ESTRO 37 Abstract book

EP-2389 Clinical comparison between OSMS data and CBCT matched values for brain metastasis with SRS W. Vingerhoed 1 1 iridium cancer network, radiotherapy, Antwerpen, Belgium Purpose or Objective To evaluate the accuracy of a video-based optical surface imaging system for motion monitoring during stereotactic treatment of brain metastasis. Precise patient positioning and thus of the PTV is a prerequisite for e ff ective treatment with SRS for brain mets. The intra fraction motion should at least be within the CTV-PTV margin used. Conventional imaging modalities used to ensure exact positioning for treatment typically involve additional radiation exposure of the patient. Patient alignment and monitoring during treatment, without additional exposure, is provided by optical 3D surface scanning and registration systems. Typical SRS brain treatments with multiple couch angels limits the ability of CBCT verification during treatment. This paper aims to correlate the data obtained by the OSMS system with the internal shifts observed by the offline CBCT matches pre- and post treatment. Material and Methods Patients treated with stereotactic radiosurgery (SRS) and fractionated stereotactic radiotherapy (FSRT) were immobilized with a thermoplastic double shell open face mask and a SRS immobilisation system (Encompass ™ Immobilisation System and Encompass ™ SRS Fiberplast Q-fix). During treatment a video-based three-dimensional optical surface monitoring system was used to monitor the motion of a region of interest. This motion monitoring was done in 6 dimensions. A tolerance of 0.2 cm for linear directions and 0.5 degrees for rotational directions was set. If the optical surface monitoring system detected an exceeding of the set tolerance, treatment stopped automatically. If necessary, the physician decided to take a new CBCT. A total of 26 patients were followed for SRS or FSRT treatment between June 2017 and October 2017 with a total of 31 fractions evaluated for intra fractional uncertainties. Both CBCT and snapshots obtained with the OSMS were acquired at the start and stop of every treatment to compare both methods. In addition the average motion and SD during treatment was monitored to investigate the validity of pre- and post measurements for assessing intra fractional motion. Results A mean intra fractional shift of 0.02 cm in vertical (stand dev of 0.01), 0.03 cm in longitudinal (stand dev of 0.03), 0.02 in lateral direction (stand dev of 0.04). Only in one patient, set tolerance was exceeded and a new CBCT was taken which showed a lateral shift of 0.19 cm. delta Vrt delta Lng delta Lat delta Pitch delta Roll delta Rot

EP-2390 Temporal Changes Of Target Volumes and Oar Volumes During High Precision Radiotherapy D. Dhar 1 1 apollo Gleneagles Hospitals, Radiation Oncology, Kolkata, India Purpose or Objective Tumor shrinkage in response to radiotherapy, change in OAR (organ at risk) volume and weight loss may impact on the dose-distribution in both target & organ at risk (OAR) and hence the therapeutic outcome in patients with Head & Neck cancer. In this study, we investigated temporal changes of the target volumes & OAR volume during radiotherapy to adopt a strategy for adaptive radiotherapy. Material and Methods Twenty-six patients who underwent radical chemo- radiotherapy (CTRT) for head & neck squamous cell carcinoma between April’12-April’13 were prospectively analyzed. After immobilization, Computed Tomography (CT) scans were taken from base of the skull to sternal angle using 3mm slices by Siemens Somatom Emotion 16 slice CT scanner. Targets and OAR were contoured using Focal Sim. Treatment Planning was done on the XiO Treatment Planning System (ELEKTA AB, Sweden). All patients underwent 3D-CRT/IMRT. The prescription dose was 59.4 Gy-70Gy @ 1.8 -2Gy/fraction. All patients underwent a CT scan after 20 fractions. Targets and organs at risk were similarly outlined on these scans as above.The cranio-caudal dimensions of the planning target volume (PTV) were kept the same. Only the part of PTV going out of body contour (due to weight loss) was changed.Volume changes of targets and OARs were analyzed using SPSS 16. Results Comparison between both CT-scan (planning CT & CT at 20th fraction.) showed changes in GTV (mean24.56ccVs19.86cc p<0.001 , 95%CI 2.86-6.54), PTV (mean280.44ccVs255.1cc p=0.48, 95% CI -3.36-6.93), right Parotid (mean27.43ccVs22.25cc p<0.001 , 95%CI 3.44-6.90) and left Parotid (mean25.38cc Vs 20.97cc p<0.001 , 95% CI 2.81-5.99) volumes. Conclusion After 4-5 weeks of adjuvant chemo-radiotherapy, it is commonly found that there is reduction in body weight due to oral mucositis. As a result, there is a change in the patient’s contour and the relative positions of the GTV/PTV and OARs may change. In the present study, we have found significant changes in GTV volume (p<0.001), and also change in OAR volume, which in this case is the parotid glands (p<0.001 for both left and right parotids). On the other hand, the total PTV volume has not been found to change significantly (p=0.48), However, for some patients we have found the PTV volume to extrude outside the body contour. Though the PTV volume does not change appreciably, our findings point to the need for re-planning after repeat CT scan, since some of the OAR’s migrate near or away from the GTV due to change in both GTV as well as OAR volumes, and due to PTV extending beyond body. It is essential to re-plan to identify dosimetric changes and to ensure adequate doses to target volumes and safe doses to normal tissues. Volumetric changes in the target volume and OARs were observed could have potential dosimetric impact when highly conformal treatment techniques are used. It Suggest that adaptive strategies, where patients are re- imaged & possibly replanned during treatment, are worth evaluating

0.01 0.20 0.02 0.22 0.04 0.20 0.19 0.90

0.10 0.14 0.16 0.60

0.20 0.27 0.29 1.30

median 0.01 0.02 mean 0.02 0.03 St Dev 0.01 0.03

Max

0.05 0.10

Conclusion The CBCT data clearly shows that the intra fractional offset of all brain metastasis patients treated with SRS or FSRT was below the institution’s predefined threshold. The OSMS data obtained during treatment still needs a more detailed evaluation. For further analysis the approach was changed and real time data during treatment is now continuously triggered, obtained, mathematically analyzed and compared with the CBCT offline calculated offset.