ESTRO 36 Abstract Book

S715 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective Dose painting radiotherapy requires accurate outlining of primary tumour volumes in the prostate. T2-Weighted (T2W) Magnetic Resonance Imaging (MRI) is the best imaging method for defining the gross tumour volume (GTV). Choline positron emission tomography (PET) is currently a controversial tracer. The image acquisition differs significantly in published studies. Many used early static imaging. One study found that 18F-choline PET/CT with late image acquisition has superior accuracy to T2W MR and functional MR alone. We investigate whether increasing 18F-Choline PET scan acquisition time from 60 (PET-60) to 90 (PET-90) minutes improves GTV TVD. Material and Methods Analysis was performed on 9 18F-Choline PET scans. Patients were injected with 370MBq of activity. Three clinicians (C1, C2 and C3) independently and without reference to each other contoured GTVs on each of the T2W-MRI, PET-60 and PET-90 scans at differing times. Scans were registered by a clinician using rigid co- registration. The treating clinicians MRI contour was used as a reference contour. The resulting PET and MRI GTVs were transferred to the PET-60 and PET-90 scans after image registration. The Dice Similarity Coefficient (DSC), Specificity (Sp) and Sensitivity (S) were calculated from contour mask voxel analysis. Results Table 1 shows the mean and range DSC, S and Sp scores on MRI, PET-60 and PET-90 for C1, C2 and C3 in comparison to the treating clinicians contour on MRI (C1). A 2 sampled T-test (P < 0.01) showed, no significant difference in the Sp, S and DSC between GTVs on PET-60 and PET-90 scans. Further to this, as shown in Figure 1, variability in GTV delineation is significant between observers in a singular case as well as across imaging modalities.

increasing the PET scan acquisition time from 60 to 90 minutes did not improve the performance of GTV TVD in comparison to MRI delineated GTV . EP-1334 Stereotactic radiotherapy with cyberknife® system in localized prostate cancer S. Falivene 1 , V. Borzillo 1 , R. Di Franco 1 , G. Totaro 1 , V. Ravo 1 , G. Quarto 2 , D. Sorrentino 2 , S. Perdonà 2 , P. Muto 1 1 Istituto Nazionale Tumori Fondazione Pascale, Radioterapia, Napoli, Italy 2 Istituto Nazionale Tumori Fondazione Pascale, Uorologia, Napoli, Italy Purpose or Objective Hypofractionated stereotactic radiotherapy (SRT) is an emerging technique in the treatment of localized prostate carcinoma (LPC). Considering that α/β ratio prostate cancer is very low (1.5), SRT is advantageous because consent to deliver higher dose/fraction on target respect conventional radiotherapy. In this study we reported our initial experience with SRT using CyberKnife® System (CK) in the treatment of LPC. Material and Methods From February 2013 to April 2016 ninety-six patients with LPC, mean age 70,6 years, were treated with CK-SRT. All patients were submitted to the eco-guided implants of 4 intraprostatic fiducial markers 7-10 days before the SRT in order to follow, to detect and to correct the intrafraction target movements. The fusion between CT scan and basal RM was made in order to optimize the contouring for treatment planning. All patients were treated with SRT in 5 fractions of 7-7,25 Gy/fraction for a total dose of 35-36,25 Gy. It was evaluated acute and late gastrointestinal and genitourinary toxicity using RTOG scale, biochemical control using mean decrease of PSA level during the different phases of follow up. In this study we have analyzed the results in the 77 patients with almost 3 months of follow up. Results All patients have completed CK SRT without severe complication. Median follow up was 17 months. Three patients died for non related cancer causes. Gastrointestinal acute toxicity G2 for perineal pain and rectal tenesmus was reported in only 13% and was decreased in all patients. Genitourinary acute toxicity G2 for urgency and nicturia was reported in only 4% and G1 for dysuria in 61% of cases which persist in 27,3% of patients. (Table 1) All patients obtained biochemical response with decrease of PSA. The PSA drop between the start of the therapy and at 21 months of follow up, was significant with p<0,01 (p=0,00001)

Conclusion Compared to MRI delineated GTVs, 18F-Choline PET GTVs are significantly different. This study found however that