Faculty Bibliography

The quantity of relevance for external beam radiotherapy is absorbed dose to water (ADW). An interferometer was built, characterized, and tested to measure ADW within the dose range of interest for external beam radiotherapy using the temperature dependence of the refractive index of water. The interferometer was used to measure radiation-induced phase shifts of a laser beam passing through a (10 × 10 × 10) cm³ water-filled glass phantom, irradiated with a 6 MV photon beam from a medical linear accelerator. The field size was (7 × 7) cm² and the dose was measured at a depth of 5 cm in the water phantom. The intensity of the interference pattern was measured with a photodiode and was used to calculate the time-dependent phase shift curve. The system was thermally insulated to achieve temperature drifts of less than 1.5 mK/min. Data were acquired 60 s before and after the irradiation. The radiation-induced phase shifts were calculated by taking the difference in the pre- and post-irradiation drifts extrapolated to the midpoint of the irradiation. For 200, 300, and 400 monitor units, the measured doses were 1.6 ± 0.3, 2.6 ± 0.3, and 3.1 ± 0.3 Gy, respectively. Measurements agreed within the uncertainty with dose calculations performed with a treatment planning system. The estimated type-A, k = 1 uncertainty in the measured doses was 0.3 Gy which is an order of magnitude lower than previously published interferometer-based ADW measurements.

PURPOSE/OBJECTIVE:To investigate why dose-rate constants for (125)I and (103)Pd seeds computed using the spectroscopic technique, Λ spec, differ from those computed with standard Monte Carlo (MC) techniques. A potential cause of these discrepancies is the spectroscopic technique's use of approximations of the true fluence distribution leaving the source, φ full. In particular, the fluence distribution used in the spectroscopic technique, φ spec, approximates the spatial, angular, and energy distributions of φ full. This work quantified the extent to which each of these approximations affects the accuracy of Λ spec. Additionally, this study investigated how the simplified water-only model used in the spectroscopic technique impacts the accuracy of Λ spec. METHODS:Dose-rate constants as described in the AAPM TG-43U1 report, Λ full, were computed with MC simulations using the full source geometry for each of 14 different (125)I and 6 different (103)Pd source models. In addition, the spectrum emitted along the perpendicular bisector of each source was simulated in vacuum using the full source model and used to compute Λ spec. Λ spec was compared to Λ full to verify the discrepancy reported by Rodriguez and Rogers. Using MC simulations, a phase space of the fluence leaving the encapsulation of each full source model was created. The spatial and angular distributions of φ full were extracted from the phase spaces and were qualitatively compared to those used by φ spec. Additionally, each phase space was modified to reflect one of the approximated distributions (spatial, angular, or energy) used by φ spec. The dose-rate constant resulting from using approximated distribution i, Λ approx,i, was computed using the modified phase space and compared to Λ full. For each source, this process was repeated for each approximation in order to determine which approximations used in the spectroscopic technique affect the accuracy of Λ spec. RESULTS:For all sources studied, the angular and spatial distributions of φ full were more complex than the distributions used in φ spec. Differences between Λ spec and Λ full ranged from -0.6% to +6.4%, confirming the discrepancies found by Rodriguez and Rogers. The largest contribution to the discrepancy was the assumption of isotropic emission in φ spec, which caused differences in Λ of up to +5.3% relative to Λ full. Use of the approximated spatial and energy distributions caused smaller average discrepancies in Λ of -0.4% and +0.1%, respectively. The water-only model introduced an average discrepancy in Λ of -0.4%. CONCLUSIONS:The approximations used in φ spec caused discrepancies between Λ approx,i and Λ full of up to 7.8%. With the exception of the energy distribution, the approximations used in φ spec contributed to this discrepancy for all source models studied. To improve the accuracy of Λ spec, the spatial and angular distributions of φ full could be measured, with the measurements replacing the approximated distributions. The methodology used in this work could be used to determine the resolution that such measurements would require by computing the dose-rate constants from phase spaces modified to reflect φ full binned at different spatial and angular resolutions.