On the conversion of dose to bone to dose to water in radiotherapy treatment planning systems

Abstract

Background and purpose: Conversion factors between dose to medium (D m,m) and dose to water (D w,w) provided by treatment planning systems that model the patient as water with variable electron density are currently based on stopping power ratios. In the current paper it will be illustrated that this conversion method is not correct. Materials and methods: Monte Carlo calculations were performed in a phantom consisting of a 2 cm bone layer surrounded by water. D w,w was obtained by modelling the bone layer as water with the electron density of bone. Conversion factors between D w,w and D m,m were obtained and compared to stopping power ratios and ratios of mass-energy absorption coefficients in regions of electronic equilibrium and interfaces. Calculations were performed for 6 MV and 20 MV photon beams. Results: In the region of electronic equilibrium the stopping power ratio of water to bone (1.11) largely overestimates the conversion obtained using the Monte Carlo calculations (1.06). In that region the MC dose conversion corresponds to the ratio of mass energy absorption coefficients. Near the water to bone interface, the MC ratio cannot be determined from stopping powers or mass energy absorption coefficients. Conclusion: Stopping power ratios cannot be used for conversion from D m,m to D w,w provided by treatment planning systems that model the patient as water with variable electron density, either in regions of electronic equilibrium or near interfaces. In regions of electronic equilibrium mass energy absorption coefficient ratios should be used. Conversions at interfaces require detailed MC calculations.

Beam modeling and beam model commissioning for Monte Carlo dose calculation‐based radiation therapy treatment planning: Report of AAPM Task Group 157

“…Reporting dose-to-water or dose-to-medium has been discussed in TG105 report 7 and issues related to correlating patient dose distributions calculated using "correctionbased" algorithms and those calculated using MC dose algorithms for outcome studies have been further investigated and remain to be a topic of discussion. 119,120 • Correction factors should be used for specific detectors used for dose measurement to account for the effects of detector size, shape, and composition. For example, inherent chamber build-up may affect entrance dose measurements and the detector geometry may cause large angular dependence.…”

Dose to medium in head and neck radiotherapy: Clinical implications for target volume metrics

“…The dosimetric accuracy of LBTE has been investigated for a range of materials and treatment geometries and techniques [6][7][8][9], the findings generally indicating improved accuracy of the LBTE algorithm over Type B algorithms. The impact of reporting to D w and D m has been discussed for lung [10][11][12], breast [13], bone [14,15] and head and neck [16][17][18]. Recommendations have also been made for reporting dose in the routine clinical setting and in clinical trials [4,19] in the context of both Monte Carlo and LBTE algorithms.…”

AAPM Task Group 329: Reference dose specification for dose calculations: Dose‐to‐water or dose‐to‐muscle?

“…). Such a simple conversion is not appropriate for media that are much different from water (e.g., bone), and this report does not attempt to address the still‐debated issue of how to handle the situations when this type of medium is of interest . Neither are we concerned with the relative accuracy of different algorithms in calculating dose in and around drastic inhomogeneities (air, lung, or bone).…”

“…This “1%” was the water phantom to muscle conversion factor historically used in clinical reference calibration. It was recently pointed out that the approach is more general compared to the one, which breaks down numerically for bone. Neither approach is valid at the interface of different materials, where detailed Monte Carlo calculations are necessary …”