Abstract Book

S833

ESTRO 37

EP-1543 Urethrogram combined CT-MRI image fusion to determine apex in IMRT for localized prostatic cancer K. Nishiyama 1 , T. Toyofuku 1 , S.I. Ikemoto 2 1 Yao Municipal Hospital, Radiation Oncology, Yao, Japan 2 Yao Municipal Hospital, Urology, Yao, Japan Purpose or Objective On CT images for radiation planning for prostatic cancer, most inferior portion of the prostate, the apex, is difficult to distinguish from adjacent structures, i.e. urogenital diaphragm. Therefore, to delineate the prostate accurately, MRI is registered to CT using pelvic bones as landmarks. After bone-to-bone matching, however, the position of the prostate reportedly differed between CT and MRI. In our study, the urethra was contrasted at CT and MRI acquisition and used as a landmark of CT-MRI registration. Because the penile urethra is directly bound to the prostatic apex, this urethra-to-urethra matching leads to accurate matching of the apex. After this matching, the position of pubic symphysis on CT and MRI was compared to examine the accuracy of bone-to-bone matching. Material and Methods The subjects were 31 patients that received IMRT for localized prostatic cancer. Every patient underwent staging MRI (stageMRI) at his first diagnosis. Before IMRT, 27 out of 31 patients were treated with androgen deprivation therapy (ADT). At simulation, patients underwent CT (simCT) of 1 mm slice thickness and three- dimensional MRI (simMRI) of 1.5 mm of slice thickness. To contrast penile urethra, iodinated contrast medium and a normal saline solution were injected into the urethra at simCT and simMRI (Fig), respectively. SimMRI and simCT were automatically and three-dimensionally registered on treatment planning system and thereafter simMRI urethra was manually registered to simCT urethra along the superior-inferior (SI) direction (arrowheads on Fig). After the matching, the distance between the simMRI apex and the superior edge of pubic symphysis (SEPS) was determined on CT [(d (CT) on Fig] and MRI [d (MRI) on Fig]. In addition, the distance between the apex and SEPS was determined on stage MRI similarly.

significantly differ between stageMRI and simMRI despite prostate shrinkage after ADT. It is concluded that the positional relation between the prostatic apex and the pubic bone is constant and pubic bone-to-bone matching can match the apex on simMRI to that on simCT along the SI direction. EP-1544 Ga-68 PSMA PET/CT in postoperative Prostate Cancer patients F. Alongi 1 , R. Mazzola 1 , S. Fersino 1 , D. Aiello 1 , A. Fiorentino 1 , F. Gregucci 1 , N. Giaj Levra 1 , F. Ricchetti 1 , M. Salgarello 1 1 Sacro Cuore Don Calabria Hospital, Radiotherapy and Radiosurgery, Negrar, Italy Purpose or Objective Currently, Choline PET/CT is under definitive assessment for staging of prostate cancer (PC) patients with Prostatic Specific Antigen (PSA) failure level up to 1.5-3 ng/ml. At lower level of PSA relapse, its accuracy is dramatically reduced. Thus, in this setting a reliable tool to detect PC recurrences is advocated. Aim of the study is to evaluate the impact of Ga-68 Prostate Specific Membrane Antigen (PSMA)-PET/CT in decision-making strategy of patients with prostate cancer (PC) who underwent radical prostatectomy (PR). Material and Methods From June 2016 to April 2017 forty consecutive patients, previously submitted to PR with a PSA value detectable in the range between 0.1 ng/mL and 2 ng/mL were recruited for Ga-68 PSMA-PET/CT restaging. Therapeutic strategy based on the Ga-68 PSMA-PET/CT evaluation was compared with the strategy that would have been proposed in case of PET not available and/or not strictly indicated. Results In twenty six out of 40 patients (65%), Ga-68 PSMA- PET/CT was considered positive for disease recurrence. Considering the site of recurrence, lymph nodes metastases were found in 20 cases, prostatic bed relapse in 5 cases, bone in 8 cases. More specifically, multiple sites of recurrence were shown in 5 cases: in 4 cases bone and lymph nodes and in one case prostatic bed and lymph nodes. Accordingly to Ga-68 PSMA-PET/CT findings, the decision making strategy was changed as follows: radiation therapy was proposed in 22 out of 40 cases (55%) while systemic therapy was suggested or modified, when compared to the previous, in 6 out of 40 patients (15%). In details, in 3 (7%) patients was administered a complete androgenic blockade, in a patient (2%) a manipulation of the androgen deprivation therapy, in a single case zoledronic acid was prescribed (2%) and finally a patient ( 2%) received an androgen- receptor–signaling inhibitor ( Enzalutamide). Conclusion In conclusion, therapeutic strategy based on the Ga-68 PSMA-PET/CT was changed in the 65% of patients analyzed compared to the strategy that would have been proposed in case of PET not available and/or not strictly indicated. Thus, Ga-68 PSMA-PET/CT seems to be a promising diagnostic tool in patients with PC who underwent PR with PSA relapse values in the window between undetectability and 2 ng/mL. Looking at the present results, further studies investigating the reliability in terms of accuracy of Ga-68 PSMA-PET/CT are needed in this scenario.

Results After the urethra-to-urethra matching, the mean distances between the MRI apex and SEPS on simCT [d (CT)] and on simMRI [d (MRI)] was 27.8 [range: 14 – 43, standard deviation (SD): 6.1] and 27.5 mm (range: 13 – 41, SD: 5.8), respectively ( P =0.21, paired t-test). The mean of the difference in SEPS level between simCT and simMRI was 0.8 mm (range: 0 – 3, SD: 0.8). The mean distances between the apex and SEPS on stageMRI was 26.7 mm (range: 13 – 37, SD: 5.9) that did not differ from that of simMRI significantly ( P =0.08). Conclusion In our study, the prostatic apex on simCT and simMRI was accurately matched using urethra-to-urethra matching. After the matching, the difference in pubic bone positions between simCT and simMRI was insignificant. In addition, the distance between the apex and SEPS did not