Statistical analysis of IMRT dosimetry quality assurance measurements for local delivery guideline.
Summary of "Statistical analysis of IMRT dosimetry quality assurance measurements for local delivery guideline."
Purpose: To establish our institutional guideline for IMRT delivery, we statistically evaluated the results of DQA (dosimetry quality assurance) measurements and derived local confidence limits using the concept confidence limit of |mean|+1.96sigma. Materials and Methods: From June 2006 to March 2009, 206 patients with head and neck cancer, prostate cancer, liver cancer, or brain tumor were treated using LINAC-based IMRT technique. In order to determine site specific DQA tolerances at a later stage, a hybrid plan with the same fluence maps as in the treatment plan was generated on CT images of a cylindrical phantom of acryl. Points of measurement using a 0.125 cc ion-chamber were typically located in the region of high and uniform doses. The planar dose distributions perpendicular to the central axis were measured by using a diode array in solid water with all fields delivered, and assessed using gamma criteria of 3%/3 mm. The mean values and standard deviations were used to develop the local confidence and tolerance limits. The dose differences and gamma pass rates for the different treatment sites were also evaluated in terms of total MU, MU/cGy, and the number of PTV's pieces.
The mean values and standard deviations of ion-chamber dosimetry differences between calculated and measured doses were -1.6+/-1.2% for H&N cancer, -0.4+/-1.2% for prostate and abdominal cancer, and -0.6+/-1.5% for brain tumor. Most of measured doses (92.2%) agreed with the calculated doses within a tolerance limit of +/-3% recommended in the literature. However, we found some systematic under-dosage for all treatment sites. The percentage of points passing the gamma criteria, averaged over all treatment sites was 97.3+/-3.7%. The gamma pass rate and the agreement of ion-chamber dosimetry generally decreased with increasing the number of PTV's pieces, the degree of modulation (MU/cGy), and the total MU beyond 700. Our local confidence limits were comparable to those of AAPM TG 119 and ESTRO guidelines that were provided as a practical baseline for center-to-center commissioning comparison. Thus, our institutional confidence and action limits for IMRT delivery were set into the same levels of those guidelines. Discussion and Conclusions: The systematic under-dosage were corrected by tuning up the MLC-related factors (dosimetric gap and transmission) in treatment planning system (TPS) and further by incorporating the tongue-and groove effect into TPS. Institutions that have performed IMRT DQA measurements over a certain period of time need to analyze their accrued DQA data. We confirmed the overall integrity of our IMRT system and established the IMRT delivery guideline during this procedure. Dosimetric corrections for the treatment plans outside of the action level can be suggested only with such rigorous DQA and statistical analysis.
This article was published in the following journal.
Name: Radiation oncology (London, England)
- PubMed Source: http://www.ncbi.nlm.nih.gov/pubmed/21439096
- DOI: http://dx.doi.org/10.1186/1748-717X-6-27
Medical and Biotech [MESH] Definitions
Quality Assurance, Health Care
Activities and programs intended to assure or improve the quality of care in either a defined medical setting or a program. The concept includes the assessment or evaluation of the quality of care; identification of problems or shortcomings in the delivery of care; designing activities to overcome these deficiencies; and follow-up monitoring to ensure effectiveness of corrective steps.
Statistical formulations or analyses which, when applied to data and found to fit the data, are then used to verify the assumptions and parameters used in the analysis. Examples of statistical models are the linear model, binomial model, polynomial model, two-parameter model, etc.
The use of statistical methods in the analysis of a body of literature to reveal the historical development of subject fields and patterns of authorship, publication, and use. Formerly called statistical bibliography. (from The ALA Glossary of Library and Information Science, 1983)
Reproducibility Of Results
The statistical reproducibility of measurements (often in a clinical context), including the testing of instrumentation or techniques to obtain reproducible results. The concept includes reproducibility of physiological measurements, which may be used to develop rules to assess probability or prognosis, or response to a stimulus; reproducibility of occurrence of a condition; and reproducibility of experimental results.
Lot Quality Assurance Sampling
A sampling methodology using small sample sizes when conducting surveys in small geographical or population-based areas or lots.
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