ESTRO 36 Abstract Book

S465 ESTRO 36 _______________________________________________________________________________________________

PO-0855 Use of the LKB model to fit urethral strictures for prostate patients treated with HDRB V. Panettieri 1 , E. Onjukka 2 , T. Rancati 3 , R. Smith 1 , J. Millar 1 1 Alfred Hospital, Alfred Health Radiation Oncology, Melbourne, Australia 2 Karolinska University Hospital, Dept of Hospital Physics, Stockholm, Sweden 3 Fondazione IRCCS- Istituto Nazionale dei Tumori, Prostate Cancer Program, Milan, Italy Purpose or Objective High-Dose-Rate brachytherapy (HDRB) is widely used in combination with external beam radiotherapy in the treatment of prostate cancer. Despite providing biochemical control similar to other techniques, due to the variety of fractionation regimes used there is no clear consensus on the dose limits for the organs-at-risk, in particular the urethra. The aim of the work has been to fit the Lyman-Kutcher- Burman (LKB) Normal Tissue Complication Probability model to clinical outcome on urethral strictures data collected at a single institution. Material and Methods Dose-volume histograms and clinical records of 262 patients were retrospectively analysed. The patients had follow-up 6, 12, 18, 24 months and then every year until 10 years after the treatment. Clinical and toxicity data were collected prospectively. The end-point was the time of the first urethrotomy, a follow-up cut-off time of 4 years was chosen and the average stricture rate was about 12.6%. The LKB NTCP model was fitted using the maximum likelihood method and used simulated annealing to find a stable solution. Since the patients were treated with 3 different fractionation regimes (18 Gy in 3, 19 Gy in 2 and 18 Gy in 2 fractions) doses were converted into EQD2 with α/β = 5 Gy. Results For this cohort of patients the risk of urethral stricture could be modelled by means of a smooth function of EUD (see Fig 1). Using the LKB model the risk of complication could be represented by a TD50 of 220 Gy, a steepness parameter m of 0.55 and a volume-effect parameter n of 2.7. The fitted model showed good correlation with the observed toxicity rates with the largest deviation shown at higher doses. The large value of n could suggest a parallel behaviour of the urethra, however further validation is required with an independent dataset.

Poster: Physics track: Intra-fraction motion management

PO-0856 Systematic baseline shifts of lymph node targets between setup and treatment of lung cancer patients M.L. Schmidt 1 , L. Hoffmann 1 , M.M. Knap 2 , T.R. Rasmussen 3 , B.H. Folkersen 3 , D.S. Møller 1 , B. Helbo 2 , P.R. Poulsen 2 1 Aarhus University Hospital, Medical Physics, Aarhus C, Denmark 2 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark 3 Aarhus University Hospital, Department of Pulmonology, Aarhus C, Denmark Purpose or Objective Internal target motion results in geometrical uncertainties in lung cancer radiotherapy. The lymph node (LN) targets in the mediastinum are difficult to visualize in cone-beam computed tomography (CBCT) scans for image-guided radiotherapy, but implanted fiducial markers enable visualization on CBCT projections and fluoroscopic kV images. In this study, we determined the intrafraction motion of mediastinal LN targets in both the setup CBCT and fluoroscopic kV images acquired during treatment delivery, and investigated the baseline shifts and treatment accuracy of LNs for ten lung cancer patients. Material and Methods Ten lung cancer patients had 2-4 fiducial markers implanted in LN targets by EBUS bronchoscope. A total of 26 markers were evaluated. The patient received IMRT with daily setup CBCT for online soft tissue match on the primary tumor. During treatment delivery, 5 Hz fluoroscopic kV images were acquired orthogonal to the MV treatment beam. Offline, the marker positions were segmented in each CBCT projection and fluoroscopic kV image. From the segmented marker positions, the 3D marker trajectories were estimated from the segmentations with sample rate of 11 Hz during CBCT acquisition and 5 Hz during treatment delivery. The 3D motion amplitude and mean position of each LN marker as well as the intrafraction baseline shifts between setup CBCT and treatment delivery were calculated. Results Figure 1 shows the internal motion of one marker at one fraction. The motion is shown relative to the mean position during the CBCT scan and corrected for the couch shift between CBCT and treatment. For this marker, the baseline shift was 4.8 mm cranially, 0.6 mm posteriorly, and 0.7 mm towards right. Figure 2a shows the distribution of intrafraction baseline shifts for all patients and LNs at all fractions. Systematic LN baseline shifts occurred between CBCT and treatment delivery in the cranial direction (mean 2.4 mm (SD 1.9 mm)) and posterior direction (0.8 mm (1.1 mm)). The frequency of cranial baseline shifts exceeding 4 mm and 6 mm were 15 % and 4 %. The baseline shifts resulted in systematic mean geometrical errors during treatment delivery of 2.8 mm (cranial) and 1.4 mm (posterior)(Figure 2b) for the LNs. These errors were substantially larger than the sub- millimeter mean errors expected from the setup CBCT based soft tissue tumor match when correcting for the applied couch shifts. In general, the largest LN motion amplitude was observed in the cranio-caudal direction both during CBCT and treatment delivery. The mean motion amplitudes during CBCT and treatment delivery agreed within 0.2 mm in all three directions.

Conclusion In this work we have fitted the LKB model to clinical outcome on urethral strictures data for patients treated with HDRB collected at a single institution. The results show that the fitted model provides a good representation of the observed data, however further analysis and independent validation are necessary to confirm its behaviour and parameters.