Abstract Book

S1242

ESTRO 37

Material and Methods This retrospective study included data of patients with biochemical failure and evidence of isolated local recurrence of PCa after radical/salvage EBRT or brachytherapy that received image-guided (IG) re-EBRT between 11/2009 and 11/2016. Biopsy was not mandatory if all elements were univocal (PSA evolution, choline-PET and MRI). Re-EBRT was delivered with IG-IMRT and/or stereotactic technology (Rapid Arc ® , VERO ® and CyberKnife ® ). Median total dose was 30 Gy in 5 (2-10) fractions. Results The data of 64 patients were analyzed, median age at re- EBRT was 73.2 years, median PSA pre-re-EBRT was 3.99 ng/ml (range: 0.37 - 21). 1 acute G≥3 genitourinary (GU) event (transitory macroscopic hematuria) and 1 late G≥3 GU (permanent reduction in bladder capacity) event were observed. No patient experienced G≥ 3 bowel toxicity. At the median follow-up of 26.1 months (3.1– 82.4), tumor progression was observed in 41 patients (64%).18 patients (28%) experienced local relapse. Median time-to-progression was 26.1 months. 2-year local control, biochemical and clinical relapse free survival rates were 75%, 40% and 53%, respectively. Biologically effective dose > 130 Gy was correlated with higher tumor control rates when compared to < 130 Gy. At the last follow-up, 30 patients (47%) show no evidence of disease, 28 patients (44%) are alive with biochemical or clinical disease and 1 is lost for follow-up. 5 patients died: 3 of disease progression, 1 for another type of tumor and 1 for unknown cause. Conclusion High-precision salvage re-EBRT for isolated local PCa recurrence is a safe, feasible and noninvasive treatment offering if biologically equivalent dose of > 130 Gy is administered, satisfactory tumor control without significant acute and late complications. Further investigation is warranted to define the optimal patient selection, dose and volume parameters. EP-2373 Pathologic pelvic lymph node displacement and the predictive value of pre-treatment imaging M. Kamphuis 1 , K.C. Crama 1 , M.A.J. De Jong 1 , M.C.C.M. Hulshof 1 , G.H. Westerveld 1 , J. Visser 1 , N. Van Wieringen 1 1 Academic Medical Center, Academic Physics, Amsterdam, The Netherlands Purpose or Objective As a result of improved imaging modalities, like FDG-PET and MRI, together with available radical radiation regimes, more patients are being treated for Pathologic Lymph Nodes (PLN). The interfraction stability of individual PLN’s is unknown. The first aim of this study was to quantify PLN displacement over the course of treatment. A moderate strong relation between elective lymph node displacement and bladder filling was described in literature. The second aim was to assess the predictive value of full and empty bladder pretreatment multi- modality imaging for PLN displacement. Material and Methods All patients (n=15) being treated for PLN in the pelvic area in a seven month period were included in this study. The number of pathological lymph nodes per patient ranged from 1-4. In total 31 lymph nodes were analyzed. Patients were imaged on a daily basis at the Linac using Conebeam-CT (CBCT). To quantify PLN displacement during treatment, we analyzed the CBCT-scans of the first five fractions.

Fractions later in the series were not always present or hard to register due to tumor shrinkage. Images were first registered based on bony anatomy using a predefined region of interest (ROI) in combination with a bony anatomy algorithm. Second, a registration of the PLN was performed using a grey-value registration using an ROI of PLN plus a margin of 5 mm excluding bony structures in the volume. Figure A shows an example of the two registration areas.

Most patients received an empty bladder pre-treatment imaging like PET. We determined whether PLN’s showed excessive displacement relative to the full bladder planning CT–scan. Excessive displacement was defined as a shift of the center of gravity of the PLN larger than 7mm (CTV-PTV margin). We also determined whether PLN’s showed excessive displacement during treatment. We calculated the positive and negative predictive values of pretreatment imaging for PLN displacement during treatment. Results

The results of the geometric uncertainties are presented in table 1. Six out of 31 (19%) PLN‘s showed excessive displacement during treatment. The four most moving PLN’s were present in only 2 patients, skewing the results. Therefore, systematic and random errors are presented with and without the two extreme cases. Moving PLN’s were found in the area of the common, the external, and internal iliac artery . The largest lymph node displacement was 34 mm. Excessive displacement was seen in 8/15 (53%) patients on pre-treatment imaging. The positive predictive value of pretreatment imaging for displacement during treatment was 38% (range 20-59%) and the negative predictive value 88% (range 50-97 %). Conclusion Most PLNs do not show large displacements (>7mm) during treatment. However, substantial lymph node movement (>30mm) can occur. Systematic surveillance of PLN patients during treatment is therefore recommended, distinguishing moving from non-moving