Accurate in vivo dosimetry of a randomized trial of prostate cancer irradiation

Purpose: To guarantee an accurate dose delivery, within +/- 2.5%, in a Phas e III randomized trial of prostate cancer irradiation (68 vs. 78 Gy) by mea ns of a comprehensive ill vivo dosimetry program. Methods and Materials: Prostate patients are generally treated in our clini c with a 3-field isocentric technique: an X-MV anteroposterior beam and 2 1 8-MV wedged laterals. All fields are shaped conformally to the PTV. Patient s were randomized between two dose levels of 68 Gy and 78 Gy. During treatm ent, the entrance and exit dose were measured for each patient with diodes. Special 2.5-mm thick steel build-up caps were applied to make the diodes a ppropriate for measurements in 18-MV photon beams as well. Portal images we re used to verify the correct position of the diodes and to detect and corr ect for gas tilling in the rectum that may influence the exit dose reading. Entrance and exit dose measurements were converted to midplane dose, which was used in combination with a depth dose correction to obtain the dose at the specification point. An action level of 2.5% was applied. Results: The added build-up for the diodes in the 18-MV beams resulted in c orrection factors that were only slightly sensitive to changes in beam setu p and comparable to the corrections used in the 8-MV beams for diodes witho ut extra build-up. The calibration factor increased almost linearly with cu mulative dose: 0.7%/kGy for the 8-MV and 1.2%/kGy for the 18-MV photon beam s. The introduction of average correction factors made the analysis easier, while keeping the accuracy within acceptable limits. In a period of 3 year s, 225 patients were analyzed, from which 8 patients needed to be corrected . The average ratio of measured and prescribed dose was 1.009 (standard dev iation [SD] 0.012) for the total group treated on two linear accelerators. When the results were analyzed per accelerator, the ratios were 1.002 (SD, 0.001) for Accelerator A and 1.015 (SD, 0.001) for Accelerator B. This diff erence could be attributed to the cumulative effect of three small imperfec tions in the performance of Accelerator B that were well within the limits of our quality assurance program. Conclusion: Diodes can be used for accurate in vivo dosimetry during prosta te irradiation in high-energy photon beams. The dose delivery in this rando mized trial is guaranteed within the 2.5% limits on an individual patient b asis. This could not be achieved without the in vivo dosimetry program, des pite our high-standard quality assurance program of treatment delivery. (C) 2001 Elsevier Science Inc.