ESTRO 38 Abstract book

S1017 ESTRO 38

Figure 2: Median "worst case" increases in rectal DV statistics when shifting target by 5mm. Error bars represent the IQR. Conclusion The implantation of the hydrogel spacer enables the use of intra-prostatic focal boosts whilst maintaining similar or reduced rectal doses to those achieved pre- implantation and also improving the robustness of these doses to PTV shifts. EP-1872 Combining multi-criteria optimisation and a hydrogel spacer for intra-prostatic focal boosts N. Laverick 1 , S. Currie 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics, Glasgow, United Kingdom Purpose or Objective To assess whether using multi criteria optimisation alone or in combination with a hydrogel spacer (SpaceOAR®, Augmenix Inc. Waltham, MA, USA) can improve plan quality in prostate patients when including an intra- Multi-criteria optimisation (MCO) is a planning tool which creates multiple treating plans for each planning objective; the aim is to enable the planner to explore the Pareto surface to find the optimum clinical planning solution. Plans created using MCO may be able to achieve lower OAR doses whilst maintaining target coverage. Hydrogel spacers are also of interest in prostate radiotherapy, aiming to lower rectal doses and therefore toxicity by increasing the separation between the prostate and rectum. Plans were created on patient (n=10) scans pre- and post- implantation of a hydrogel spacer which treated the prostate to 60Gy and seminal vesicles to 47Gy using the Centre’s RapidPlan model (Varian Medical Systems, Palo Alto, CA, USA). Boost volumes were identified on both sets of scans with the aid of MRI imaging and further plans created treating these volumes to 78Gy. Finally, MCO was used to further optimise all plans. Plan quality was compared through target coverage and doses to bladder and rectum and paired two-tailed t-tests were used to examine for statistical significance. Results PTV coverage and bladder doses were similar across all plan types. When MCO was applied to the pre- hydrogel plans, the rectal doses were significantly reduced across all dose levels (p<0.01) for both plans with and without the focal boost. Applying MCO to the focal boost plans reduced doses to the same level as the standard plans without the boost. When the hydrogel spacer was implanted, MCO was found to only significantly affect the lower dose metrics (V24Gy and V32Gy) and the mean dose. However, this is likely due to the extremely low volumes of rectum receiving the higher dose levels when the spacer is present. Figures 1 and 2 show the median pre- and post-implantation dose statistics. The combination of using the hydrogel spacer and MCO in the focal boost plans resulted in significantly lower rectal doses than were achieved in the standard pre- implantation plan without the boost (p<0.01). prostatic focal boost. Material and Methods

Figure 1: Median Rectal DV statistics pre-implantation of hydrogel spacer. Error bars represent IQR

Figure 2: Median rectal DV statistics post-implantation, Error bars represent IQR Conclusion Using multi-criteria optimisation allows for boosting of an intra-prostatic volume up to 78Gy without significantly increasing rectal doses. Rectal doses are further reduced when MCO and the hydrogel spacer are used in combination. EP-1873 Reducing OAR doses in prostate patients: use of a hydrogel spacer and multi-criteria optimisation N. Laverick 1 , D. Church 1 , S. Currie 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics, Glasgow, United Kingdom Purpose or Objective To assess the impact of using a hydrogel spacer (SpaceOAR®, Augmenix Inc. Waltham, MA, USA) with and without multi-criteria optimisation on bladder and rectum doses in moderately hypo-fractionated prostate treatments. Material and Methods Plans were created using Eclipse v15.5 (Varian Medical Systems, Palo Alto, CA, USA) on ten patient scans pre- and post-implantation of the hydrogel spacer treating the prostate to 60Gy in 20 fractions in accordance with the CHHiP trial protocol. All plans were optimised using the Centre’s RapidPlan TM model. Finally, the post- implantation plans were further optimised using multi- criteria optimisation. Plans were compared in terms of target coverage and doses to bladder and rectum. Paired two-tailed t-tests were used to examine for statistical significance. Results PTV coverage was similar between all plan types. The table below shows the mean rectal doses across the ten patients. Statistically significant differences between plan types are highlighted in green.