C. Greco

PO-0918: Validation of freeware-based mid-ventilation CT calculation for upper abdominal cancer patients

ESTRO 35 2016 ______________________________________________________________________________________________________ ca. patients treated in our institution. First, pattern statistics were compared to population data in literature to establish validity of the data used for testing. Second, patterns representing highest irregularity were selected: variance in amplitude (1), periodicity (2), and a pattern with a baseline drift (3). A periodical computer generated sinusoid (4) was used for comparison. Patterns were fed into a QUASARTM Respiratory Motion Phantom (Modus Medical), with “lung tumour insert” (cork/polystyrene). Each pattern was scanned 5 times using a 16 slice lightspeed RT series scanner (General Electric). “Lung tumour” contours were extracted using auto segmentation of average (AVE) and MIP CT data. Contour volumes were compared using Dice coefficients (DC) and to expected volumes.

PO-0927: Minimizing intra-fractional prostate motion during hypofractionated radiotherapy using beacon transponders

benefit, whereas gating, real tracking and MidV principle showed inferior advantage. The dose benefit showed statistical significance (p<0.05) for 9 patients in the case of ideal tracking, 8 patients for real tracking, 6 for gating and 5 for MidV. Dose benefit was also organ-specifically averaged and mean values, standard deviations and p-values are shown in Tab. 1 for spinal cord, liver, ipsilateral kidney, ipsilateral lung and stomach.

EP-1320: VMAT versus fixed-angle IMRT for IMRT breast planning

planar dose measurements made normal to the beam (MapCheck2 in mounting frame (IMF)) together with integrated measurements made in a MapPhan phantom on the treatment couch. Both sets of QA had been analysed using a gamma index approach (2%/2mm/10%). Plans were selected for further analysis where either a result from the MapPhan measurement had a significantly lower pass rate than the normal plane average (3 plans), or the pass rate averaged over all normal planes was less than 97% (8 plans). Careful measurements were made of the position of a square radiation field (a) on the MapCheck2 in the IMF and (b) at the isocentre, together with radiation output at the isocentre as the gantry angle was varied. A correction was applied to the MapCheck2 measured dose planes based on the above measurements. The corrected dose planes were then used within 3DVH (Sun Nuclear, USA) to apply a perturbation to the planned dose distribution both in the patient and in the MapPhan QA plan (for validation of the methodology by comparison with MapCheck2 measurements). A comparison of the 3D dose distributions generated by 3DVH on the patient dataset using measured dose planes that had been (a) corrected and (b) not corrected for the linac performance characteristics. Results: Measured geometrical corrections to the dose planes were all less than 2mm, and radiation output corrections were less than 1%. Validation of the methodology by comparing MapCheck2-measured dose planes with 3DVH-perturbed planned dose planes with and without corrections for linac performance characteristics, showed that the gamma analysis pass rate increased when linac performance characteristics were taken into account. In patient dose distributions the differences caused by taking into account the radiation isocentre and variation in output with gantry angle were small. Looking at DVHs of organs-at-risk, the differences in all cases were very small. However for CTVs and PTVs, dose coverage was reduced by taking into account these performances characteristics. The D99% and 95% were reduced on average by1% (0.3-2.4%) and 0.7% (0.4-1.2%) respectively. Conclusions: An analysis of IMRT QA results has been confirmed that neither planar dose measurements nor integrated measurements in a phantom are a good predictor of the affect on the patient distribution. However it has been shown that it is necessary to take additional linac performance characteristics into account when making a patient dose distribution analysis.

PO-0790: DVH measurements for VMAT

area. Different analysis techniques have been proposed to increase the accuracy of radiochromic films dose distribution measurements. The aim of this work is to compare the results obtained whit different analysis techniques in assessing dose distribution for IMRT photon beams pre-treatment verification. Materials and Methods: Gafchromic®EBT3 films have been calibrated irradiating 5x5 cm film pieces with a 6 MV linac photon beam at different dose levels in a range from 10 to 400 cGy at 5cm depth in PMMA phantom and SSD 95 cm. Then 40 IMRT clinical beams have been verified by gafchromic films with the same irradiation setup. Films have been scanned with a Epson 10000XL flatbed scanner 24 hours after irradiation and dose distributions have been assessed using an home-made software. Our software allows to perform analysis in 4 different ways: red channel (R) analysis, red channel analysis with the correction for the scanner non-uniformities (RC), the red/blue channels (RB) analysis and the 3 channel (RGB) analysis using formulas proposed by Mayer (Med. Phys. 2012). The films absolute dose distributions obtained have been compared with the calculated ones by means of 3%(local)/3mm gamma analysis. Results: Gamma analysis pass rates obtained with RGB analysis (98.0±2.7) are higher than pass rates obtained with all the other analysis approaches, while the lowest mean pass rate (88.9±13.3) has been obtained, as is was expected, evaluating the dose distribution using the R analysis. Comparing RB and RC techniques, the last one provide better results (96.5 ± 3.4 vs 94.1 ± 7.2). Moreover standard deviations of mean values are inversely proportional to gamma pass rates meaning that methods giving higher pass rates are also more consistent. Conclusions: The newly proposed three channels analysis allows to take in account different source of inaccuracy increasing the gafchromic films capability to measure IMRT dose distributions.