S. Vieira

Fast and accurate leaf verification for dynamic multileaf collimation using an electronic portal imaging device.

A prerequisite for accurate dose delivery of IMRT profiles produced with dynamic multileaf collimation (DMLC) is highly accurate leaf positioning. In our institution, leaf verification for DMLC was initially done with film and ionization chamber. To overcome the limitations of these methods, a fast, accurate and two-dimensional method for daily leaf verification, using our CCD-camera based electronic portal imaging device (EPID), has been developed. This method is based on a flat field produced with a 0.5 cm wide sliding gap for each leaf pair. Deviations in gap widths are detected as deviations in gray scale value profiles derived from the EPID images, and not by directly assessing leaf positions in the images. Dedicated software was developed to reduce the noise level in the low signal images produced with the narrow gaps. The accuracy of this quality assurance procedure was tested by introducing known leaf position errors. It was shown that errors in leaf gap as small as 0.01-0.02 cm could be detected, which is certainly adequate to guarantee accurate dose delivery of DMLC treatments, even for strongly modulated beam profiles. Using this method, it was demonstrated that both short and long term reproducibility in leaf positioning were within 0.01 cm (1sigma) for all gantry angles, and that the effect of gravity was negligible.

Two-dimensional measurement of photon beam attenuation by the treatment couch and immobilization devices using an electronic portal imaging device

In our institution, an individualized dosimetric quality assurance protocol for intensity modulated radiotherapy (IMRT) is being implemented. This protocol includes dosimetric measurements with a fluoroscopic electronic portal imaging device (EPID) for all IMRT fields while the patient is being irradiated. For some of the first patients enrolled in this protocol, significant beam attenuation by (carbon fiber) components of the treatment couch was observed. To study this beam attenuation in two-dimensional, EPID images were also acquired in absence of the patient, both with and without treatment couch and immobilization devices, as positioned during treatment. For treatments of head and neck cancer patients with a 6 MV photon beam, attenuation of up to 15% was detected. These findings led to the development of new tools and procedures for planning and treatment delivery to avoid underdosages in the tumor.

Dosimetric verification of x-ray fields with steep dose gradients using an electronic portal imaging device.

Regions with steep dose gradients are often encountered in clinical x-ray beams, especially with the growing use of intensity modulated radiotherapy (IMRT). Such regions are present both at field edges and, for IMRT, in the vicinity of the projection of sensitive anatomical structures in the treatment field. Dose measurements in these regions are often difficult and labour intensive, while dose prediction may be inaccurate. A dedicated algorithm developed in our institution for conversion of pixel values, measured with a charged coupled device camera based fluoroscopic electronic portal imaging device (EPID), into absolute absorbed doses at the EPID plane has an accuracy of 1-2% for flat and smoothly modulated fields. However, in the current algorithm there is no mechanism to correct for the (short-range) differences in lateral electron transport between water and the metal plate with the fluorescent layer in the EPID. Moreover, lateral optical photon transport in the fluorescent layer is not taken into account. This results in large deviations (>10%) in the penumbra region of these fields. We have investigated the differences between dose profiles measured in water and with the EPID for small heavily peaked fields. A convolution kernel has been developed to empirically describe these differences. After applying the derived kernel to raw EPID images, a general agreement within 2% was obtained with the water measurements in the central region of the fields, and within 0.03 cm in the penumbra region. These results indicate that the EPID is well suited for accurate dosimetric verification of steep gradient x-ray fields.

Treatment planning with intensity modulated particle therapy for multiple targets in stage IV non-small cell lung cancer

Intensity modulated particle therapy (IMPT) can produce highly conformal plans, but is limited in advanced lung cancer patients with multiple lesions due to motion and planning complexity. A 4D IMPT optimization including all motion states was expanded to include multiple targets, where each target (isocenter) is designated to specific field(s). Furthermore, to achieve stereotactic treatment planning objectives, target and OAR weights plus objective doses were automatically iteratively adapted. Finally, 4D doses were calculated for different motion scenarios. The results from our algorithm were compared to clinical stereotactic body radiation treatment (SBRT) plans. The study included eight patients with 24 lesions in total. Intended dose regimen for SBRT was 24 Gy in one fraction, but lower fractionated doses had to be delivered in three cases due to OAR constraints or failed plan quality assurance. The resulting IMPT treatment plans had no significant difference in target coverage compared to SBRT treatment plans. Average maximum point dose and dose to specific volume in OARs were on average 65% and 22% smaller with IMPT. IMPT could also deliver 24 Gy in one fraction in a patient where SBRT was limited due to the OAR vicinity. The developed algorithm shows the potential of IMPT in treatment of multiple moving targets in a complex geometry.

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.

SU-E-T-573: The Robustness of a Combined Margin Recipe for Uncertainties During Radiotherapy

PURPOSE To investigate the variability of a safety margin recipe that combines CTV and PTV margins quadratically, with several tumor, treatment, and user related factors. METHODS Margin recipes were calculated by monte-carlo simulations in 5 steps. 1. A spherical tumor with or without isotropic microscopic was irradiated with a 5 field dose plan2. PTV: Geometric uncertainties were introduced using systematic (Sgeo) and random (sgeo) standard deviations. CTV: Microscopic disease distribution was modelled by semi-gaussian (Smicro) with varying number of islets (Ni)3. For a specific uncertainty set (Sgeo, sgeo, Smicro(Ni)), margins were varied until pre-defined decrease in TCP or dose coverage was fulfilled. 4. First, margin recipes were calculated for each of the three uncertainties separately. CTV and PTV recipes were then combined quadratically to yield a final recipe M(Sgeo, sgeo, Smicro(Ni)).5. The final M was verified by simultaneous simulations of the uncertainties.Now, M has been calculated for various changing parameters like margin criteria, penumbra steepness, islet radio-sensitivity, dose conformity, and number of fractions. We subsequently investigated A: whether the combined recipe still holds in all these situations, and B: what the margin variation was in all these cases. RESULTS We found that the accuracy of the combined margin recipes remains on average within 1mm for all situations, confirming the correctness of the quadratic addition. Depending on the specific parameter, margin factors could change such that margins change over 50%. Especially margin recipes based on TCP-criteria are more sensitive to more parameters than those based on purely geometric Dmin-criteria. Interestingly, measures taken to minimize treatment field sizes (by e.g. optimizing dose conformity) are counteracted by the requirement of larger margins to get the same tumor coverage. CONCLUSION Margin recipes combining geometric and microscopic uncertainties quadratically are accurate under varying circumstances. However margins can change up to 50% for different situations.

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.