ESTRO 37 Abstract book

S1059

ESTRO 37

Purpose or Objective To evaluate effects of inaccurate OF measurements on Fractioned Stereotactic Brain Radiotherapy (FSBRT) VMAT plans. Dependence on TPS, Linear Accelerators and treatment targets (volume and shape) were evaluated. Material and Methods Six centres were enrolled for a multi-institutional study. FSBRT VMAT plans were elaborated using four TPS (RayStation, Pinnacle, Monaco and Eclipse) and 6 MV photon beams. These TPS were coupled with two Elekta Beam Modulator, one Elekta MLCI2, and three Varian TrueBeam linear accelerators. The multi-institutional study was drawn on different phases. The first phase was oriented to establish a baseline for each center (dependent on TPS-Linac coupling and planner’s experience). For the purpose a CT data set of one patient underwent VMAT treatment for a single brain metastases was selected and anonymized for data sharing. The planning target volume (PTV) and organs at risks (OARs) were previously contoured by a single radiation oncologist. Planning rules (PTV coverage, OARs sparing), beams geometry, energy and calculation grid (1mm) were univocally fixed. DICOM_RT dose files, extracted by each TPS were imported into PlanIQ software (SUN NUCLEAR corporation) and analyzed. A quantitative total scorecard based on clinical goals (Quality Assurance goals) was used to measure the performance of each treatment plan. The second phase was oriented to evaluate the impact of inaccurate OF dose measurements on intra-center calculated dose. OFs for the smallest fields sizes used, was changed for known increments from the OF baseline values, treatments units re-commissioned in the TPS and plans re-calculated. The PlanIQ analysis were compared between the first and the second phase. Finally the dependence of OF values on TPS-Linac coupling was evaluated. Results PlanIQ analysis was performed identifying plan quality metric components and setting its goals (eg. at least 95% of PTV volume with 100% of the prescription dose, PTV Conformality Index, OARs sparing in terms of max dose or percentage of volume). A cumulative raw score for each center was obtained to evaluate the performance reached (Fig.1).

Protección Radiológica, Madrid, Spain 2 Hospital Universitario 12 de Octubre, Servicio de Oncología Radioterápica. Sección de Radiofísica., Madrid, Spain Purpose or Objective To compare left breast treatment plans with concomitant boost employing 3DCRT, 4-field IMRT, 7-field IMRT and VMAT. Material and Methods Treatment plans were created in Eclipse v.11 TPS (Varian Medical Systems, Palo Alto. California) using a 6 MV Varian Unique. AAA (Analytical Anisotropic Algorithm) algorithm was used. DVO v11.0.31 and PRO v11.0.31 optimizers were employed for IMRT and VMAT. A cohort of 10 patients wasselected. The dose prescription was 40.05 Gy for breast PTV and 4.5 Gy for boost PTV in 15 fractions. For each case four different plans were created, one for each planning technique used. For the first three techniques the two main tangential angulations were chosen to be the same. 3DCRT included one reduced segment for each field to avoid hot spots. 4F IMRT included two more oblique fields angled 30º from the tangential angulations. 7F IMRT included five more fields located in an equally spaced angular distribution between the tangentials. Two partial arcs were used for VMAT. Start and finish angles coincided with those utilized for tangentials. Homogeneity index (HI) and Paddick conformity factor (CF) were evaluated for breast and boost PTVs. HI was defined as (D 2% -D 98% )/ D 50% . CF was defined as V c 2 /(V·V 100 ) where V c stands for 100% isodose volume contained inside the PTV, V is the PTV volume and V 100 is the 100% isodose volume. For both lungs D avg , V 5Gy and V 20Gy were compared. Regarding the heart, V 23Gy, D max and D avg were considered. D max and D avg were obtained for the contralateral breast. A two-tailed t-test was carried out to elucidate whether the discrepancies obtained were significant. Results Results obtained for OARs and PTVs are shown in tables 1 and 2. VMAT diminished heart V 23 and D max (p<0.022 and p<0.013). Ipsilateral lung V 5 and D avg were reduced using 3DCRT (p<3.4·10 -4 ), the improvement seen in V 20 by using VMAT being insignificant. Contralateral lung V 5 can be reduced by using 3DCRT or 4F IMRT (p<1.5·10 -4 ). 3DCRT achieved by far the lowest D max and D avg (p<0.0015 and p<3.35·10 -7 ). The best values obtained for HI and CF were achieved for 7F IMRT (p<6.6·10 -4 and p< 1.2·10 -5 ). The boost PTV CF is significantly better for 7F IMRT and VMAT (p<0.011) with respect to 4F IMRT and 3DCRT.

Table 1: Results for OAR constraints evaluated for different techniques.

Conclusion All plans passed our fixed QA goals, with slight differences depending on TPS, machine and planner ’s experience. For the second phase, a preliminary study is under way with the aim of defining increment values of SF OFs (datasets from AIFM national studies with different detectors was taken as a reference to establish OF incremental values depending on field size). Moreover the recruitment of centers is under review for the study purposes. EP-1947 Dosimetric comparison of four left breast treatment modalities with concomitant boost. A. Prado 1 , A. Milanés 2 , M. Manzano 1 1 Hospital Universitario 12 de Octubre, Radiofísica y

Table 2: Results obtained for HI and CF for breast and boost PTVs.