Purpose: To determine the difference between expected and measured dose for patients prescribed a mantle treatment for Hodgkin's disease and estimate the range of dose at critical sites and between different treatment centres.
Methods and materials: Twenty three radiotherapy centres were surveyed with regard to the accuracy of dose delivery to a custom-built upper-torso phantom. Thermoluminescent dosimeters were used to monitor the delivered dose at sites such as mid-plane, spinal cord, neck, axilla and lung.
Results: The intended dose to the phantom at each centre was 1 Gy to central axis mid-plane. Of the centres surveyed, the median measured dose to this region was 0.96 Gy with a minimum of 0.92 and a maximum of 1.00 Gy. Median dose to the axilla region was low (0.90 Gy) whereas median dose to the blocked lung and neck region were higher than expected, 0.18 Gy and 1.25 Gy, respectively. The 95% confidence interval on the reported relative dose using the 4000 thermoluminescence dosimetry readings in this study was +/- 1.5%
Conclusion: In this controlled experiment, using conventional methods to calculate dose, there was a surprising variability in the dose delivered at the central axis mid-plane position. This was traced to lack of uniformity in the use of equivalent squares to calculate the output factor. The measured doses to axilla and lung are explained by photon and electron scattering effects. Centres, where dose compensation was included, had a superior dose homogeneity in the neck. The off-axis dose calculations depend on computer planning software but the magnitude of these differences is secondary to that of central axis mid-point dose differences. Improved consistency of dose calculation techniques between centres would enable more reliable dose response evaluation from multicentre clinical studies of Hodgkin's disease.