The aim of this study was to develop a deformable image registration (DIR)-based offline ART protocol capable of identifying significant dosimetric changes in the first treatment fractions to determine when adaptive replanning is needed. A total of 240 images (24 planning CT (pCT) and 216 kilovoltage cone-beam CT (CBCT)) were prospectively acquired from 24 patients with prostate adenocarcinoma during the first three weeks of their treatment (76 Gy in 38 fractions). This set of images was used to plan a hypofractionated virtual treatment (57.3 Gy in 15 fractions); correlation with the DIR of pCT and each CBCT allowed to translate planned doses to each CBCT, and finally mapped back to the pCT to compare with those actually administered. In 37.5% of patients, doses administered in 50% of the rectum (D50) would have exceeded the dose limitation to 50% of the rectum (R50). We first observed a significant variation of the planned rectal volume in the CBCTs of fractions 1, 3, and 5. Then, we found a significant relationship between the D50 accumulated in fractions 1, 3, and 5 and the lack of compliance with the R50. Finally, we found that a D50 variation rate [100 × (administered D50 − planned D50/planned D50)] > 1% in fraction three can reliably identify variations in administered doses that will lead to exceeding rectal dose constraint.
Introduction. Several medical physicists have moved to a statistical process control-oriented approach to learn more about the intrinsic performance of radiotherapy equipment. Our aim is to report how gauge repeatability and reproducibility (R&R) and sensitivity and specificity studies can provide optimal specifications for radiotherapy-beam calibration checks. Methods. A team of three medical physicists performed gauge R&R studies on 6 megavolt (MV) photon, and 4, 9, and 15-megaelectron volt electron-beams. The operator order was randomized to reduce time-related biases. Checks were performed accurately using ionization chambers with a water phantom. These gauge R&R studies allowed us to determine the proportion of variation due to the repeatability, operator, and beam-tobeam components associated with repeated beam-calibration checking. We then calculated the conditional probabilities of misclassifying the check results as a false accept (β=1-sensitivity) and a false reject (δ=1-specificity) by changing the beam calibration specifications. Using this information we plotted the receiver operating characteristic curves in order to identify the sensitivity and specificity settings that maximize detection. Results. The main component of variation was due to the operator; the repeatability component was less than 30%. The intrinsic variation component for 6 MV photonbeams was approximately 20%, whereas it was over 34% for electron-beams. The optimal specification bands expressed as a percentage of the mean were ±0.7% for 6 MV photons and ±0.9% for electron-beams. Optimal sensitivity was over 50%, whereas specificity was not less than 0.995. The very low δ indicates that the system is unlikely to reject a correct beam calibration. However, the high β shows that should we require high levels of precision and therefore adopt such optimized specifications, the probability of accepting a drifted beam calibration would be fairly high. Therefore, the combination of 'medical physicist, water phantom, and ionization chamber' is an excellent tool for correctly adjusting beam calibrations to their specifications; moreover, this procedure could also be further improved in order to detect drifts in beam calibrations if the specifications required were to tighten in the future. Conclusion. Calculating the optimal specifications is feasible and leads to a very high detection specificity, but with low sensitivity. The operator-associated variation component contributed the most to the overall variation in check results. The beam calibration procedure might need to be adapted if the demand for precision in radiotherapy increases.
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