A six-year review of more than 13,000 patient-specific IMRT QA results from 13 different treatment sites

Our plain-English summary of A six-year review of more than 13,000 patient-specific IMRT QA results from 13 different treatment sites, by K.B. Pulliam, D. Followill, L. Court, et al (Journal of Applied Clinical Medical Physics, 2014).

The study

• Authors: K.B. Pulliam, D. Followill, L. Court, L. Dong, M. Gillin, K. Prado, S.F. Kry
• Journal: Journal of Applied Clinical Medical Physics
• Published: 2014;15(5):4935
• Type of article: Review
• Cited by: 34 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Due to a lack of information regarding the current clinical experience of IMRT QA for a large and varied plan population, we reviewed our patient-specific IMRT quality assurance (QA) results for 13,003 treatment plans from 13 distinct treatment sites from a six-year period. QA records were reviewed for dose difference (single point with ion chamber measurement; ± 3% agreement criteria) and percentage of pixels passing relative dose gamma analysis (film measurement; 90% passing 5%(global)/3 mm agreement criteria) from 2005 through 2011. Plan records were analyzed for trends with measurement date and treatment site. Plans failing to meet QA tolerance criteria were evaluated for follow-up clinical action (i.e., if repeat measurements were performed). The mean difference (± SD) between ion chamber point measurements and calculated doses was -0.29% ± 1.64% (calculated values being slightly higher) and, regarding planar dose evaluations, the mean percentage of pixels passing the gamma criteria of 5%(global)/3 mm was 97.7% (lower 95th percentile: 92.2%). 97.7% and 99.3% of plans passed the point dose and planar dose verification, respectively. We observed statistically significant differences (p< 0.05) in both point dose and planar dose verification measurements as a function of treatment site (particularly for stereotactic spine and mesothelioma sites) and measurement date (average agreement improved with time). However, despite improved dosimetric agreement, the percentage of failing plans has remained nearly constant at 2.3%.

In plain English

This article was not a study of asbestos exposure or asbestos-related disease. It reviewed one hospital's quality-assurance records for intensity-modulated radiation therapy (IMRT) at The University of Texas MD Anderson Cancer Center. The records covered 13,003 treatment plans from January 2005 to July 2011 across 13 treatment sites, one of which was mesothelioma. Before treatment, calculated doses were checked against measurements in a test phantom, using an ion chamber and, from late 2005, film. Overall, 301 plans (2.3%) had at least one failing ion chamber measurement, and 95 plans (0.7%) failed the film check. Mesothelioma plans had a mean ion chamber difference of 2.60%, a failure rate of 21.2%, and the poorest film results. Their mean differences, along with those of stereotactic spine plans, were statistically distinct from all other sites. Ion chamber agreement improved over time, but ion chamber failure rates stayed essentially constant, while film failure rates decreased. The authors concluded that QA programs with no plan failures may not be sensitive to plan errors. They noted that their recording method underrepresented borderline failures.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review of one hospital's patient-specific radiation therapy (IMRT) quality-assurance records. Calculated doses were checked against measurements made in a QA phantom, not in patients.
• Who was studied: 13,003 IMRT treatment plans (13,308 point dose and 12,677 planar dose measurements) from 13 treatment sites, including mesothelioma, at The University of Texas MD Anderson Cancer Center, January 2005 to July 2011
• Exposure: No asbestos exposure was studied. The study compared calculated and measured radiation doses by treatment site (including mesothelioma) and by date, using an ion chamber and, from late 2005, film.
• Main results:
  • Across all sites, 301 plans (2.3% of plans) had at least one failing point dose (ion chamber) measurement.
  • Mesothelioma (MESO) plans had a mean point dose difference of 2.60%, and stereotactic spine (IMSSRT) plans had -1.59%. These were the largest mean differences of any site.
  • Point dose failure rates were 21.2% for mesothelioma plans and 16.1% for stereotactic spine plans (the text's figure; the table gives 15.8%).
  • Mean point dose differences in the stereotactic spine and mesothelioma sites were statistically distinct from those in all other treatment sites (p less than 0.05).
  • Mesothelioma plans had the poorest film (gamma) results: a mean of 94.4% of pixels passing and a lower 95th percentile of 86.4%.
  • Across all sites, 95 plans (0.7%) failed the film (gamma) criteria.
  • Point dose agreement improved over time, but point dose failure rates stayed essentially constant.
  • Film (planar) failure rates decreased over time, unlike point dose failure rates.
  • Of the 301 plans that failed the initial point dose measurement, 191 eventually passed at a different measurement location under normal irradiation conditions.
  • Plans with no follow-up after multiple failures were typically mesothelioma, stereotactic spine, or large-volume pediatric cases, where the planning system had known limitations.
  • No clinical treatment plan was ever changed because a plan failed the film (gamma) criterion.
• The authors' conclusion: The authors concluded that plan failure rates stayed nearly constant at a nontrivial rate (2.3%) despite improved agreement, so QA programs with no plan failures may use techniques or action levels that are not sensitive to plan errors.
• Limitations the authors state:
  • For borderline results, only the final passing remeasurement was recorded, so failed point dose measurements near the tolerance limit were underrepresented.
  • The failure rate may not apply to other hospitals.
  • The film (gamma) criteria may have been too lax, and no dose thresholding was used.
  • The film check was more sensitive to dosimeter and processor problems than to treatment plan problems.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelioma.

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