Per-beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors

Abstract

Purpose: The purpose of this work is to determine the statistical correlation between per-beam, planar IMRT QA passing rates and several clinically relevant, anatomy-based dose errors for per-patient IMRT QA. The intent is to assess the predictive power of a common conventional IMRT QA performance metric, the Gamma passing rate per beam.

Methods: Ninety-six unique data sets were created by inducing four types of dose errors in 24 clinical head and neck IMRT plans, each planned with 6 MV Varian 120-leaf MLC linear accelerators using a commercial treatment planning system and step-and-shoot delivery. The error-free beams/plans were used as "simulated measurements" (for generating the IMRT QA dose planes and the anatomy dose metrics) to compare to the corresponding data calculated by the error-induced plans. The degree of the induced errors was tuned to mimic IMRT QA passing rates that are commonly achieved using conventional methods.

Results: Analysis of clinical metrics (parotid mean doses, spinal cord max and D1cc, CTV D95, and larynx mean) vs. IMRT QA Gamma analysis (3%/3 mm, 2/2, 1/1) showed that in all cases, there were only weak to moderate correlations (range of Pearson's r-values: -0.295 to 0.653). Moreover, the moderate correlations actually had positive Pearson's r-values (i.e., clinically relevant metric differences increased with increasing IMRT QA passing rate), indicating that some of the largest anatomy-based dose differences occurred in the cases of high IMRT QA passing rates, which may be called "false negatives." The results also show numerous instances of false positives or cases where low IMRT QA passing rates do not imply large errors in anatomy dose metrics. In none of the cases was there correlation consistent with high predictive power of planar IMRT passing rates, i.e., in none of the cases did high IMRT QA Gamma passing rates predict low errors in anatomy dose metrics or vice versa.

Conclusions: There is a lack of correlation between conventional IMRT QA performance metrics (Gamma passing rates) and dose errors in anatomic regions-of-interest. The most common acceptance criteria and published actions levels therefore have insufficient, or at least unproven, predictive power for per-patient IMRT QA.

Figure 1

Schematic of the methodology of data generation for the correlation study.

Figure 2

Sample DVH differences between the induced-error beam models (dashed lines) and the virtual measurement beam models (solid lines). These are the results for patient plan no. 22 (of 24).

Figure 3

Magnitude of errors in the maximum cord dose and the cord D1cc vs the conventional IMRT QA performance metric of passing rate (%) averaged over all beams per plan, shown for three different sets of Gamma parameters.

Figure 4

(a) Magnitude of errors in the mean contralateral parotid dose vs conventional IMRT QA performance metric of Gamma passing rate (%) averaged over all beams per plan. (b) Magnitude of errors in the mean ipsilateral parotid dose vs conventional IMRT QA performance metric of Gamma passing rate (%) averaged over all beams per plan. Data are shown for three different sets of Gamma parameters.

Figure 5

Magnitude of errors in the mean larynx dose vs the conventional IMRT QA performance metric of passing rate (%) averaged over all beams per plan, shown for three different sets of Gamma parameters

Figure 6

Magnitude of errors in CTV60’s D95 dose vs the conventional IMRT QA performance metric of passing rate (%) averaged over all beams per plan, shown for three different sets of Gamma parameters

Figure 7

Distribution of errors in two critical anatomy dose metrics for three types of errors induced. (a) Errors in CTV D95 (low range of errors, overall) and (b) errors in contralateral parotid mean dose (higher range of errors).

Figure 8

Generalized illustration of regions of false negatives (high passing rates despite critical patient dose errors) and false positives (low QA passing rates but with noncritical patient dose errors) when correlating critical patient dose errors to conventional IMRT QA Gamma passing rates. In this schematic, the critical dose error threshold is “E” and the standard acceptance criteria for Gamma passing rates is “C.”