On the implementation of the plan‐class specific reference field using multidimensional clustering of plan features and alternative strategies for improved dosimetry in modulated clinical linear accelerator treatments

Abstract

Purpose The plan‐class specific reference field concept could theoretically improve the calibration of radiation detectors in a beam environment much closer to clinical deliveries than existing broad beam dosimetry protocols. Due to a lack of quantitative guidelines and representative data, however, the pcsr field concept has not yet been widely implemented. This work utilizes quantitative plan complexity metrics from modulated clinical treatments in order to investigate the establishment of potential plan classes using two different clustering methodologies. The utility of these potential plan clusters is then further explored by analyzing their relevance to actual dosimetric correction factors. Methods Two clinical databases containing several hundred modulated plans originally delivered on two Varian linear accelerators were analyzed using 21 plan complexity metrics. In the first approach, each database’s plans were further subdivided into groups based on the anatomic site of treatment and then compared to one another using a series of nonparametric statistical tests. In the second approach, objective clustering algorithms were used to seek potential plan clusters in the multidimensional complexity‐metric space. Concurrently, beam‐ and detector‐specific dosimetric corrections for a subset of the modulated clinical plans were determined using Monte Carlo for three different ionization chambers. The distributions of the dosimetric correction factors were compared to the derived plan clusters to see which plan clusters, if any, could help predict the correction factor magnitudes. Ultimately, a simplified volume averaging metric (SVAM) is shown to be much more relevant to the total dosimetric correction factor than the established plan clusters. Results Plan groups based on the site of treatment did not show noticeable distinction from one another in the context of the metrics investigated. An objective clustering algorithm was able to discriminate volumetric modulated arc therapy (VMAT) plans from step‐and‐shoot intensity‐modulated radiation therapy plans with an accuracy of 90.8%, but no clusters were found to exist at any level more specific than delivery modality. Monte Carlo determined correction factors for the modulated plans ranged from 0.970 to 1.104, 0.983 to 1.027, and 0.986 to 1.009 for the A12, A1SL, and A26 chambers, respectively, and were highly variable even within the treatment modality plan clusters. The magnitudes of these correction factors were explained almost entirely by volume averaging with SVAM demonstrating positive correlation with all Monte Carlo established total correction factors. Conclusions Plan complexity metrics do provide some quantitative basis for the investigation of plan clusters, but an objective clustering algorithm demonstrated that quantifiable differences could only be found between VMAT and step‐and‐shoot beams delivered on the same treatment machine. The inherent variability of the Monte Carlo determined correction factors could not be explained solely by the modalit...

A clinical validation of the MR‐compatible Delta4 QA system in a 0.35 tesla MR linear accelerator

“…For each treatment plan investigated, 24 modulation complexity metrics were calculated to determine whether any specific metric or metrics correlated with the gamma pass rates. The metrics and means of calculation are summarized in Table 2 based on the work of Desai et al 14 The small aperture score (2mm) metric was not calculated in this work since no clinical plans investigated involved MLC opening widths of 2 mm or smaller. An in‐house Matlab® (Mathworks, Natick, MA) script was used to record and calculate all metrics by extracting data from the headers of the DICOM‐RT plan files.…”

“…Summary of complexity metrics calculated modified from the work of Desai et al14 with permission from Medical Physics. Comparison of (a) global gamma 3%/3mm pass rates and (b) global gamma 2%/2mm pass rates across all investigated plans for both the ArcCHECK-MR as well as the Delta4 Phantom+ MR. performance of the Delta 4 Phantom+ MR was assessed through measurement of 36 clinical cases treated on a ViewRay MRIdian linac.…”

Characteristics and limitations of a secondary dose check software for VMAT plan calculation

“…These metrics were calculated based on work by Desai et al. and included the average absolute leaf position off the central axis (ALPCA), mean aperture displacement (MAD), and modulation complexity score (MCS), among others 13 …”

“…tive DLGC was assessed by performing a collective DLGC optimization while excluding single plans from the analysis, in order to assess the potential impact of test plan selection.2.C | Plan-specific DLGC correlation to plan complexity metricsThe optimal values for the plan-specific DLGC were assessed relative to several plan metrics to assess whether any relevant correlation between DLGC values and plan metrics was present. These metrics were calculated based on work by Desai et al and included the average absolute leaf position off the central axis (ALPCA), mean aperture displacement (MAD), and modulation complexity score (MCS), among others 13. …”

Reference dosimetry of modulated and dynamic photon beams

“…The most recent study available on dynamic photon beam reference dosimetry was led by Desai (2019), Desai et al (2019Desai et al ( , 2020. To investigate a practical definition of PCSR beams, the features of 758 plans separated into tumour sites and machine models were analyzed retrospectively: ten sites on a TrueBeam™ STx (627 VMAT plans) and eight sites on Clinac ® iX (131 step-and-shoot and VMAT plans).…”