ESTRO 36 Abstract Book

S452 ESTRO 36 2017 _______________________________________________________________________________________________

Material and Methods Twenty patients, ten right-sided and ten left-sided, were selected by including all patients receiving partial breast radiotherapy between the 1/6-2016 and the 19/9-2016 at our institution. All patients were treated with forward planned tangential step-and-shoot 6MV fields and with 18MV fields used partly for larger breasts. The ten left- sided patients were treated in breath hold using ABC from Elekta. For each patient an additional plan was generated, using two small (30-40 degrees) 6 MV FFF VMAT fields with tangential like beam angels. Dose planning was done in Pinnacle 9.10 and the Auto-Planning module was used for generation of the VMAT plans. Mean doses to target regions and organs and risk were compared using paired t- tests. Results VMAT plans were generated fast with a median time for complete plan generation by Auto-Plan of 10,5 min (range: 9 min – 12 min) with further adjustments needed for 7/20 patients (5 min -15 min additional time). Mean doses to target regions and organs at risk are shown in the table. The doses were similar from both plans except for the dose to the ipsilateral lung being statistically significant lower from the VMAT plans. Dose volume histograms for the ipsilateral lung and the PTV are shown in figure a) and b) respectively. As shown, the dose to ipsilateral lung was lower for all dose levels in the VMAT plans even though the coverage of the PTV was better. The measured delivery time of all VMAT fields were 14,5 s (range: 10 s - 22 s). As a result all VMAT plans could potentially be delivered within two breath holds (our threshold for maximum breath hold duration is 25 s). In comparison the median number of breath holds required for the ten left- sided patients treated in breath hold in the forward planned treatment was 4 (range: 2 – 7).

PO-0844 Dosimetric Evaluation of MLC and Fixed Cone for Patients in the Prone Position with CyberKnife S.K. Ahn 1 , J.H. Cho 1 , K.C. Keum 1 1 Yonsei Cancer Center- Yonsei University, Department of Radiation Oncology- Yonsei Cancer Center- Yonsei University Health System- Seoul- Korea, Seoul, Korea Republic of Purpose or Objective The constraints of systems using fixed cones have been improved with the recent introduction of the multileaf collimator (MLC) to the CyberKnife® system. This study evaluated the dosimetric impact of the MLC in stereotactic body radiation therapy for spine lesions, with the patient Sixteen patients with spinal tumors, who were treated with CyberKnife® M6 TM , were placed in a body fixer and scanned with four-dimensional computed tomography (4DCT). A total of 32 treatment plans were set up with two fixed cones (ray tracing algorithm) and MLC (finite-sized pencil- beam algorithm), using the MultiPlan® System. A total of 24 Gy in four fractions was prescribed to the 78%–83% isodose line, encompassing at least 95% of the planning target volume (PTV). The XSight® prone tracking method was used for target tracking, and the Synchrony® Respiratory Tracking System was used for motion tracking. For the PTV, the maximum dose, homogeneity index (HI), and conformity index (CI) were analyzed. For the spinal cord and bowel, the maximum dose (D 0.03 cc) was analyzed. The other analyzed parameters included monitor unit, treatment time, beam number, and node number. Results Regardless of the type of collimator, the difference among the maximum dose, HI, and CI values of the PTV was 3.1±2%, while the maximum dose of the spinal cord and bowel was 9.7±4.5%, indicating clinically insignificant differences. For the other parameters, the values of the treatment plan using MLC were lower by 53.8±8.4% for MU, by 39.5±7.5% for treatment time, by 49.3±7.3% for beam number, and by 49.7±7.1% for node number, compared to the use of fixed cones. The differences were larger when the tumors were greater in size. Conclusion There are dosimetric advantages to evaluating patients in the prone position for lesions that are anatomically located in the back, such as spinal tumors. However, MLC, which has fewer treatment nodes and a shorter treatment time, is also useful in the prone position because the maintenance of positional reproducibility is critical. PO-0845 Automatic treatment planning of FFF VMAT for breast cancer: fast planning and fast treatment E.L. Lorenzen 1 , K.L. Gottlieb 1 , C.R. Hansen 1 , H.R. Jensen 1 , J.D. Jensen 2 , M.H. Nielsen 2 , M. Ewertz 2 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 2 Odense University Hospital, Department of Oncology, Odense, Denmark Purpose or Objective Forward planned tangential radiotherapy with wedges or few segments is the standard technique in many centres for radiotherapy after breast conserving surgery. Helical techniques such as Thomotherapy and VMAT can be used to increase conformity but may increase the volume receiving low doses and the treatment planning can be time-consuming. In the present study we evaluate FFF VMAT using automated planning by comparison with manually planned tangential radiotherapy on its plan quality as well as its efficiency in both treatment planning and delivery. in the prone position. Material and Methods