Abstract Book

S1252

ESTRO 37

recommendations based on the postoperative prostate volume. Results In intraoperative conditions, an implant of 73 seeds is generated, satisfying the TG-137 recommendations. However, significant increase in prostate dose heterogeneity and urethra toxicity is observed in the postoperative dosimetry at day 30. The prostate V150 and V200 metrics are increased 26% and 11%, respectively. Moreover, the urethra D10 and D30 are increased by 38 Gy and 33 Gy, respectively. Applying the proposed strategy during planning, the implant optimization is adapted to the postoperative prostate volume. In the adapted plan, comprised of 67 seeds, the prostate dose heterogeneity and urethra toxicity are reduced. The difference for both prostate V150 and V200 is 0.1%, compared to the initial intraoperative plan. Moreover, the urethra D10 is 2 Gy lower in the adapted plan and D30 difference is negligible. Conclusion A strategy for adapting the intraoperative treatment plan to edema was evaluated in a proof-of-concept scenario. It is shown that toxicity can be reduced if postoperative edema resorption is taken into consideration during planning. EP-2266 The first use of a biodegradable balloon spacer device for high dose rate prostate brachytherapy R. Alonzi 1 , P. Costa 2 , L. Benjamin 1 , C. Hardiman 1 , G. Lowe 1 , A. Fischer 1 , R. Lad 1 , H. Killen 1 Purpose or Objective HDR prostate brachytherapy, either as monotherapy or in combination with external beam therapy, provides excellent tumour control across a range of risk groups. The rectum is in close proximity to the prostate and is an organ at risk (OAR) for acute and late radiation effects, including proctitis, faecal incontinence, diarrhoea, rectal bleeding, fistula and stricture. The risk of toxicity to the rectum has, at times, meant having to compromise CTV dosimetry in order to reduce rectal dose. Due to the very high dose gradients associated with HDR brachytherapy, a device that creates a small increase in the distance between the prostate and rectum is likely to result in large reductions in rectal dose. We have tested the BioProtect Balloon Implant biodegradable (BPBI) spacer 1 Mount Vernon Cancer Centre, Department of Radiotherapy, Northwood, United Kingdom 2 Porto CUF Institute- Porto, Department of Radiotherapy, Porto, Portugal The BPBI spacer device was inserted in two patients prior to HDR brachytherapy for intermediate risk localised prostate cancer (both patients had T 2a N 0 , Gleason 3+4 disease). The device was introduced prior to HDR catheter implantation under general anaesthetic using trans-rectal ultrasound guidance in real time, in accordance with the manufacturer’s guidelines. The practical aspects of applicator insertion in the presence of a BPBI spacer device were assessed and rectal dosimetry was compared to current standards. Results BPBI spacer device insertion was straightforward and took 5-10 minutes to perform. Subsequent HDR brachytherapy device for this purpose. Material and Methods

catheter implantation was not hampered by the presence of the balloon. A distance of at least 1cm was achieved between the rectum and the prostate following spacer insertion in both cases. Figure 1 shows the balloon in place on the T 2 -weighted planning MRI scan. The displacement of the rectum relative to the prostate can be clearly seen on the sagittal image. Compared to standard implants at our centre, rectal D 2cc values for the two cases were reduced by 39.4% and 36% and V 10Gy values were reduced by 77.9% and 64.3%. Optimum PTV doses were achieved, allowing further dose escalation to the GTV (orange line on the dose-volume histogram in Figure 1). Neither patient was aware of the presence of the balloon after the implant.

Conclusion Insertion of a BPBI spacer device is feasible and only increases the duration of the brachytherapy procedure by a few minutes. The fact that neither patient could feel the implant afterwards is probably due to the fact that the balloon remains relatively deformable, unlike the gel based spacer systems which harden into a solid mass. The BPBI spacer device clearly and reliably increases the distance between the prostate and the rectum due to the pre-determined balloon geometry which is not operator dependent. Excellent dose distributions were achieved in both cases with far less rectal dose than would have been possible without the balloon. The use of the BPBI spacer device has the potential to significantly reduce rates of rectal toxicity and allow for further dose escalation to the prostate. EP-2267 Adverse events after HDR brachytherapy as monotherapy in localized prostate cancer. S. Kariya 1 , K. Kobayashi 1 , I. Yamasaki 2 , S. Ashida 2 , K. Inoue 2 , T. Yamagami 1 1 Kochi Medical School, Department of Radiology, Nankoku, Japan 2 Kochi Medical School, Department of Urology, Nankoku, Japan Purpose or Objective The aim of this study is to investigate if the clinical and dosimetric factors affected the incidence of adverse events after a single implant, after high dose rate (HDR) brachytherapy as monotherapy in localized prostate cancer. Material and Methods One hundred five patients, 3 with low-risk (T stage < or = 2a, PSA < or = 10 ng/ml, and Gleason score (GS) < or = 6), 35 with intermediate-risk (T stage = 2b or 2c, PSA > 10 and < or = 20 ng/ml, GS = 7), and 67 with high-risk (T stage > or = 3a, PSA > 20 ng/ml, GS > or = 8) prostate cancer who underwent HDR brachytherapy as monotherapy (no external beam radiotherapy) using 27 Gy/2 fractions in one day from July 2011 to September 2015 were analyzed prospectively. Patient age ranged 54 to 82 (median 72 ） years old. The median follow-up period was 56 months (range 24 – 75).Acute and late toxicities were assessed as per Common Terminology Criteria for Adverse Events (CTCAE), Version 4.03. The dosimetric factors affecting the acute or late grade 2 to 3 gastrointestinal