ESTRO 36 Abstract Book

S515 ESTRO 36 _______________________________________________________________________________________________

PO-0929 Needle Migration in HDR Brachytherapy for Prostate Cancer evaluated by Serial MRI a nd Photos S. Buus 1 , M. Lizondo 2 , S. Hokland 3 , S. Rylander 3 , E. Pedersen 4 , L. Bentzen 1 , K. T anderup 3 1 Aarhus University Hospital, Department o f Oncology, Aarhus C, Denmark 2 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain 3 Aarhus University Hospital, Department of Medical physics, Aarhus C, Denmark 4 Aarhus University Hospital, Department of Radiology, Aarhus C, Denmark Purpose or Objective Needle migration in high dose rate brachytherapy (HDR- BT) for prostate cancer may lead to insufficient target coverage and increased dose to organs at risk. The aim of this study was to assess the magnitude of needle migration in HDR-BT with serial MRI and photos. Material and Methods 12 patients with high risk prostate cancer treated with EBRT and two separate boosts of HDR-BT were included in the study. In order to fixate the needles, a thin silicone pad was placed within the template, which was fixated to perineum with 4 sutures. Following US guided needle implant, patients were placed in supine positi on on an MRI couch on trolley for the rest of the procedure. Three MRIs were performed; one for planning (MRI1), one immediately before HDR-BT (MRI2), and one after HDR-BT (MRI3). All MRIs were a transversal T2-weighted turbo spin-echo with 2 mm slice thickness and 1.2 x 1.49 mm resolution. The position of the template was marked with indian ink on the thighs of patients, and photos of the perineum were taken after each MRI. MRI2 and MRI3 were co-registered to MRI1 to match the prostate. Coordinates of each needle tip defined on all three MRIs were used to calculate the migration for each needle. An average needle migration of ≤3 mm was considered "acceptable". On photos, movement of the template relative to the ink markings was regarded as needle migration, which was scored as either "acceptable" or "considerable" from MRI1 to MRI2 and from MRI1 to MRI3. Scoring of needle migration with MRI and photos was compared. An analysis was performed to examine whether posterior needles were more prone to migrate compared with anterior needles.

17/19 procedures from MRI1 to MRI2 and in 13/18 procedures from MRI1 to MRI3. Concordance between scoring by photo and MRI was found in 24/37procedures. Average needle migration was 2.9 ± 1.6 mm for anterior needles and 3.6 ± 1.5 mm for posterior needles (students t-test, p=0.08) Conclusion Needle migration was of acceptable magnitude measured from MRI1 to MRI2, but of considerable magnitude from MRI1 to MRI3. Insufficient concordance between scoring by photo and MRI indicates that visual inspection is inadequate for evaluating implant stability. A likely explanation for the lack of concordance between for photos and MRI is the developing oedema following needle insertion. PO-0930 CT to TRUS based Prostate HDR: what is the optimal dosimetric margin to use? F. Lacroix 1 , M. Lavallée 1 , E. Vigneault 1 , W. Foster 1 , A.G. Martin 1 1 Centre Hospitalier Universitaire de Québec- L'Hôtel- Dieu de Québec, Department of radio-oncology, Quebec, Canada Purpose or Objective The contouring volume variability resulting from delineating the target with Computed Tomography (CT) or Transrectal Ultrasound (TRUS) results in a 30 to 50% increase in volume when contouring a prostate on CT versus TRUS due to the poor soft tissue contrast of CT. This may have a significant dosimetric impact when moving from a CT to a TRUS based prostate high-dose rate (HDR) brachytherapy planning as the treated volumes are susceptible to differ significantly. This study aims at determining the proper dosimetric margin to apply when going from CT to TRUS based planning in order to compensate for this volume difference. By doing so, we aim to treat the same volume of prostatic tissue in CT or TRUS and insure a constancy in quality of care for prostate cancer patients treated with HDR. Material and Methods Twenty-seven prostate cancer patients were given a 15Gy HDR boost using a TRUS-based catheter insertion and planning approach. A 2 mm isotropic dosimetric margin was used for the TRUS planning. An average of 17 catheters were implanted. Without moving patients still under general anesthesia, a CT on rails located inside the operating room was used to image the pelvis. Three experienced radiation oncologists specialized in brachytherapy delineated the prostate on the resulting CT images and an offline, independent CT based planning was performed. A 1 mm isotropic dosimetric margin was used in CT planning. The prostate volume, 15Gy volume and V100 of the prostate were then collected and compared for the US and CT based plans. Results The average prostate, 15Gy volumes and V100 are presented in table 1. Table 1: Average prostate volume, 15Gy volume and V100 for TRUS and CT based planning Modality Average prostate volume (CC) Average 15 Gy volume (CC) V100 (100%) TRUS 38.0 50.2 96.3 CT 44.3 54.2 96.0 The average TRUS volume is 16.5% smaller than the average CT volume. When using a 2 mm dosimetric margin, the volume receiving 15Gy is smaller by 8% in TRUS compared to CT based planning. The V100 are almost identical with both modalities. The standard deviation on the TRUS prostate volume is slightly lower (10.6) than on CT (11.2).

Results A median of 16 needles (14 - 21) were used for each HDR- BT procedure. Serial photos were taken in 19/24 procedures. MRI2 was performed in 24/24 procedures and MRI3 in 22/24 procedures. MRI evaluated needle migration was median 2.2 mm per needle (-0.8 - 4.4) from MRI1 to MRI2, median 2.6 mm per needle (0 - 10) from MRI2 to MRI3, and median 3.9 mm per needle (0.3 - 9.8) from MRI1 to MRI3. Needle migration evaluated by MRI was found "acceptable" in 23/24 procedures from MRI1 to MRI2, and in 7/22 procedures measured from MRI1to MRI3. Needle migration evaluated by photo was found "acceptable" in