No abstract

In IMRT, the method for a quantitative comparison of two-dimensional dose distributions is still under development. The gamma evaluation method proposed by Low et al is the most accepted approach and has been adapted by many groups. Based on the concept of Low et al we developed a software tool with an intelligent search algorithm to minimize the calculation time. For the interpretation of deviations a y angle distribution and other tools (dose difference map, profiles, y area histograms, etc) are integrated in the software package. Ten hybrid plans are included in the verification study containing 6 IMRT head and neck cases, 2 IMRT prostate cases and one IMRT paravertebral case as well as a standard uniform intensity conformal 4 field box treatment for comparison. IMRT plans are realized with a segmental MLC delivery technique. The fields of a hybrid plan are applied at once and dose distributions are measured with films in three planes of a verification phantom. All y vector calculations are based on a 3% dose criterion and a 3 mm DTA acceptance criterion. The mean value gamma(mean) (mean value in the y distribution) of the various IMRT plans is 0.45+/-0.10 (1 SD). On average, the percentage of points exceeding the acceptance criteria of gamma < or = 1 (gamma > 1) is 5.8+/-5.4% (1 SD). The mean value of gamma 1% (1% of points have an equal or higher gamma value) is 1.47+/-0.59 (1 SD) for IMRT plans. In 5 out of 27 planes, gamma > 1 is substantially larger than the average. This is also indicated in gamma area histograms. Planes with large areas outside the tolerance criteria were further evaluated using gamma angle distributions. This additional information indicates that the large areas with high gamma values are dominated by the dose difference. It is shown that the deviations are influenced by tongue and groove effects. From the statistical evaluation of gamma values (e.g. gamma area histogram), acceptance criteria for IMRT hybrid plans can be defined. For the interpretation of the gamma maps, distributions of the gamma angle and traditional evaluation methods, such as dose profiles, are still very useful.

No abstract

Experimental methods are commonly used for patient-specific intensity-modulated radiotherapy (IMRT) verification. The purpose of this study was to investigate the accuracy and performance of independent dose calculation software (denoted as 'MUV' (monitor unit verification)) for patient-specific quality assurance (QA). 52 patients receiving step-and-shoot IMRT were considered. IMRT plans were recalculated by the treatment planning systems (TPS) in a dedicated QA phantom, in which an experimental 1D and 2D verification (0.3 cm(3) ionization chamber; films) was performed. Additionally, an independent dose calculation was performed. The fluence-based algorithm of MUV accounts for collimator transmission, rounded leaf ends, tongue-and-groove effect, backscatter to the monitor chamber and scatter from the flattening filter. The dose calculation utilizes a pencil beam model based on a beam quality index. DICOM RT files from patient plans, exported from the TPS, were directly used as patient-specific input data in MUV. For composite IMRT plans, average deviations in the high dose region between ionization chamber measurements and point dose calculations performed with the TPS and MUV were 1.6 +/- 1.2% and 0.5 +/- 1.1% (1 S.D.). The dose deviations between MUV and TPS slightly depended on the distance from the isocentre position. For individual intensity-modulated beams (total 367), an average deviation of 1.1 +/- 2.9% was determined between calculations performed with the TPS and with MUV, with maximum deviations up to 14%. However, absolute dose deviations were mostly less than 3 cGy. Based on the current results, we aim to apply a confidence limit of 3% (with respect to the prescribed dose) or 6 cGy for routine IMRT verification. For off-axis points at distances larger than 5 cm and for low dose regions, we consider 5% dose deviation or 10 cGy acceptable. The time needed for an independent calculation compares very favourably with the net time for an experimental approach. The physical effects modelled in the dose calculation software MUV allow accurate dose calculations in individual verification points. Independent calculations may be used to replace experimental dose verification once the IMRT programme is mature.