Patient Specific Quality Assurance of IMRT: Quantitative Approach Using Film Dosimetry and Optimization

Shin, Kyung Hwan; Park, Sung Yong; Park, Dong Hyun; Shin, Dongho; Park, Dahl; Kim, Tae Hyun; Pyo, Hongryull; Kim, Joo Young; Kim, Dae Yong; Cho, Kwan Ho; Huh, Sun Nyung; Kim, Il Han; Park, Charn Il

Abstract

PURPOSE: Film dosimetry as a part of patient specific intensity modulated radiation therapy quality assurance (IMRT QA) was performed to develop a new optimization method of film isocenter offset and to then suggest new quantitative criteria for film dosimetry.
MATERIALS AND METHODS
Film dosimetry was performed on 14 IMRT patients with head and neck cancers. An optimization method for obtaining the local minimum was developed to adjust for the error in the film isocenter offset, which is the largest part of the systemic errors.
RESULTS
The adjust value of the film isocenter offset under optimization was 1 mm in 12 patients, while only two patients showed 2 mm translation. The means of absolute average dose difference before and after optimization were 2.36 and 1.56%, respectively, and the mean ratios over a 5% tolerance were 9.67 and 2.88%. After optimization, the differences in the dose decreased dramatically. A low dose range cutoff (L-Cutoff) has been suggested for clinical application. New quantitative criteria of a ratio of over a 5%, but less than 10% tolerance, and for an absolute average dose difference less than 3% have been suggested for the verification of film dosimetry.
CONCLUSION
The new optimization method was effective in adjusting for the film dosimetry error, and the newly quantitative criteria suggested in this research are believed to be sufficiently accurate and clinically useful.