Abstract Book

S966

ESTRO 37

Material and Methods COMPASS QA system, with the MatriXX ionization chamber detector array (IBA Dosimetry) was implemented for pre- treatment QA of VMAT plans optimized with Monaco TPS and delivered by an Elekta Synergy LINAC. Open fields on homogeneous (RW3) and inhomogeneous (Gammex) phantoms were tested to verify the discrepancies of beam modelling and the differences between the dose calculation algorithms of COMPASS and Monaco (Figure 1). Then, as proposed by AAPM TG119, local CL on the 3%,3mm gamma passing rate (γ PR ) for VMAT QA were established using TG119 test plans implemented on a cylindrical phantom, as previously done for VMAT QA on a Delta4 phantom. Mean dose (D mean ) to the PTVs and to the OARs were also compared, and local CL on the dose differences were set. The TPS dose distribution was compared with the dose distribution calculated in COMPASS on the patient/phantom anatomy from the RT- PLAN (CC) and with the dose reconstructed from measurements performed with MatriXX (CR). CC was also compared with CR (Figure 2). CL were finally reviewed by analysing QA results of 41 H&N and 10 prostate VMAT plans.

has differences up to 3% between CC and CR dose distributions. QA of H&N and prostate plans confirmed that ±2% can be set as CL when evaluating ΔD mean (Fig.2). The average dose to the main OARs (parotids for H&N, bladder and rectum for prostate plans) had higher variability among TPS, CC and CR distributions, suggesting to set the CL at ±5%.

Conclusion COMPASS was implemented in pre-treatment verification of VMAT plans and local confidence limits based both on gamma PR and on mean dose differences in the PTV and main OARs, calculated on the patient anatomy, can be set for clinical routine QA. EP-1799 Determination of the angular dependence of a Pin Point ion chamber. A. Prado 1 1 Hospital Universitario 12 de Octubre, Radiofísica y Protección Radiológica, Madrid, Spain Purpose or Objective Due to the specific geometry and design of each type of detector, its response when exposed to a radiation beam might vary with the angle of incidence. In the present work this angular dependence is quantified for a Pin Point 31014 ion chamber (PTW). Material and Methods Measurements were performed on a 6 MV Varian Unique. The ion chamber used (PTW Pin Point 31014) along with a PTW UNIDOS electrometer were utilized. A home-made phantom made of high density polyethylene with a cavity adapted to the size of the ion chamber used was employed (fig. 1). This cavity was constructed so as to make the center of the phantom coincide with the reference point of the chamber. The phantom is comprised of a half sphere joined to a cylindrical region. The center of the phantom was located at 10 cm depth from the spherical region surface.

Results For all the open fields in the RW3 phantom, the |mean|+1.5SD per cent dose difference between reference TPS and CC (CR) in X and Y profiles at 10cm depth were within 2% (3%), 16% (25%) and 60% (60%) respectively for inner, penumbra and outer beam regions (Fig.1B). Repeated measurements showed that fields smaller than 5x5cm 2 appear to be more sensitive to MatriXX positioning, thus influencing CR distribution. In Gammex phantom, there was a maximum difference of 3.5% in bone tissue and 1.7% in lung tissue, whereas the differences for densities near water were within ±1.5% (Fig.1A). In the AAPM-TG119 test plans, the CLγ PR (TPS vs.CC) was 99%, while it was 98% for clinical plans since patient heterogeneities lowered the agreement in very high/low density regions. CLγ PR (CC vs.CR) was 99% for the QA of both test and clinical plans (Fig. 2). Regarding DVH comparison, the discrepancies between the average dose to the PTVs (ΔD mean ) of AAPM-TG119 plans were within ±2% for all the test plans, except for the multitarget plan, in which ΔD mean to the low-dose PTV

Figure 1: Home-made phantom used with a cavity adapted to the Pin Point ion chamber size. The phantom center was positioned at the unit isocenter. The phantom spherical region pointed upwards, being the chamber perpendicular to the couch plane. In this way, it was possible to rotate the gantry between 90º and 270º without moving the couch. In each measurement 200 MU were delivered using a 600 MU/min dose rate and a 4x4 cm 2 field size. Gantry angles