ESTRO 38 Abstract book

S444 ESTRO 38

OARs were segmented on a fused CT/MR set acquired with organ motion-mitigation devices in place, namely, an endorectal air-filled balloon (150 cc) and a transponder- loaded Foley catheter. The CTV included the prostate gland and seminal vesicles and an isotropic 2 mm margin was added for the PTV. Validation a 2 mm margin expansion was performed in the first 60 patients. The urethral wall was anatomically identified through the catheter. All the other OARs were clearly identified on the MR image set. Strict QA procedures to validate the plan were carried out before final approval. At the time of treatment, anatomical reproducibility was achieved by recapitulating the positioning of the endoluminal devices. Precise PTV targeting was performed by CBCT and real- time motion management was obtained via beacon transponders, ensuring treatment delivery within the 2 mm PTV margin. Toxicities were graded according to the NCI CTCAE v.4, and QoL was assessed by EPIC and IPSS questionnaires. Tumor response was assessed by PSA and MRI at 6 and 12 months post-treatment. Results On-line tracking with 2 mm motion tolerance was achieved in all cases. At a median follow-up of 33 months (range, 12-62), acute G2 GU and GI toxicities were 2% and 0%, respectively. Late G2 GU and GI toxicities were 1% and 0%, respectively. No G3 toxicity occurred. An initial 11% and 6% drop in the GU and GI EPIC domain scores, respectively, was observed at 1 month post treatment, returning to baselines by 3 months. Similarly, at one month median IPSS values increased from 7 to 11 also returning to baseline by 3-months. Post-treatment PSA kinetics show a rapid decline, reaching a median ≤1 ng/mL by 18 months. A total of 8 patients have relapsed biochemically (Phoenix definition). All relapses occurred in patients with intermediate risk disease between 24 and 36 months post- treatment. Actuarial probability of freedom from biochemical relapse at 48 months is 100% and 97% for low and intermediate risk patients, respectively. MR response at 6 months post-treatment correlated with long term bNED. 60 months bNED was 99% in patients with a negative DW-MR vs. 82% for those with persistent disease, respectively ( p <0.01).

PO-0845 What is the dosimetric benefit of daily position control imaging for prostate cancer radiotherapy? M. Splinter 1,2,3 , T. Bostel 2,4 , C. Lang 1,2,3 , P. Häring 1,2,3 , N. Bougatf 5 , J. Debus 2,3,6 , N.H. Nicolay 2,7 1 German Cancer Research Center DKFZ, Division of Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 German Cancer Research Center, Clinical Cooperation Unit "Radiation Oncology", Heidelberg, Germany ; 3 Heidelberg Institute for Radiooncology, National Center for Radiation Research in Oncology, Heidelberg, Germany ; 4 University Medical Center Mainz, Department of Radiation Oncology, Mainz, Germany ; 5 Heidelberg Ion Beam Therapy Center, Department of Radiation Oncology, Heidelberg, Germany ; 6 University Hospital Heidelberg, Department of Radiation Oncology, Heidelberg, Germany ; 7 Freiburg University Medical Center, Department of Radiation Oncology, Freiburg, Germany Purpose or Objective For prostate cancer patients, interfractional differences in bladder and rectum volumes as well as positioning inaccuracies result in dose deviations in the target volume (TV) and OARs; however, regarding the dose exposure associated with regular CT imaging, the optimal compromise between dose sparing and positioning accuracy remains to be elucidated. Therefore, the influence of daily vs. weekly CT-based positioning imaging on dosimetric deviations between accumulated applied and prescribed doses was analyzed. Material and Methods The data of twenty prostate carcinoma patients (ten patients with primary radiation therapy for prostate cancer, ten patients with salvage treatments to the prostatic bed after previous prostatectomy), who underwent treatment (IMRT plan, 76.5Gy in 34fx for primary and 68Gy in 34fx for salvage treatments) were investigated (680 fractions in total). Prior to each daily fraction, position-control imaging (fx-CT) was carried out using a diagnostic in-room CT scanner (SOMATOM ® Emotion Syngo, SIEMENS). For each patient, both daily and once-a- week (d1, d8, d15, d22, d29) fx-CTs were rigidly registered to the planning CT using the position correction vectors. TVs and all OARs were contoured and volumetrically compared to the structures of the planning CT. By using a deformable registration algorithm, doses were tracked, and applied dose were compared to the planned dose for each structure in dependence of the frequency of CT positioning imaging. Gamma analyses of the total dose distribution were performed at the tolerance level of 3 %/3 mm for the γ<1 test, within the region receiving doses >10% of the maximum dose. Results Volumetric changes and positional variations especially of the bladder revealed significant dose deviations between accumulated applied dose and planned dose for primary treatments and salvage treatments.

Conclusion These trial outcomes indicate that 5x9Gy SBRT can be consistently and safely delivered and is associated with excellent biochemical, imaging and QoL measures.

Doing once-a-week fx-CTs effected a decrease of the applied D 50 (0.54±0.45Gy; 4.79±7.84Gy) and D 95