ESTRO 36 Abstract Book

S854 ESTRO 36 _______________________________________________________________________________________________

2 Az. Ospedaliero Universitaria di Modena, Radiation Oncology, Modena, Italy

(MU) objective value with the MaxMU dimensionless parameter. We aimed to minimize MU for prostate RA plans as a function of the MaxMU value. Material and Methods We retrospectively evaluated 40 prostate RA plans optimized with the PRO algorithm of Eclipse v.11. Prescribed doses were 57 Gy/ 59.5 Gy for PTV1 (including prostate and seminal vesicles), and 15 Gy/17.5 Gy for a simultaneous boost to the prostate only, for 30/35 fractions. For each patient, the original plan optimized without a MaxMU constraint ( base plan ) was reoptimized ( reoptimized plan) for six values of MaxMU: 1000, 800, 700, 600, 500 and 400. Differences in dosimetric parameters of PTV and OAR between the base and reoptimized plans were analyzed with a paired samples t- test. For PTVs the mean dose, D2%, D98%, V95% and (D5%- D95%) were evaluated. For all OAR (rectum, bladder and femoral heads) the mean dose was considered. Furthermore, in rectum, V50, V60, V70; in bladder, D67%, V30 and in femoral heads and bladder, D2%. Results For MaxMU ≤ 0.75×MU base_plan , the mean reduction of MU was of -2.3% (p<0.001). For MaxMU between 0.75×MU and MU, the mean reduction of MU was -1.3% (p=0.01). For MaxMU ≥ MU, the mean MU increased by 1.2% (p<0.001). Base plans with MU > 650 showed a MU mean reduction of -2.2% (p<0.001). The maximum mean decrease was obtained for MaxMU=500 (-3.7%; p<0.001). Base plans with MU < 650 showed a MU mean increase of 1.7% (p<0.001). However, for some plans the MU decreased (see graph). No clinically relevant differences were found for the analyzed dosimetric parameters.’

Purpose or Objective For Total Body Irradiation (TBI) very few experiences of dose calculation using Treatment Planning Systems (TPS) have been reported. The estimation of local dose inhomogeneity and sparing of critical organs is usually performed based spreadsheet calculation fine-tuned by in- vivo measurements. No reports exist for treatment planning with a TPS and dose accumulation based on deformable registration. We report the implementation of these techniques using RayStation ® commissioned to perform any type of treatment plan and to estimate actual dose distribution as delivered with a Volumetric Modulated Arc Therapy (VMAT)-based TBI technique. Material and Methods For TPS commissioning, the PTW ® water tank was positioned at 170cm from the LINAC (Elekta Synergy ® ). PDD, profile and output factors were acquired (8.5x8.5, 10x10, 17x17, 34x34, 51x51, 68x68, 68x8.5, 68x17, 68x34, 68x51 cm 2 ). The TBI plan was calculated using a total body CT in supine and prone position and the inverse planning and 3DCRT module available in the TPS. A dose of 600cGy in 6 fractions was prescribed to the midline of the patient outline as contoured in supine and prone position. A longitudinal VMAT-technique with 8 arcs [330°÷30° with step of 10°] was simulated using 6MV photons. The ANACONDA algorithm was applied to perform the elastic image registration (0.15cm of grid size) between the two CTs. The deformed vectors fields obtained were used to warp the dose grid allowing, the dose summation between the supine and the prone treatment. Results The relative weighting factors of the beams were experimentally obtained and confirmed by the calculation of the TSP; a specific MU/fx was set for each gantry angle into the range of [100÷191]MU/fx, based on the previously published VMAT-technique. For the supine and prone treatments, 607cGy and 544cGy were recorded at the respective prescription points in the separate treatment, respectively. The summation of both dose distributions witch dose warping using the SumDoses tool developed as described above resulted in an average dose of 1154cGy with quite uniform irradiation. Conclusion This study assessed the possibility to apply a dedicated TPS with hybrid algorithms to the novel VMAT-TBI technique. A real-time dosimetry was obtained simulating the patient treatment in both supine and prone setup and a cumulative dose was analyzed using deformation and a summation of the dose grids. With this planning approach, lung sparing can be performed as before with individualized lung blocks but this approach would also provide the possibility to directly modulate dose reductions over the lungs (provided appropriate patient positioning can be assured). After full commissioning of the TPS for extended SSD-conditions within the requirements of the TBI-delivery technique, an optimized comprehensive treatment planning approach would be available for this treatment paradigm. EP-1585 A practical method to reduce monitor units in prostate cancer RapidArc plans D. Sánchez-Artuñedo 1 , S. Jiménez-Puertas 1 , M. Sancho- Navarro 1 , M. Hermida-López 1 1 Hospital Universitari Vall d'Hebron, Servei de Física i Protecció Radiològica, Barcelona, Spain Purpose or Objective While optimizing a RapidArc (RA) plan with Eclipse Progressive Resolution Optimizer (PRO) algorithm (Varian Medical Systems), it is possible to select a monitor units

Conclusion Prostate cancer Rapid Arc plans may be divided in two groups based on the MU of the base plan. For plans with MU < 650, MU tend to increase if MaxMU parameter is used, but not in all cases (see graph). Due to this variability, we recommend to use this parameter and to compare the obtained plan with the base plan. For plans with MU > 650, a mean MU decrease was obtained being maximum for MaxMU=500. We recommend in this case to reoptimize using this value. The MU reduction was achieved without compromising plan quality. EP-1586 ART and VMAT–the benefits in bone marrow sparing for patients with bladder cancer M. Poncyljusz 1 , D. Blatkiewicz 1 , J. Chorazy 1 , B. Czyzew 1 , P.F. Kukolowicz 1 , M. Piziorska 1 1 The Maria Skłodowska-Curie Memorial Cancer Centre and Institute of Oncology, Department of Medical Physics, Warsaw, Poland