Abstract Book

S1075

ESTRO 37

EP-1976 An automated tool for perpendicular beam incidence in electron radiation therapy H. Afsharpour 1 , M. Desbiens 2 1 Peel regional cancer center, Radiation Oncology, Mississauga, Canada 2 CISSS Monteregie-Centre, Radio-Oncologie, Greenfield Park, Canada Purpose or Objective The purpose of this work is to introduce an automatic method for determining the required gantry and couch rotations for a perpendicular beam incidence on patient’s surface for electron radiation therapy. Introduction Electron beam radiation therapy is an efficient method for eradicating tumours near the skin. This technique is gaining popularity because of the widespread availability of linear accelerators in expense of orthovoltage photon beams. Unless otherwise stated, a basic requirement for an electron beam is to have a perpendicular incidence on the surface to reproduce, as much as possible, the commissioning data measurement conditions (water tank). But not all cases could be handled by rotating the gantry only and usually a couch rotation is also required. Thus, one of the challenges for planners is to find out the good combination of gantry and couch angle to produce a perpendicular beam incidence on patient’s surface being the skin, a bolus or any other equipment. Currently, planners can only rely on the state-of-the-art 3D visualization tools in the TPS for this endeavor which is time-consuming and not accurate. The purpose of this work is to provide the planners with an automatic tool (a script) to determine the proper gantry and couch rotation angles to achieve a perpendicular incidence on the skin, quickly and accurately. Material and Methods Two major treatment planning systems namely Pinnacle 3 (Philips) and Eclipse (Varian) were considered in this study. Each patient is scanned and the scans are sent to the TPS. A physician contours the CTV/PTV to define the extent and the depth of treatment. A planner will then place the isocenter at the center of PTV projected to the skin’s surface. Next step is to define four points at the surface vis-à-vis the four extremities of the PTV being the superior, inferior, right and left directions. The coordinates of those points are then input to the script which will immediately give instructions for gantry and couch rotations for achieving a perpendicular beam incidence. Results Two cases are reported as examples for the usefulness of this macro in our clinical routine. First case was a right breast boost with a surgical cavity placed medially (Fig. 1). The second case was a scalp skin lesion (Fig. 2) with a non-standard patient setup (prone with face turned towards left). Using the 3D visualization tool, it took about 10 minutes for a planner to finalize the beam setup. However, this tool allowed us to achieve an accurate beam setup within a minute.

Figure 2; A scalp skin cancer treatment. The position of the lesion and the initial setup of the patient made it particularly challenging to find the good gantry and couch rotation angles using the 3D visualization tool. Conclusion This tool has proved to be an efficient way to produce fast and accurate perpendicular beam arrangements for electrons patients. It has shown great usefulness for challenging patient anatomies. EP-1977 Comparative study of Diffusion and T2- Weighted MRI for delineation and planning-Cervix brachytherapy R. Kumar 1 , G. Narayanan 2 1 Vydehi Institute of Medical Sciences, Radiation Oncology, gurgaon, India 2 Vydehi Institute of Medical Sciences, Radiation Oncology, Bangalore, India Purpose or Objective Brachytherapy (BT) plays a crucial role in the management of invasive cervix cancer from stages I to IV. GEC ESTRO recommends the use image-based brachytherapy as it has resulted in better local control and organ sparing . The use of T2W MRI is the gold standard for image-guided adaptive brachytherapy but few studies have shown uncertainties, poor interobserver variabilities and low conformity index for target delineation. Functional imaging with DWI can improve cervical tumor distinction as it is more sensitive than T2W MRI even in detecting parametrial invasion. Addition of DWI as a supplement to T2W imaging has also decreased the interobserver variability. Hence we intended to compare target delineation using these two imaging modalities. Material and Methods A total of 50 patients with squamous cell carcinoma who were suitable for intracavitary brachytherapy were included for the study from Jan 2015 to june 2016. Patients were subjected to MRI based brachytherapy post EBRT using GEC ESTRO working group guidelines. Along with a T2W sequence a diffusion weighted sequence was also taken using b values 0, 400, and 800. After importing the images to brachy vision software by varian, GTV, HRCTV and IRCTV were contoured on both T2W MRI and DWI along with bladder rectum and sigmoid. D2cc for all the organs at risk (OAR) and HRCTV D90, D100 were recorded after generating 2 plans, one for each imaging modality. Treatment was given using the plan generated from T2W images Results GTVBT, HRCTV and IRCTV contours on DWI were different when compared with T2W, mean GTVBT on T2W and DWI was 5.25 and 5.23 respectively (p value 0.8). Mean HRCTV on T2W and DWI was 28.3 and 27cc respectively (p value .003) with Mean HRCTV on DWI being significantly lower. Mean IRCTV on T2W and DWI was 68.1 and 65cc respectively with mean IRCTV on DWI being significantly lower (p value .003). planning on the above volumes resulted in a superior coverage in terms of HRCTV D90 and D100 for DWI based plan, HRCTV D90 – 735.1 and 741cGy for T2W and DWI respectively (p value .006), HRCTV D100- 441.05 and 444.5 for T2W and DWI plans

Figure 1; A breast boost plan with a superior-medial surgical bed.