ESTRO 36 Abstract Book

S456 ESTRO 36 _______________________________________________________________________________________________

beam-on time.

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

Results The mean equivalent doses in 2-Gy fractions to 40% (EQD40%) of the volumes of the bilateral hippocampus were for 9.90/5.31 Gy for coVMAT/VMDWAT, respectively. The EQD40% for VMWAT were < 7.3 Gy, which is the threshold predicting cognitive impairment, as defined by Gondi et al., and were significantly lower than those for coVMAT. The mean equivalent doses in 2-Gy fractions to 2, 10, 20, 30, 50, 80, 98 % (EQD2-98%) of the volumes of the bilateral hippocampus was also significantly lower than those of coVMAT. VMDWAT also significantly reduced the EQD40% and EQD2-98% of the left hippocampus. While the normal brain volume receiving 5 Gy (V5) was significantly larger in VMDWAT, as compared to coVMAT, the normal brain volume receiving 10, 15, 20, 25, 30, 35, 40, 45, and 50 Gy (V10–50) was significantly smaller in VMDWAT. The conformity and homogeneity indices were significantly better in VMDWAT. The mean treatment time of VMDWAT was significantly longer than that of to VMDWAT (67.1/70.1 seconds in coVMAT/VMDWAT, respectively).

Conclusion VMDWAT significantly reduced the doses to the bilateral and left hippocampus compared to coVMAT. The target conformity and homogeneity were significantly better in VMDWAT. Although the treatment time and V5 of the normal brain was increased in VMDWAT, V10–50 of the normal brain was significantly decreased in VMDWAT. VMDWAT could be a promising treatment technique for pituitary adenomas and craniopharyngiomas.