sup-inf field length (22cm), but a suitable node-negative patient was identified. The prescription, contours and reference plan were created as per EMBRACE II guidelines. The primary RTP scan was MR and bulk density overrides were applied to enable dose calculation. Daily adaptation was carried out via the Adapt-to-Shape (ATS) workflow; deformed structures were reviewed and edited prior to a full plan reoptimisation. Results Contouring was initially time consuming, in part due to the 1mm online MR slice thickness, but improved with experience, with treatment sessions eventually concluding in under an hour – see Figure 1.

Deformation was acceptable but could perhaps be improved if online MR and RTP slice thickness matched. Only gross changes were made; edits to fix small imperfections were omitted, as these became irrelevant post target margin expansion. However, bladder and bowel bag contours were rigidly propagated. A drinking protocol was utilised so the bladder would fill during the planning process and eventually match or slightly