PubMed · 7003074
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W G Myers. 1980. PET = positron+electron transmutation.. https://pubmed.ncbi.nlm.nih.gov/7003074/
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PURPOSE: In breast radiotherapy, delivery of manually-calculated electron boosts limits retrospective dose-response analyses as dose distribution is unavailable. This work evaluates the feasibility of reconstructing dose distributions from manually planned electron boosts in breast-conserving radiotherapy. METHODS: Only 72 out of 198 breast cancer patients had complete stored dose distributions from sequential electron boosts in the REQUITE study. Arbitrary data from 70/72 patients were used to develop and validate dose reconstruction method. Twenty patients were used to determine optimal parameters for Monte-Carlo-based (MC) electron dose reconstruction on RayStation (v.11B-R), considering CT-calibration curve, MC-history number, andcalculation grid resolution. Remaining 50 patients were used to quantify dose reconstruction accuracy. The similarity between reconstructed and stored dose was evaluated using 3D-gamma index and dosimetric parameters extracted from breast and tumour bed contours. Dose difference location was evaluated using dose-location histogram. RESULTS: Calculation grid resolution significantly impacted electron dose distribution (p < 0.01), where the finest grid (0.15 cm) showed highest similarity to stored doses. CT-calibration curve and MC-history number had a negligible influence on dose reconstruction. Dosimetric difference between reconstructed and stored doses was < 1 Gy for breast and tumour bed. Reconstructed dose was achieved > 90% gamma passing rate in the validation set. However, around 2.5 Gy dose differences were observed at the skin and tissue interface regions. CONCLUSIONS: Retrospective electron boost dose reconstruction is feasible with acceptable accuracy, and could increase data completeness in large cohort studies. Caution is advised when assessing dose near tissue interface and further validation is needed outside the REQUITE dataset.
This paper characterizes the functional dependence of the giant dipole resonance neutron yield produced by electrons in terms of the atomic number (Z) and thickness (T) of the target. The yields were calculated by integrating, over the photon energy, the product of the differential photon track length and published photoneutron cross sections. The EGS4 Monte Carlo code and analytical formulas were used to calculate the differential photon track length. In thick targets, the Giant Dipole Resonance neutron yield approaches a saturation value as target thickness T increases to 10 radiation lengths. A formula, 8 x 10(-6) x (Z1/2 + 0.12 Z3/2 - 0.001 Z5/2) n electron-1 MeV-1, developed from EGS4 calculations, estimates thick-target neutron yields for incident electron energies Eo above 50 MeV. Giant dipole resonance neutron yields, calculated by several analytic formulas for the differential photon track length, are compared with EGS4 calculations. Modifications to the analytic formulas are suggested. A scaling function is derived to estimate, from the thick-target formula, neutron yields produced in thin targets.
Organ and effective doses per unit of fluence are calculated through Monte Carlo simulation of radiation transport (MCNP code, version 4) for the mathematical adult male phantom, ADAM, placed in vacuum and irradiated in the AP direction with broad unidirectional electron beams. The electrons are monoenergetic in the energy range of 0.1 tp 10 MeV. Effective dose per unit of fluence increases with increasing electron energy from 8x10(-14) to 2x10(-10) Sv cm(-2). Corresponding effective dose equivalents are also calculated. At the high end of the energy range considered, the present values agree reasonably well with the results of a series of calculations performed with the MIRD-5 male phantom in the 5 to 46 MeV electron energy range, as published in ICRU Report 43 (1988). The difference between effective dose and effective dose equivalent is relatively small for electron energies between 1.5 and 10 MeV. However, the difference increases rapidly if the electron energy decreases. Below 0.5 MeV, effective dose equivalent is about a factor of 100 less than effective dose. In this low energy range, annual dose limits are reached sooner if effective dose instead of effective dose equivalent is used as parameter of risk assessment. Taking the directional dose equivalent as a substitute for effective dose introduces an unnecessarily large safety factor.