Quantitative analysis of patient-specific dosimetric IMRT verification

Abstract

Patient-specific dosimetric verification methods for IMRT treatments are variable, time-consuming and frequently qualitative, preventing evidence-based reduction in the amount of verification performed. This paper addresses some of these issues by applying a quantitative analysis parameter to the dosimetric verification procedure. Film measurements in different planes were acquired for a series of ten IMRT prostate patients, analysed using the quantitative parameter, and compared to determine the most suitable verification plane. Film and ion chamber verification results for 61 patients were analysed to determine long-term accuracy, reproducibility and stability of the planning and delivery system. The reproducibility of the measurement and analysis system was also studied. The results show that verification results are strongly dependent on the plane chosen, with the coronal plane particularly insensitive to delivery error. Unexpectedly, no correlation could be found between the levels of error in different verification planes. Longer term verification results showed consistent patterns which suggest that the amount of patient-specific verification can be safely reduced, provided proper caution is exercised: an evidence-based model for such reduction is proposed. It is concluded that dose/distance to agreement (e.g., 3%/3 mm) should be used as a criterion of acceptability. Quantitative parameters calculated for a given criterion of acceptability should be adopted in conjunction with displays that show where discrepancies occur. Planning and delivery systems which cannot meet the required standards of accuracy, reproducibility and stability to reduce verification will not be accepted by the radiotherapy community.

Clinical implications of the anisotropic analytical algorithm for IMRT treatment planning and verification

“…The highest discrepancies were seen at the Posterior point, outside the high dose region, although all measurements there agreed with the AAA doses to within 2.1% or 1.3mm distance to agreement. Only for one measurement (at the Right measurement point) did the level of agreement exceed 3% or 3mm, a commonly-used action level for IMRT verification [20,27]. The agreement between the AAA and measurement was comparable to that reported between other CS algorithms and measurement or Monte Carlo simulation [13][14][15][16].…”

Practical application of Octavius®‐4D: Characteristics and criticalities for IMRT and VMAT verification

“…The first important issue is related to how the phantom is conceived, as it allows to consider different typology of metrics for the γ ‐index. The 2D approach considers each slice as independent of the surrounding volume, with the drawback that results are strongly dependent on the chosen plane, without a certain significant correlation between the magnitude of errors of different plans 19. Such an aspect is then an undesirable characteristic of the 2D γ .…”

Clinical implementation and error sensitivity of a 3D quality assurance protocol for prostate and thoracic IMRT

“…However, if the error was due to all MLC leaves being shifted uniformly to the right or left, the ability to fix such error might lead to false positive QA pass. Another disadvantage of 2D gamma analysis is that it has been previously found to be measurement acquisition plane‐dependent (6) and unable to detect certain errors (5) that would render a given treatment plan clinically unacceptable.…”

“…Nelms et al (5) introduced four types of errors and found lack of correlation between 2D gamma analysis passing rates and dose errors introduced to anatomic regions of interest. Budgell et al (6) concluded that, when 2D gamma analysis is performed using 3% dose difference and 3 mm DTA, the result and the error detectability is strongly dependent on the plane chosen for measurement acquisition, and no correlation could be found between the levels of errors in different verification planes. Both research teams suggest that moving from 2D to 3D gamma analysis might solve those issues.…”