Feasible measurement errors when undertaking in vivo dosimetry during external beam radiotherapy of the breast

In vivo dosimetry is a proven reliable method of checking overall treatment accuracy, allowing verification of dosimetry and dose calculation as well as patient treatment setup. We conducted a pilot study to assess the clinical utility of in vivo dosimetry in our department. Diodes (calibrated for typical treatment conditions) were used to record entrance dose measurements on 62 patients representing a variety of treatment sites. Measurements were compared with predictions from the planning system, with results found to be in tolerance for the majority of treatment sites. However, large discrepancies were encountered for measurements performed during breast irradiation (up to 16% for lateral tangential fields). The sensitivity of the recorded entrance dose to the positioning error of the diode placement was examined. The sensitivity of diode signal to small changes in position were compared with feasible variations in other parameters (e.g., dosimetry, FSD at setup). For the breast irradiation technique considered, wedges are used for the majority of fields. It was found that a proportion of error was predominantly due to the use of wedges and the presence of significantly nonuniform patient contours. In combination with diode placement errors, this resulted in increased measurement error. Correct diode placement is critical to ensure accurate data collection. The results of this study indicate the importance of separating errors due to measurement technique from actual treatment/setup errors.