Quality assurance in radiation oncology. A study of feasibility and impact on action levels of an in vivo dosimetry program during breast cancer irradiation

Abstract

Background and purpose: The study was aimed at investigating the feasibility and accuracy of an in vivo quality assurance program in radiotherapy. Breast irradiation was found to be a relevant clinical model due to the fairly good uniformity of the irradiated tissue.

Materials and methods: The investigation was based on an extension of the method described by Leunens et al. (Leunens, G., van Dam, J., Dutreix, A. and van der Schueren, E. Quality assurance in radiotherapy by in vivo dosimetry. 1. Entrance dose measurements, a reliable procedure. Radiother. Oncol. 17: 141-151, 1990; Leunens, G., van Dam, J., Dutreix, A. and van der Schueren, E. Quality assurance in radiotherapy by in vivo dosimetry. 2. Determination of the target absorbed dose. Radiother. Oncol. 19: 73-87, 1990; van Dam, J. and Marinello, G. Methods for in vivo dosimetry in external radiotherapy. Physics for clinical radiotherapy. ESTRO Booklet n. 1 (Garant), 1994), determining the absorbed dose at any point on the central axis from a measurement of entrance and exit doses with individually calibrated and corrected diodes. Treatment accuracy (delta) was quantified as the ratio of the measured and the expected isocentre dose from the treatment planning system (TPS).

Results: A preliminary study was carried out on a Plexiglas slab phantom to test the method ending with a frequency distribution of delta with a mean of 0.04 +/- 0.05% and a standard deviation (SD) of 0.83 +/- 0.04%. In the in vivo study, 101 patients irradiated with two tangential fields were included in the protocol over a 1-year period. The total number of patient set-ups analyzed was 421 giving a distribution of delta with a mean of -1.3 +/- 0.2% and an SD of 2.7 +/- 0.1% without any correction. Taking into account temperature effects and set-up errors as SSD accuracy and diodes positioning it was possible to implement an off-line correction method leading to a final distribution with a mean of -1.9 +/- 0.2% and an SD of 2.4 +/- 0.1%. Individual cases with large deviations were detected and evaluated and actions were undertaken whenever possible.

Conclusions: The study showed that diodes can be easily used by radiographers in an accurate in vivo quality assurance (QA) program and that an accuracy level of 3% at 1 SD can be reached on average. Attention and action levels can also be identified and careful evaluation of positioning and morphological variations during treatment should be part of a comprehensive QA program.