Abstract Book

S964

ESTRO 37

Purpose or Objective To obtain S cp

Purpose or Objective For in-vivo dose verification, a back-projection (BP) model was applied on measured images using an electronic portal imaging device (EPID), which was positioned behind the patient. This model was calibrated for each combination of photon beam energy, linear accelerator (linac) and EPID. In this study, we compared the full commissioning method (FCM) and a novel template based commissioning method (TBCM). With the FCM, the complete parameter set of the BP model was created for each EPID. For the TBCM, a template parameter set (TCM) with non-EPID specific parameters was used and only EPID specific parameters were newly created. The commissioning workload per model was 2 h for FCM and 10 min for TBCM. Material and Methods We found our BP model parameters using an iterative fitting procedure, where EPID reconstructed dose distributions were fitted to ionization chamber (IC) values. A novel photon beam specific TCM was established with a fitting that uses an improved BP algorithm, including dose rate correction, and an input of a large pool of data from 4 different EPIDs. Previously, a TCM was created with the non-EPID specific parameters of only one FCM. We created 12 TBCMs and 12 FCMs for 6 EPIDs attached to 6 SL20i linacs with 6 and 10 MV beams. The model fits were assessed with the percentage dose difference between reconstructed and IC dose at the isocenter (ΔD isoc (%)) for a series of square fields from 3x3 to 23x23 cm 2 on a phantom. Furthermore, TBCMs and FCMs were used to reconstruct dose for 134 clinical treatment plans consisting of EPID data of 782 IMRT beams and 533 VMAT arcs. Planned and reconstructed dose were compared with the dose difference at the dose reconstruction point (ΔD DRP (%)) and the gamma (γ) evaluation parameters with (3 %, 3 mm): γ pass rate (%γ < 1), γ mean and the maximum 1% γ. Results The largest difference in the mean ΔD isoc (%)(TBCM – FCM) for the field size series was 1.06% (6 MV series B4a). ΔD isoc (%)(TBCM – FCM) was positive (compare Fig 1a with b, and c with d), since a dose rate correction was used in the BP algorithm for the fit of the TCM and not for the FCM. Also, the σ of the TBCM series was equal or a little more than for the FCM series .

values for a 5 mm diameter cone using a scanning chamber method (SCM) utilizing different number of scanning positions. Material and Methods Measurements were performed on a 6MV Varian Unique to which a 5 mm diameter cone was attached. A PTW UNIDOS electrometer along with a Pin Point 31014 ion chamber were employed. A Blue Phantom 2 (Iba Dosimetry) water tank of 50 x 50 x 50 cm 3 was utilized. A stereotactic diode (Iba Dosimetry) was used to obtain the 5 mm cone profile, essential to perform this methodology. The SCM is based on the superposition principle, according to which a beam can be expressed as a sum of smaller beams. This multiple beam configuration along with a static detector position can be shown to be equivalent to a configuration in which the beam is still and the detector changes its position. A grid of measurement points centered in the cone field was created, varying the number of points in the grid and the distances between them. Hence, the total dose from the multiple beam field is expressed as: axis beam dose, D(r,d max ) is the central axis dose and r is the cone field size. Measurements were performed using different beam number (NB) and distinct neighboring beam distances (NBD). The sum of all field contributions is D tot . The f(x i ) factor for each detector position is the value of the measured profile normalized to unity at distance x i of its maximum. D(r,d max ) was computed for each case through equation (1). S cp is defined as the ratio of D(r,d max ) to D(10,d max ), which was measured with the same Pin Point ion chamber. In addition, an S cp value was obtained measuring D(r,d max ) and D(10,d max ) directly and calculating the ratio between them so as to compare it with the SCM value. Results Table 1 shows S cp values obtained for each configuration. Table 1: Results obtained for different configurations of number of beams and neighboring distance. S cp results are consistent from 441 to 9 beams (discrepancies with respect to the average value lower than 1%). For 5 beam configuration this difference is greater (6.3%). This suggests that a further reduction on the number of beams could yield a higher value and, in consequence, a worst result for the S cp . The average value compared with the direct measurement value (0.509) entails a 22% increase. Conclusion The SCM diminishes the influence of volume averaging effect on the final S cp result. This can be seen in the 22% increase achieved with respect to direct measurement value. Obtained results agree quite well with data published in the literature, so they are considered as satisfactory. Although the SCM could be a bit tedious to perform, it clearly provides an excellent result for the S cp value of such a narrow beam. EP-1796 Template-based commissioning model for in vivo EPID transit dosimetry K. Landheer 1 , R. Van Oers 1 , I. Olaciregui-Ruiz 1 , A. Mans 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands where N is the number of beams, f(x i ) is the ratio of the dose at distance x i from the beam center to the central