ESTRO 2020 Abstract Book

S1085 ESTRO 2020

boost in 76 patients (108-110 Gy). Day 30 urethra V125 as a % of dose did not differ between technique or intent. D90 was higher for HELP monotherapy (monoHELP) than for MUS (monoMUS), 117.2% vs. 110.2% (P < 0.001) with no difference in D90 between boost with MUS (boostMUS) and boost with HELP (boostHELP). There were no significant differences in grade 2+ AUT between groups, and no difference between grade 2+ LUT for monoMUS vs. monoHELP (9.5% vs. 14.9%, p=0.24), or boostMUS vs. boostHELP (20% vs. 29%, p=0.8). Three total patients had grade 3 LUT, with no grade 4 GU toxicity. Median time to recovery of IPSS < 5 points of baseline was 6 months (IQR 3-12 months) and did not differ with monotherapy vs. boost in the entire cohort (P = 0.2808). There was a trend towards longer time to recovery of IPSS in monoHELP vs. monoMUS patients (P = 0.0569), but not for boost patients for HELP vs. MUS (P = 0.65). On multivariable logistic regression, the only factor predicting IPSS recovery was pre-treatment prostate volume (P=0.0436). Kaplan-Meier estimated bPFS at 5 years was 98.3% for monotherapy and 96.1% for boost, with no difference between techniques.

gold fiducial markers were implanted before MRI and planning CT for accurate fusion and image guided irradiation. Prostate with involved extraprostatic tissue and proximal seminal vesicles were used as CTV. PTV margins were 3 mm in all directions and 1mm posteriorly to spare the anterior rectum wall. Boost was delivered as 3 fractions of 7Gy (ED 2Gy; α/β-3 - 42Gy; BED; α/β-3 -70Gy). Toxicity was scored according to the RTOG scale. Results The median follow-up in the group of HDR brachytherapy boost was 40.7 (12.3;73) months. Grade II late genitourinary toxicity detected in 22%. Grade II rectal toxicity mentioned in 10.5%. Grade III was not mentioned neither for rectal nor for genitourinary toxicity. The median follow-up in the group of SBRT boost was 19.2 [12.1; 49] months. Genitourinary toxicity was moderate: grade II in 8 cases (16.3%), grade III – in 1 men (2.1%). Surprisingly high frequency (3 cases; 6.1%) of severe (grade III-IV) rectal toxicity was mentioned 9-16 months after the end of SBRT boost. Colostomy was performed in 2 patients with therapy resistant rectal pain, severe bleeding and ulcerations of anterior rectal wall (grade IV toxicity). Rectal pain and ulceration detected in another patient who is still on conservative therapy with limited surgical interventions (grade III). Comparison of dosimetric plans demonstrated that average value of maximum dose to 2 cm³ of anterior rectal wall (D 2cm³ rectum ) was 35.7% higher with SBRT than with HDR brachytherapy boost. All 3 patients with grade III-IV rectal toxicity received 50Gy of whole pelvis radiotherapy. The cumulative D 2cm³ rectum (α/β- 4) values were 83.5Gy, 84.1Gy - for grade IV and 84.9Gy - for grade III toxicity. Conclusion In our patients HDR brachytherapy boost for prostate characterized by higher cumulative dose to the prostate and significantly less toxicity. Risk of severe (grade III-IV) rectal toxicity significantly increased when cumulative D 2cm³ rectum was close to 83-84 Gy and higher. OC-1029 Validation of the bladder neck as a significant OAR at prostate seed brachytherapy based on D2cc N. Wallace 1 , M.F. Jamaluddin 1 , P.J. Kelly 1 1 Cork University Hospital, Radiation Oncology, Cork, Ireland Purpose or Objective We report our single institution experience of outcomes after beginning to measure the D2cc to the bladder neck at prostate brachytherapy seed implantation. The three metrics examined were: -urinary toxicity pre and post implementation of D2cc bladder neck constraint -acute and late urinary toxicity for bladder neck D2cc < and > 50% -whether the D2cc changed with time as operators gained experience Material and Methods Following the publication by Harthout et al (1) which identified the bladder neck D2cc as the best predictor of acute and late urinary toxicity, we began contouring the bladder neck volume as defined in the paper with a view to maintaining the dose to this structure as low as achievable. We retrospectively analysed the final 100 patients treated with a prostate seed implant, using 160Gy prescribed dose to the prostate, before making this change and the first 100 treated after the routine contouring of the the bladder neck. Data gathered included dosimetric statistics and records of acute and late toxicities after the procedure. All patients were analysed for the incidence of acute (within 12 months of implant) toxicities. The median follow-up was 29 months (IQR 22-35) in the group prior to introduction of the OAR and 12 months (IQR 7-13) afterwards. Patients with less than 12 months’ follow-up were excluded from the analysis of late toxicities.

Conclusion There was acceptable grade 2+ AUT and LUT with MUS and HELP, as well as excellent bPFS for all our subgroups. The trend to longer time to recover IPSS in the monoHELP vs. monoMUS can be explained by higher mean D90 with monoHELP with no significant difference in urethra V125, suggesting D90 or another metric may better predict persistent AUT. Higher doses with monoHELP may reflect smaller variation of needle placement or use of non- stranded seeds vs. MUS. Controlling for D90, there was no significant difference in AUT, LUT or bPFS between MUS and HELP groups in our cohort. OC-1028 HDR brachytherapy vs stereotactic boost in treatment of prostate cance: comparison of safety profile S. Novikov 1 , M. Gotovchikova 2 , N. Ilin 2 , R. Novikov 2 , Y. Merezhko 2 , J. Melnik 2 , S. Kanaev 2 1 Prof. N.N. Petrov Research Institute of Oncology, Radiotherapy, St. Petersburg, Russian Federation ; 2 Prof. N.N. Petrov Research Institute of Oncology, Radiotherapy, St Petersburg, Russian Federation Purpose or Objective to compare toxicity of high dose rate (HDR) brachytherapy and stereotactic body radiotherapy when used to deliver the boost to the prostate after whole pelvis irradiation. Material and Methods Since 2013 in our Institute patients with high and very high risk prostate cancer are treated by androgen deprivation therapy with whole pelvis radiotherapy to 46-50Gy and subsequent HDR brachytherapy boost to the prostate: 2 fractions of 10Gy (equivalent dose (ED 2Gy α/β-3) – 52Gy, biologically effective dose (BED; α/β-3) – 86.7Gy) or 1 fraction of 15 Gy (ED 2Gy; α/β-3 - 54Gy; BED; α/β-3 -90Gy). Eighty-nine patients were treated according to this protocol from 2013 to 2017. Since 2016 in 46 patients with significant comorbidities SBRT was preferred as boost technique because of minimally invasive nature of the procedure. In all cases