ESTRO 2021 Abstract Book

S1283

ESTRO 2021

was calculated as 19.4% (p=0.009). In the projected dose analysis performed on the last MVCT image before ART, the maximum point dose values of all target volumes increased. No statistically significant difference was calculated in the target volumes, parotids and spinal cord dose when the pre-ART and post-ART doses were summed at the end of the treatment. The average weight loss rate was %10,7 (%5,6-%20,7) between the first and last day of treatment. Significant volumes reductions were observed both in PTVs (PTV70=25,69%, PTV60=8.03% and PTV56=10,73%) and parotids (29,38% in left and 29,28% in right parotid). Conclusion The use of MVCT images to monitor both anatomical and dosimetric variations is a very practical tool and very helpful to do automated dose monitoring/determining a need for ART. PO-1558 MR Linac Stereotactic Prostate:Accumulated dose comparison of adaptive versus non adaptive treatment J. de Leon 1 , D. Crawford 2 , L. Hogan 2 , Z. Moutrie 2 , C. Pagulayan 2 , C. Loo 2 , M. Heinke 2 , S. Sampaio 3 , S. Alvares 2 , A. Johnson 2 , K. Simon 2 , T. Twentyman 2 , M. Jameson 2 1 GenesisCare St Vincent, New South Wales, Radiation Oncology, New South Wales, Australia; 2 GenesisCare, St Vincent, Radiation Oncology, New South Wales, Australia; 3 GenesisCare, St Vincent, Radiation Oncology, New South Wales , Australia Purpose or Objective The MR Linac (MRL) offers improved soft tissue visualisation to guide daily adaptive radiotherapy treatment. When treating with this system there are broadly two modes available; adapt to position (ATP) whereby a virtual isocentre shift is applied daily to account for patient positioning and adapt to shape (ATS) which is a fully adaptive process consisting of recontouring and re-planning while the patient is on the treatment couch. We analysed the cumulative dose for our initial 9 stereotactic prostate patients treated with an ATS workflow on the MRL and compared this to a non adaptive treatment and the predicted dose on reference plans. Materials and Methods Nine patients were treated with stereotactic prostate radiotherapy on the MRL. Patients were prescribed 36.25Gy to PTV and CTV could be boosted to 38 to 40Gy. 4 patients were treated without hydrogel and one had hydrogel inserted, but it did not provide any rectal separation. Organ at Risk (OAR) constraints were as per published guidelines. All patients were treated with an ATS workflow which consisted of recontouring and replanning based on daily MRI images. All delivered fractions were analysed offline comparing the accumulated dose using an ATS workflow to a non adaptive workflow and to the reference plan (figure 1). Figure 1 Workflow for performing deformable dose accumulation

Results Target organ coverage was higher in the reference plan compared to the ATS or non adaptive workflow. ATS target organ coverage was consistently higher than that achieved using a non adaptive approach. OAR constraints were consistently met using an ATS workflow as opposed to a non adaptive workflow (figure 2) . Figure 2 Example prostate SBRT case for ATS Vs No adaption and reference plan