IMPORTANCE OF IN-VIVO DOSIMETRY AS PART OF A QUALITY ASSURANCE PROGRAM IN TANGENTIAL BREAST TREATMENTS

Purpose: The investigation of the accuracy and reproducibility in the daily dose delivered in tangential breast treatments with in vivo dose measurements. Methods and Materials: In vivo dose measurements perfor med on the tangential treatment fields of 35 breast cancer patients ar e analysed for three units: a 6 MV linear accelerator, an old Cobalt u nit and a new Cobalt unit. The results are plotted in frequency distri butions. Deviations on the mean are often the expression of a systemat ic error in one of the core procedures of a department. A large spread of the results around the mean indicates a high burden of random set- up errors and/or systematic errors in individual patients. The reprodu cibility in dose delivery is studied by comparing repetitive checks to their respective mean for investigation of random day-to-day variatio n. Results: A small systematic error on the entrance dose (+ 1.4%) is detected on the old Cobalt unit due to a discrepancy between measured and published percentage depth dose values. An unexpected systematic o verdosage (+ 6%) is detected after implementation of a new software fo r dose calculation, proving that treatment quality is a process needin g continuous monitoring. The transmission measurements demonstrate a s ystematic error in dose delivery of 1.5 to 3% due to the assumption th at the breast is water equivalent when calculating the dose. The large spread of the transmission measurements (s(a) = 7.7%) shows that the weakest point in the treatment preparation chain is inaccurate acquisi tion of external body contours, leading to systematic errors in dose d elivery for specific patients. The standard deviation for the reproduc ibility is 3.1% for the old Cobalt unit, vs. 1.6% on the other units, demonstrating the influence of staffing and mechanical characteristics of the units on daily precision in dose delivery. Conclusion: In vivo dosimetry is an important tool in a departmental quality assurance pr ogram to detect systematic errors in dose delivery, to identify inadeq uate treatment situations, to investigate weak points in the chain of treatment preparation and to ensure accurate dose delivery for individ ual patients. The predictive value of a single check for the accuracy in dose delivery during the whole treatment series is high for reprodu cible treatment methodologies.