ESTRO 2021 Abstract Book

S1561

ESTRO 2021

Conclusion No significant difference in plan quality or plan robustness was observed between non-robust and robust optimised plans for the 8 patients used in this study. Robust optimisation planning was found to be more time consuming than PTV-based planning with no apparent benefit to target coverage or OAR dose. The robust evaluation module was a useful tool in plan assessment and would be beneficial to quantify plan robustness for lung SABR.

PO-1831 Reducing the source axis distance to 80 cm - a prostate cancer planning study B. Polat 1 , L. Binder 1 , F. Exner 1 , P. Kessler 1 , M. Flentje 1 , K. Bratengeier 1 1 University of Würzburg, Department of radiation oncology, Würzburg, Germany

Purpose or Objective In prostate cancer, especially a steep dose gradient is needed to spare organs at risk for escalating the dose to the prostate. A planning study to explore opportunities of a reduced patient-source-distance (SAD, virtual isocentre) to increase IMRT plan quality is presented. We investigated if such an approach can be achieved on a standard LINAC and a commercial treatment planning software. Materials and Methods To simulate a LINAC with a SAD of 80 cm we implemented the concept of a virtual isocenter. Prostate cancer cases were selected for this planning study. Twenty one patients were treated with a clinically accepted reference plan using a simultaneous integrated boost (SIB) with a total dose of 60 and 76 Gy in 33 fractions. Additionally, plans using 9 and 15 equidistant coplanar beams were created by direct machine parameter optimization (DMPO) in Pinnacle 3 treatment planning system. We further calculated fluence optimized (FO) plans for 90 irradiation angles, simulating intensity modulated arc therapy without limitations by MLC-sequencing. A sum of the set of weighted objectives (composite objective value, COV) was used as an indicator for overall plan quality. For each patient an identical set of objectives was applied for each of the three plan types. Rectal dose was evaluated by D2cc, V20Gy - V60Gy in 10Gy steps and up to V75Gy in 5Gy steps. For the PTV we used the D99%. Results Median patient age was 71.2 years. D’Amico risk group was low in 22 %, intermediate in 65 % and high in 13% of patients. Overall plan quality (COV) was significantly improved for all virtual isocentre techniques comparing SAD 80cm with standard SAD 100cm (p<0.05, Wilcoxon rank test). This resulted in a reduction of high dose rectum volumes (V65Gy, V70Gy, V75Gy and D2cc) for 15 beam DMPO plans and to a lesser degree for 9 beam (D75 Gy) and 90 beam FO plans (D65Gy). The minimum dose in the PTV (D99%) was kept constant. Conclusion With the implementation of a virtual isocentre IMRT concept, we could demonstrate a significant improvement in plan quality. Applicability, efficiency, application security and quality assurance has to be investigated in combination with a dedicated couch which is able to deliver adequate precision of positioning. In a next step, we have to prove if this method translates into a clinical benefit for prostate cancer patients. PO-1832 An experimental verification of the Advanced Collapsed cone Engine for HDR skin brachytherapy R. Stansbridge 1 , R. Caines 1 , C. Lee 1 1 The Clatterbridge Cancer Centre, Physics, Liverpool, United Kingdom Purpose or Objective The Advanced Collapsed cone Engine (ACE) in Oncentra Brachy v4.6 (Elekta, Sweden) is a model-based dose calculation algorithm developed to address tissue heterogeneity effects disregarded in TG-43 calculations. For HDR brachytherapy skin treatments in particular, the presence of a tissue-air interface at the skin surface is not considered under TG-43 calculations as uniform density is assumed throughout the irradiated volume. Following a previous skin planning study in which clinically significant differences between ACE and TG-43 were demonstrated, the aim of this investigation was to