T. Nuver

Cone-beam CT-based adaptive planning improves permanent prostate brachytherapy dosimetry: An analysis of 1266 patients.

Purpose To evaluate adaptive planning for permanent prostate brachytherapy and to identify the prostate regions that needed adaptation. Methods and materials After the implantation of stranded seeds, using real‐time intraoperative planning, a transrectal ultrasound (TRUS)‐scan was obtained and contoured. The positions of seeds were determined on a C‐arm cone‐beam computed tomography (CBCT)‐scan. The CBCT‐scan was registered to the TRUS‐scan using fiducial gold markers. If dose coverage on the combined image‐dataset was inadequate, an intraoperative adaptation was performed by placing remedial seeds. CBCT‐based intraoperative dosimetry was analyzed for the prostate (DSymbol, VSymbol, and VSymbol) and the urethra (DSymbol). The effects of the adaptive dosimetry procedure for Day 30 were separately assessed. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Results We analyzed 1266 patients. In 17.4% of the procedures, an adaptation was performed. Without the dose contribution of the adaptation Day 30 VSymbol would be < 95% for half of this group. On Day 0, the increase due to the adaptation was 11.8 ± 7.2% (1SD) for DSymbol and 9.0 ± 6.4% for VSymbol. On Day 30, we observed an increase in DSymbol of 12.3 ± 6.0% and in VSymbol of 4.2 ± 4.3%. For the total group, a DSymbol of 119.6 ± 9.1% and VSymbol of 97.7 ± 2.5% was achieved. Most remedial seeds were placed anteriorly near the base of the prostate. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Conclusion CBCT‐based adaptive planning enables identification of implants needing adaptation and improves prostate dose coverage. Adaptations were predominantly performed near the anterior base of the prostate.

OC-0086: Dosimetric inter- and intra-observer contouring and registration variability for prostate brachytherapy

physiological methods and validated questionnaires. Ten single nucleotide polymorphisms (SNPs) have previously been suggested to be predictive of late radiation induced toxicity by GWAS studies or candidate gene studies. The objective of this study was to test the ten SNPs in this unique cohort. The strength of the reportings as well as the clinical data available served as the rationale for using our rather small cohort as a validation cohort. Materials and Methods: The patients in this cohort have received EBRT 70-78 Gy for prostate cancer with curative intent. Functional toxicity endpoints have been examined by sigmoidoscopy, manometry, endoanal ultrasonography and impedance planimetry in an earlier study. Objective endpoints include the Vienna Rectoscopy Score (VRS), cross sectional area (CSA) of rectum at distension, maximum resting pressure (MRP) and maximum squeezing pressure (MSP) of anal sphincters. The subjective measure RTAnorectal dysfunction score (RT-ARD) was obtained from the questionnaires. Biological material from the patients in this cohort is available from an established research biobank. The SNPs were investigated in DNA from fibroblasts with TaqMan SNP assays. Statistical analyses was carried out with Stata13. For each SNP (table I) the reported risk allele was identified and due to the small sample size a risk-allele average was created for each patient. This risk-allele average was correlated to RT-ARD, VRS, MRP, MSP and CSA by Spearmans rank correlation.

SU-FF-J-68: First Clinical Results of An Adaptive Off-Line Radiation Scheme Using Cone-Beam CT Scans for Treatment of Prostate Cancer

Purpose: We developed an adaptive scheme for prostate cancerradiotherapy based on kV cone‐beam‐CT (CBCT)images that are obtained on the machine during the first six treatment days. The aim of this scheme is to improve knowledge of the average prostate position and average rectum shape and safely reduce the PTV margin. Method and Materials: CBCT‐scans, acquired our on Elekta Synergy systems, were first matched on the planning CT scan using the pelvic bones. Automatic grey‐value matching was then used to match the prostates of the CBCT‐scans to the prostate of the planning CT scan. The mean of the obtained translations and rotations was used to move the prostate of the planning CT scan to its average position. Subsequently, the rectal wall was delineated in the CBCT‐scans, and coordinates of corresponding points of the 7 rectums were averaged to obtain the average rectal wall. Based on average prostate and rectum a new IMRTtreatment plan was made with a reduced PTV margin of 7 mm. Weekly CBCT‐scans were made to verify that the new PTV encompasses the prostate. Results: So far, 16 patients were successfully treated with our adaptive treatment scheme. For 85% of the CBCT‐scans a successful grey‐value match was obtained, the other scans were discarded. For 88 out of 89 verification scans the prostate was inside the PTV. The mean dose received by the rectum reduced on average by 7.6%, and the equivalent uniform dose (a=12) by 1.5%. Conclusion: This is the first routine clinical application of soft tissue image guidance for the prostate using kV CBCT. Contrary to adaptive schemes that use implanted markers, our method is non‐invasive and improves localization of both prostate and rectum. Conflict of Interest: Elekta, Inc financially supported part of this study.