ESTRO 38 Abstract book

S1016 ESTRO 38

EP-1871 Can the use of a hydrogel spacer enable intra-prostatic boosts without increasing rectal doses? N. Laverick 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) on plan quality in prostate patients when including an intra- Treatment plans were created on scans of ten patients pre- and post-implantation of the hydrogel spacer. All plans used VMAT with two coplanar arcs to treat the prostate to 60Gy in 20 fractions (PTV60) and the seminal vesicles to 47Gy (PTV47). All plans were optimised using the Centre’s RapidPlan model (Varian Medical Systems, Palo Alto, CA, USA). A boost volume was then identified for each patient and outlined on both the pre- and post- scan sets with the aid of MRI imaging. These boost volumes were planned to two separate dose prescriptions, 67Gy and 78Gy. 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. For the plans without the focal boost and the 78Gy boost plans, the robustness of the OAR doses was also assessed by inducing a 5mm target shift in all directions and recording the poorest rectal and bladder doses. Results Target coverage was not affected by the presence of the hydrogel spacer; however rectal doses were reduced post- implantation. As Figure 1 shows, the volume of rectum receiving both intermediate and higher doses is reduced with the spacer. The implantation of the spacer has allowed boosting of in intra-prostatic volume to 78Gy whilst still achieving lower rectal doses than the treatment plans produced on the pre-implantation scans with no boost volume. Using p<0.01 as the limit for statistical significance, significant increases were found in rectal dose when boosting without the hydrogel spacer, whereas increases with boosting with the spacer in place were not significant. The mean dose to the rectum, which is associated with faecal incontinence and high stool frequency following radiotherapy, is also reduced; the reduction is such that similar levels (p = 0.04) are achieved for the pre- implantation with no boost (median 19.6Gy) and the post- implantation with 78Gy boost (median 17.0Gy). No overall change in bladder dose was seen. prostatic focal boost. Material and Methods

Figure 1: Median values of rectal DV statistics pre and post implantation of hydrogel spacer for different boost dose prescriptions. Error bars represent the IQR. As seen in Figure 2, the worst-case increase in rectal doses was reduced when using the hydrogel spacer, especially the volume of rectum receiving the higher dose levels which are associated with rectal bleeding. No meaningful change in the robustness of bladder dose was observed.

Conclusion SEMAR dedicated UH-ED curves don’t seem necessary to compute dose with SEMAR on CT. When beams do not enter through metallic implants, slight dosimetric differences caused by UH modification are negligible. When PTV encompasses metal and beams pass through it, PTV coverage may be strongly modified. Further investigations (Acuros® XB dose calculation, Monte Carlo simulation) need to be done to improve the real irradiation knowledge.