The main objectives of this study are to (1) analyze the sensitivity of various gamma index passing rates using different types of detectors having different resolutions and (2) investigate the sensitivity of various gamma criteria in intensity‐modulated radiation therapy (IMRT) and volumetrically modulated arc therapy (VMAT) quality assurance (QA) for the detection of systematic multileaf collimator (MLC) errors using an electronic portal imaging device (EPID) and planar (MapCheck2) and cylindrical (ArcCheck) diode arrays. We also evaluated whether the correlation between the gamma passing rate (%GP) and the percentage dose error (%DE) of the dose–volume histogram (DVH) metrics was affected by the finite spatial resolution of the array detectors. We deliberately simulated systematic MLC errors of 0.25 mm, 0.50 mm, 0.75 mm, and 1 mm in five clinical nasopharyngeal carcinoma cases, thus creating 40 plans with systematic MLC errors. All measurements were analyzed field by field using gamma criteria of 3%/3 mm, 3%/2 mm, 3%/1 mm, and 2%/2 mm, with a passing rate of 90% applied as the action level. Our results showed that 3%/1 mm is the most sensitive criterion for the detection of systematic MLC errors when using EPID, with the steepest slope from the best‐fit line and an area under the receiver operating characteristic (ROC) curve >0.95. With respect to the 3%/1 mm criterion, a strong correlation between %GP and %DE of the DVH metrics was observed only when using the EPID. However, with respect to the same criteria, a 0.75 mm systematic MLC error can go undetected when using MapCheck2 and ArcCheck, with an area under the ROC curve <0.75. Furthermore, a lack of correlation between %GP and %DE of the DVH metrics was observed in MapCheck2 and ArcCheck. In conclusion, low‐spatial resolution detectors can affect the results of a per‐field gamma analysis and render the analysis unable to accurately separate erroneous and non‐erroneous plans. Meeting these new sensitive criteria is expected to ensure clinically acceptable dose errors.

Stereotactic radiosurgery requires sub-millimetre accuracy in patient positioning and target localization. Therefore, verification of the linear accelerator (linac) isocentre and the laser alignment to the isocentre is performed in some clinics prior to the treatment using the Winston-Lutz (W-L) test with films and more recently with images obtained using the electronic portal imaging devices (EPID). The W-L test is performed by acquiring EPID images of a radio-opaque ball of 6 mm diameter (the W-L phantom) placed at the isocentre of the linac at various gantry and table angles, with a predefined small square or circular radiation beam. In this study, the W-L test was performed on two linacs having EPIDs of different size and resolution, viz, a TrueBeam™ linac with aS1000 EPID of size 40 × 30 cm(2) with 1024 × 768 pixel resolution and an EDGE™ linac having an EPID of size 43 × 43 cm(2) with pixel resolution of 1280 × 1280. In order to determine the displacement of the radio-opaque ball centre from the radiation beam centre of the W-L test, an in-house MATLAB™ image processing code was developed using morphological operations. The displacement in radiation beam centre at each gantry and couch position was obtained by determining the distance between the radiation field centre and the radio-opaque ball centre for every image. Since the MATLAB code was based on image processing that was dependent on the image contrast and resolution, the W-L test was also compared for images obtained with different beam energies. The W-L tests were performed for 6 and 8 MV beams on the TrueBeam™ linac and for 2.5 and 6 MV beams on the EDGE™ linac with a higher resolution EPID. It was observed that the images obtained with the EPID of higher resolution resulted in same accuracy in the determination of the displacement between the centres of the radio-opaque ball and the radiation beam, and significant difference was not observed with images acquired with different energies. It is concluded that the software based on morphological operations provided an accurate estimation of the displacement of the ball centre from the radiation beam center.

Aims: To evaluate the efficacy of the deep inspirational breath-hold (DIBH) technique and its dosimetric advantages over the free breathing (FB) technique in cardiac (heart and left anterior descending artery [LAD]) and ipsilateral lung sparing in left-sided post-mastectomy field-in-field conformal radiotherapy. DIBH is highly reproducible, and this study aims to find out its dosimetric benefits over FB. Materials and Methods: Nineteen left-sided mastectomy patients were immobilized using breast boards with both arms positioned above the head. All patients had 2 sets of planning CT images (one in FB and another in DIBH) with a Biograph TruePoint HD CT scanner in the same setup. DIBH was performed by tracking the respiratory cycles using a Varian Real-Time Position Management system. The target (chest wall and supraclavicular region), organs at risk (OARs; ipsilateral lung, contralateral lung, heart, LAD, and contralateral breast), and other organs of interests were delineated as per the RTOG (Radiation Therapy Oncology Group) contouring guidelines. The single-isocenter conformal fields in the field treatment plans were generated with the Eclipse Treatment Planning System (Varian Medical Systems) for both FB and DIBH images, and the doses to the target and OARs were compared. The standard fractionation regimen of 50 Gy in 25 fractions over a period of 5 weeks was used for all patients in this study. Results and Discussion: The target coverage parameters (V95, V105, V107, and D_mean) were found to be 97.8 ± 0.9, 6.1 ± 3.4, 0.2 ± 0.3, and 101.9 ± 0.5% in the FB plans and 98.1 ± 0.8, 6.1 ± 3.2, 0.2 ± 0.3, and 101.9 ± 0.4% in the DIBH plans, respectively. The plan quality indices (conformity index and homogeneity index) also showed 1.3 ± 0.2 and 0.1 for the FB plans and 1.2 ± 0.3 and 0.1 for the DIBH plans, respectively. There was a significant reduction in dose to the heart in the DIBH plans compared to the FB plans, with p values of nearly 0 for the V5, V10, V25, V30, and D_mean dosimetric parameters. The difference in ipsilateral lung doses between FB and DIBH showed statistically significant p values, and the differences in mean doses were found to be 7, 15.7, 11.8, and 10.7% for V5, V20, V30, and D_mean, respectively. There was a significant reduction in dose to the LAD in the DIBH compared to the FB plans. Conclusions: DIBH resulted in significant reductions in doses to the heart, LAD, and lungs, since with this technique there was an increase in the distance between the target and the OARs. With appropriate patient selection and adequate training, the DIBH technique is acceptable and achievable for radiotherapy to the chest, and therefore should be considered for all suitable patients, as this could result in fewer radiotherapy-related complications. However, this technique is time-consuming, since the setup is complex, results in an increased time for treatment delivery, and needs patient cooperation and technical expertise.