This study aimed to provide a detailed evaluation and comparison of a range of modulated beam evaluation metrics, in terms of their correlation with QA testing results and their variation between treatment sites, for a large number of treatments. Ten metrics including the modulation index (MI), fluence map complexity, modulation complexity score (MCS), mean aperture displacement (MAD) and small aperture score (SAS) were evaluated for 546 beams from 122 intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment plans targeting the anus, rectum, endometrium, brain, head and neck and prostate. The calculated sets of metrics were evaluated in terms of their relationships to each other and their correlation with the results of electronic portal imaging based quality assurance (QA) evaluations of the treatment beams. Evaluation of the MI, MAD and SAS suggested that beams used in treatments of the anus, rectum, head and neck were more complex than the prostate and brain treatment beams. Seven of the ten beam complexity metrics were found to be strongly correlated with the results from QA testing of the IMRT beams (p < 0.00008). For example, values of SAS (with multileaf collimator apertures narrower than 10 mm defined as 'small') less than 0.2 also identified QA passing IMRT beams with 100% specificity. However, few of the metrics are correlated with the results from QA testing of the VMAT beams, whether they were evaluated as whole 360° arcs or as 60° sub-arcs. Select evaluation of beam complexity metrics (at least MI, MCS and SAS) is therefore recommended, as an intermediate step in the IMRT QA chain. Such evaluation may also be useful as a means of periodically reviewing VMAT planning or optimiser performance.

The dosimetric leaf gap (DLG) is a beam configuration parameter used in the Varian Eclipse treatment planning system, to model the effects of rounded MLC leaf ends. Measuring the DLG using the conventional sliding‐slit technique has been shown to be produce questionable results for some volumetric modulated arc therapy (VMAT) treatments. This study therefore investigated the use of radiochromic film measurements to optimize the DLG specifically for the purpose of producing accurate VMAT plans using a flattening‐filter‐free (FFF) beam, for use in treating vertebral targets using a stereotactic (SABR, also known as SBRT) fractionation schedule. Four test treatments were planned using a VMAT technique, to deliver a prescription of 24 Gy in 3 fractions to four different spine SABR treatment sites. Measurements of the doses delivered by these treatments were acquired using an ionization chamber and radiographic film. These measurements were compared with the doses calculated by the treatment planning system using a range of DLG values, including a DLG identified using the conventional sliding‐slit method (1.1 mm). An optimal DLG value was identified, as the value that produced the closest agreement between the planned and measured doses (1.9 mm). The accuracy of the dose calculations produced using the optimized DLG value was verified using additional radiochromic film measurements in a heterogeneous phantom. This study provided a specific initial DLG (1.9 mm) as well as a film‐based optimization method, which may be used by radiotherapy centers when attempting to commission or improve an FFF VMAT‐based SABR treatment programme.

Given that there is increasing recognition of the effect that sub-millimetre changes in collimator position can have on radiotherapy beam dosimetry, this study aimed to evaluate the potential variability in small field collimation that may exist between otherwise matched linacs. Field sizes and field output factors were measured using radiochromic film and an electron diode, for jaw- and MLC-collimated fields produced by eight dosimetrically matched Varian iX linacs (Varian Medical Systems, Palo Alto, USA). This study used nominal sizes from 0.6 × 0.6 to 10 × 10 cm(2), for jaw-collimated fields, and from 1 × 1 to 10 × 10 cm(2) for MLC-collimated fields, delivered from a zero (head up, beam directed vertically downward) gantry angle. Differences between the field sizes measured for the eight linacs exceeded the uncertainty of the film measurements and the repositioning uncertainty of the jaws and MLCs on one linac. The dimensions of fields defined by MLC leaves were more consistent between linacs, while also differing more from their nominal values than fields defined by orthogonal jaws. The field output factors measured for the different linacs generally increased with increasing measured field size for the nominal 0.6 × 0.6 to 1 × 1 cm(2) fields, and became consistent between linacs for nominal field sizes of 2 × 2 cm(2) and larger. The inclusion in radiotherapy treatment planning system beam data of small field output factors acquired in fields collimated by jaws (rather than the more-reproducible MLCs), associated with either the nominal or the measured field sizes, should be viewed with caution. The size and reproducibility of the fields (especially the small fields) used to acquire treatment planning data should be investigated thoroughly as part of the linac or planning system commissioning process. Further investigation of these issues, using different linac models, collimation systems and beam orientations, is recommended.

IntroductionThis study examines and compares the dosimetric quality of radiotherapy treatment plans for prostate carcinoma across a cohort of 163 patients treated across five centres: 83 treated with three-dimensional conformal radiotherapy (3DCRT), 33 treated with intensity modulated radiotherapy (IMRT) and 47 treated with volumetric modulated arc therapy (VMAT).MethodsTreatment plan quality was evaluated in terms of target dose homogeneity and organs at risk (OAR), through the use of a set of dose metrics. These included the mean, maximum and minimum doses; the homogeneity and conformity indices for the target volumes; and a selection of dose coverage values that were relevant to each OAR. Statistical significance was evaluated using two-tailed Welch's T-tests. The Monte Carlo DICOM ToolKit software was adapted to permit the evaluation of dose metrics from DICOM data exported from a commercial radiotherapy treatment planning system.ResultsThe 3DCRT treatment plans offered greater planning target volume dose homogeneity than the other two treatment modalities. The IMRT and VMAT plans offered greater dose reduction in the OAR: with increased compliance with recommended OAR dose constraints, compared to conventional 3DCRT treatments. When compared to each other, IMRT and VMAT did not provide significantly different treatment plan quality for like-sized tumour volumes.ConclusionsThis study indicates that IMRT and VMAT have provided similar dosimetric quality, which is superior to the dosimetric quality achieved with 3DCRT.

This study assessed the validity of the conversion from percentage depth dose (PDD) to tissue maximum ratio (TMR) using BJR Supplement 25 data for flattened and flattening filter free (FFF) beams. PDD and TMR scans for a variety of field sizes were measured in water using a Sun Nuclear Corporation 3D SCANNER™ on a Varian TrueBeam linear accelerator in 6 MV, 10 MV and 6 MV FFF beams. The BJR Supplement 25 data was used to convert the measured PDDs to TMRs and these were compared with the directly measured TMR data. The TMR plots calculated from PDD were within 1% for the 10 MV and 6 MV flattened beams, for field sizes 3 cm × 3 cm to 40 cm × 40 cm inclusive, at depths measured beyond the depth of maximum dose. The disagreement between the measured and calculated TMR plots for the 6 MV FFF beam increased with depth and field size to a maximum of 1.7% for a 40 cm × 40 cm field. The results found in this study indicate that the BJR Supplement 25 data should not be used for field sizes larger than 20 cm × 20 cm at depths greater than 15 cm for the 6 MV FFF beam. It is advised that PDD to TMR conversion for FFF beams should be done with phantom scatter ratios appropriate to FFF beams, or the TMR should be directly measured if required.

Post-implant CT and dosimetry did not alter patients' management after real-time intraoperative planning. However, we recommend that it still be employed for difficult cases or if there are any concerns identified in real-time planned dosimetry.

Purpose: The accurate completion of reliable quality assurance measurements of complex intensity‐modulated and rotational radiotherapy treatments demands the use of a dosimetry system with both high resolution and minimal angular dependence. This study demonstrates the utility of Gafchromic EBT2 radiochromic dosimetry film for the quality assurance of complex radiotherapy treatments. Methods: This study establishes that the dose‐response of EBT2 film has minimal angular and depth dependence and then demonstrates how the film can be used to perform quality assurance measurements for BrainLab stereotactic IMRT, Varian RapidArc and TomoTherapy treatments. Results: For all treatment plans, the proportion of dose points agreeing with the film measurements to within gamma(3%,3mm) was found to be above 95%, with all points agreeing within 5%, indicating that the treatments were delivered with an acceptable level of accuracy. The film images also provided additional information on low‐level dose variations including: the location and extent of dose from inter‐leaf leakage (for RapidArc); the location and extent of helical field junctioning effects, or threading (for TomoTherapy); and the existence of small regions of under‐dosage from very small treatment segments (for stereotactic IMRT). Conclusions: The fact that these film measurements detected small‐scale dosimetric effects that were not apparent in the dose planes exported from the treatment planning systems confirms both the value of high‐resolution dosimetry and the importance of designing complex radiotherapy treatments with the maximum achievable level of target dose homogeneity.