PubMed · 5377676
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G Porter. 1969. Molecules in microtime.. https://doi.org/10.1259/0007-1285-42-503-801
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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.
PURPOSE: An innovative technique is used to harvest backscatter electrons for the treatment of superficial small lesions of skin, oral cavity, and rectum where a significant dose gradient and maximum surface dose is desired. METHODS AND MATERIALS: Backscatter electrons are harvested out of the primary electron beams from the linear accelerators. The design consists of a short cylindrical cone that fits snugly over a long cylindrical electron cone. The short cylindrical cone has a thick circular plate of high atomic number medium (Pb) attached to the distal end, and a lateral slit of variable length and width. The width of the slit could be closed as desired by rotating the two cones and the length can be increased by lowering the short cylindrical cone. Primary electrons strike the Pb plate perpendicularly and produce backscatter electrons that pass through the lateral slit for treatment. Using film and a parallel plate ion chamber, backscattered electron dose characteristics are studied. RESULTS: The depth dose characteristic of the backscatter electron is very similar to that of the 0.2 mm Al half-value layer x-ray beam that is commonly used for the intracavitary and superficial lesions. The backscatter electron energy is nearly constant and effectively < or = 1 MeV from the clinical megavoltage beams. The backscatter electron dose rate of 0.32-0.8 Gy/min could be achieved from modern accelerators without any modification. The beam flatness is dependent on the slit size and the depth of treatment, but is satisfactory to treat small lesions. CONCLUSIONS: The measured data for backscatter electron energy, fluence, depth dose, flatness, dose rate, and absolute dose indicates that the harvested backscattered electrons are suitable for clinical use.