Quality assurance in radiotherapy by in vivo dosimetry. 2. Determination of the target absorbed dose

Combined entrance and exit dose measurements were performed with semiconductor detectors on patients, treated for neck and oral cavity malignancies. Transmission measurements showed the important influence of contour inaccuracies and tissue inhomogeneities. In 39.6% (21/53) of the checked contours, the discrepancy between the contour diameter used for routine treatment planning and the actual patient diameter was 1 cm or more, and in this group a systematic tendency for patient diameter underestimation due to the procedure was detected. When the X-ray beam passed through important bone structures such as the mandibular bones or the vertebral body, large discrepancies of 10% and more between the measured and the expected transmission were found. The target absorbed dose was determined from the transmission and entrance dose measurement. A systematic underdosage of about 2% at midline level was found to be due to an inaccuracy in the algorithms of the treatment planning system. Underdosages of 5% or more at midline were detected in more than 20% (47/230) of the measurements. In all cases, the reason for erroneous dose delivery was identified. Entrance dose measurements were previously demonstrated to be useful for the assessment of uncertainties related to treatment machine, patient set-up and treatment planning system (part 1). Transmission measurements (the ratio of the exit to the entrance dose measurement) are shown to be very useful to evaluate uncertainties related to patient data such as contour errors and tissue inhomogeneities as well as to the algorithms of the planning system. The influence of these errors on the target absorbed dose can be estimated and corrections can be applied for each individual patient.