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.
A dedicated electron-only intra-operative suite has just been completed at the Massachusetts General Hospital. This suite is located on the 3rd floor with the shielding consisting entirely of lead and borated polyethylene, except for concrete in the floor and ceiling to support the lead and on the finished floor. The radiation protection barriers for this facility were calculated on the basis of a maximum permissible dose of 10 microSv wk(-1) for photons and 10 microSv wk(-1) for neutrons, based on a quality factor of 10 (this factor is specified in the regulations of the Commonwealth of Massachusetts). This even split was predicated on the basis that, except for the primary beam, the neutron leakage from the machine was generally about the same as for x rays. The initial survey showed that the neutron dose equivalent outside all barriers, except below the floor, was within the calculated values, < 10 microSv wk(-1). However, the neutron dose equivalent measured directly below the floor was found to be 35 microSv wk(-1) for the highest electron energy, a difference of > 25 microSv wk(-1). The neutron dose equivalent, for neutrons produced by bremsstrahlung photon interactions in the lead and based on a 3% photon background in the electron beam, was calculated to be 25 microSv wk(-1), in good agreement with this difference.
In plants and algae, photosystem II uses light energy to oxidize water to oxygen at a metalloradical site that comprises a tetranuclear manganese cluster and a tyrosyl radical. A model is proposed whereby the tyrosyl radical functions by abstracting hydrogen atoms from substrate water bound as terminal ligands to two of the four manganese ions. Molecular oxygen is produced in the final step in which hydrogen atom transfer and oxygen-oxygen bond formation occur together in a concerted reaction. This mechanism establishes clear analogies between photosynthetic water oxidation and amino acid radical function in other enzymatic reactions.