Abstract Book

ESTRO 37

S575

requirements for high-quality FBP are mpMRI and TRUS- MRI guided biopsy. They result in a map of cancer distribution within different parts of the gland. There are at least several commercially available software systems to guide TRUS-MRI fusion biopsy. Unfortunately their cost exceeds the financial abilities of many brachytherapy departments. The goal of this study was to check the feasibility of TRUS-MRI fusion prostate biopsy using ONLY brachytherapy TPS. Material and Methods From 12.2016 to 09.2017 we performed TRUS-MRI fusion biopsies of prostate in 27 pts. The main indications for this procedure were: repeat biopsy after previously negative biopsy (14pts), suspicion of local recurrence of PCa after EBRT (8pts), other (4pts). The median age was 72 (51-84) yo. The median PSA level was 9,5 (2,13- 170) ng/ml. We defined 35 suspected lesions on MRI images. 6 lesions were scored PIRADS3, 13- PIRADS4, 7- PIRADS5, 9- were not scored with PIRADS (pts with suspicion of radio- recurrent tumors). MRI images were analyzed with Oncentra Brachy TPS (Elekta AB). In this system we also delineated prostate, suspected lesions and 5mm margins for every lesion. Prepared images were subsequently sent to Oncentra Prostate TPS (Elekta AB). This system was used to perform TRUS-MRI fusion transperineal biopsy with following steps: 1. acquisition of 3D TRUS images of prostate; 2. fusion of MRI and TRUS images; 3. preplanning of the desired position of biopsy cores; 4. guidance of targeted biopsy, biopsy of 5mm margin, systematic biopsy of left and right lobe. Each biopsy core was named with template position and sent separately to Pathology Department. All data from pathology report were entered into Oncentra Prostate TPS resulting in 3D map of cancer distribution within the gland. Results We obtained verification of PCa in 88% pts. The median volume of lesion was 0,53 (0,1-8,4) cc, of lesion with 5mm margin 2,95 (2-22) cc. The median number of biopsy cores was 22 (17-27). From 35 lesions 57,6% were positive for PCa, with PIRADS3, 4, 5 and no PIRADS lesions being positive in 16,6%, 58,3%, 71,4% and 66% of cases, respectively. Among pts with PCa verification 31,8% were positive only within lesion, 59% within lesion and 5mm margin, 68,2% within lesion, margin and occupied lobe. If biological significance of PCa was added to this analysis the above-mentioned values changed to 41%, 68,1% and 77,3%, respectively. Only in 5 pts biologically significant cancer outside occupied lobe was found. One patient experienced major toxicity (AUR). Conclusion Transperineal TRUS-MRI fusion prostate biopsy is feasible with the use of brachytherapy TPS which are available at the majority of cancer centres. There is no need for extra expenses related to commercially available systems. We hope this study will help brachytherapy community to move towards focal treatment in selected pts with PCa. PO-1025 Effects of antecedent androgen deprivation therapy in prostate seed implant brachytherapy dosimetry R. Nakamura 1 1 Iwate Medical Univerrsity, Department of Radiology, Morioka, Japan Purpose or Objective Seed implant brachytherapy (BT) for organ-confined prostate cancer (OCPC) is frequently combined with androgen deprivation therapy (ADT) to enhance control of

unfavorable disease. It is believed that antecedent ADT induces morphological changes in the prostate that cause considerable difficulty in appropriate needle insertion during implant. To clarify the effect on quality of dose delivery, a retrospective analysis was performed using DVH analysis. Material and Methods A total of 580 OCPC patients were treated in our institute between January 2005 and December 2014 with BT alone (243:BT-pts) or BT after ADT (337:BTH-pts). Patients were classified as low/intermediate/high risk (282/255/43, respectively). BT was performed with real- time, intraoperative transrectal US-guided transperineal seed insertion with modified peripheral loading at a prescribed dose of 160Gy. ADT mostly consisted of combined androgen blockade for a median duration of 6 months (range:1 to 88 months). Mean comparative DVH parameters in BT-pts and BTH-pts included prostate volume (pV), the dose covering 90% of the pV (pD 90 ), and the pV covered by 160Gy or 240Gy (pV 100 , pV 150 ) measured during and 30 days after implantation, as determined by US (US-plan) and CT (CT-plan), respectively. These comparisons were also performed in subgroups of BTH-pts according to the duration of ADT (<12 months:BTH <12 vs. >12 months:BTH >12 ). Results The mean and standard deviation of DVH parameters according to US-plan and CT-plan in all patients, BT-pts, and BTH-pts are listed below.

BT-pts (243)

BTH-pts (337)

all (570)

p

pV US-plan (cc) pV CT-plan (cc)

0.000

28.1±10.6 30.9±11.5 25.5±10.6

28.8±7.0 29.7±5.9

26.4±7.8

0.001

US-plan

pD 90 (Gy) pD 90 (Gy)

192±16.4 193±17.0 191±15.7

0.178

CT-plan

179±23.6 183±23.7 173±22.3

0.000

US-plan

pV 100

96.8±6.6 96.8±6.6

96.7±6.5

0.826

(%)

CT-plan

pV 100

93.2±6.4 93.9±6.4

92.2±6.3

0.001

(%)

US-plan

pV 150

0.638

62.6±14.0 65.6±10.2 63.9±9.5

(%)

CT-plan

pV 150

0.000

57.4±13.4 66.2±13.3 57.4±12.4

(%)

In general, pD 90 (prostate-DVH values) in the CT-plan were smaller than those in the US-plan. In a comparison between BT-pts and BTH-pts, there was no significant difference in prostate-DVH values in the US- plan. However, the results in BTH-pts were significantly less than those in BT-pts in the CT-plan. Change in mean pV in the US-plan and CT-plan was reciprocal in BT-pts and BTH-pts, with statistical significance in both cases (p=0.000). The pV was smaller in the BTH >12 group (23.1 ± 8.9 cc) than in the BTH <12 group (28.6±7.8 cc). The prostate-DVH values were smaller in the BTH >12 group than in the BTH <12 group:pD 90 :168 ± 24.7 vs. 175 ± 21.7Gy; pV 100 :89.3 ± 10.3 vs 92.9 ± 5.0%; pV 150 :52.6±16.1 vs , pV 100 , and pV 150

58.9±12.6%. Conclusion

Antecedent ADT induces additional degradation of prostate dose and coverage. The pV increase in the CT-