Institutional Patient-specific IMRT QA Does Not Predict Unacceptable Plan Delivery

Abstract

Purpose To determine whether in-house patient-specific IMRT QA results predict the Imaging and Radiation Oncology Core (IROC)-Houston phantom results. Methods and Materials IROC Houston’s IMRT head and neck phantoms have been irradiated by numerous institutions as part of clinical trial credentialing. We retrospectively compared these phantom results with those of in-house IMRT QA (following the institution’s clinical process) for 855 irradiations performed between 2003 and 2013. The sensitivity and specificity of IMRT QA to detect unacceptable or acceptable plans was determined relative to the IROC Houston phantom results. Additional analyses evaluated specific IMRT QA dosimeters and analysis methods. Results IMRT QA universally showed poor sensitivity relative to the head and neck phantom i.e., poor ability to predict a failing IROC Houston phantom result. Depending on how the IMRT QA results were interpreted, overall sensitivity ranged from 2% to 18%. For different IMRT QA methods, sensitivity ranged from 3% to 54%. Although the observed sensitivity was particularly poor at clinical thresholds (e.g., 3% dose difference or 90% of pixels passing gamma), receiver operator characteristic analysis indicated that no threshold showed good sensitivity and specificity for the devices evaluated. Conclusions IMRT QA is not a reasonable replacement for a credentialing phantom. Moreover, the particularly poor agreement between IMRT QA and the IROC Houston phantoms highlights surprising inconsistency in the QA process.

Can a commercially available EPID dosimetry system detect small daily patient setup errors for cranial IMRT/SRS?

“…13 This finding is a strong indication that standardized pass-fail criteria could be useful in standardizing quality of care across clinics. Nelms et al concur: "Many forms of relevant systematic errors can go undetected when the currently prevalent metrics for IMRT/ [volumetric arc therapy] commissioning are used.…”

Are simple IMRT beams more robust against MLC error? Exploring the impact of MLC errors on planar quality assurance and plan quality for different complexity beams

“…Some studies showed the result of planar QA could not predict clinically relevant patient dose error 21 , 22 , 23 , 24 . To overcome the shortcomings of planar QA, some new patient‐specific QA methods have been developed 25 , 26 , 27 .…”

Modeling the Agility MLC in the Monaco treatment planning system

“…The discrepancy between the vendor‐provided model and that determined in‐house most likely has a diffuse explanation, partially associated with our choice of QA equipment and partially associated with the natural limitations of dose distribution comparison as a method for tuning a head model (12) . The spatial resolution of the ArcCHECK makes the visibility of small structure in the distributions difficult, and due to the higher physical density of the device with respect to water, the absolute calibration of the device requires careful consideration in Monaco with regard to dose‐to‐medium, dose‐to‐water, and effective relative electron density assigned to the image‐set 11 , 13 , 14 , 15 .…”

“…It is likely that the vendor‐assisted procedure could have provided a parameter set that produced calculated point doses closer to those found in ion chamber measurements if we had chosen a water‐equivalent QA system initially. However, given the lack of ion chamber measurements in the ExpressQA package, the reliance on using the absolute calibration of such a QA system remains a concern, and the use of planar comparison metrics with a single pseudoclinical field in the ExpressQA package places limits on the generalizability of any statements of head model accuracy using this methodology 12 , 16 …”