PubMed Health⌕ Search
Home/Archive/PubMed

PubMed · 8912381

Variation in calculated effective source-surface distances with depth.

P M Ostwald·T Kron
View original source ↗Export metadata ↓

Abstract

Effective source-surface distances (ESSD) are assessed at the depth of maximum dose in electron beams. This study investigated the variation of the ESSD with the depth of measurement. The dose was measured with the range of SSDs 100-130 cm, using a water-equivalent parallel-plate ion chamber in solid water. ESSDs were calculated for electron beams in the energy range 4-20 MeV and were found to vary with depth. The surface ESSD varied from 68 cm for 4 MeV to 82 cm for 16 MeV, but increased with depth to a maximum value, which was found at approximately half the practical range (Rp), at 0.3Rp for 4 MeV and at 0.6Rp for 20 MeV. Beyond this depth the ESSD decreased towards the end of the practical range. Without an electron applicator, the ESSD was higher at the surface. For smaller field sizes, the depth of the maximum ESSD increased towards Rp, and ESSD values increased. The 20 MeV beam in the 6 cm x 6 cm2 field showed a difference of 31 cm between the surface ESSD and the maximum ESSD. The ESSD calculated at the maximum dose depth (Dmax) may be used with reasonable accuracy for calculation of the dose in the therapeutic range, except at larger SSDs or when high-energy beams are used in small fields Depth-dose distributions under these conditions should be compared with measured results.

Explore related subjects

ElectronsModels, TheoreticalRadiotherapyRadiotherapy DosageComparative StudyJournal Article

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

P M Ostwald, T Kron. 1996. Variation in calculated effective source-surface distances with depth.. https://doi.org/10.1088/0031-9155%2F41%2F10%2F015

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

PubMed2026

Technical note: reconstructing dose distributions from manually planned electron boosts in breast radiotherapy.

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&#xa0;<&#xa0;0.01), where the finest grid (0.15&#xa0;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&#xa0;<&#xa0;1&#xa0;Gy for breast and tumour bed. Reconstructed dose was achieved&#xa0;>&#xa0;90% gamma passing rate in the validation set. However, around 2.5&#xa0;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.

Electrons↗
PubMed1998

Enhanced electron-density envelopes by extended solvent definition.

Extended delineation of water molecules, monitored using Rfree values, afforded considerable improvement in quality of electron-density maps for structure determination of mammalian class I and E. coli class II aldolases. Augmented solvent definition results in an additional decrease in Rfree values of 3-4% and is reflected in significantly enhanced electron-density envelopes enabling tracing of amino-acid sequences through regions of otherwise discontinuous or weak electron density.

Electrons↗
PubMed1998

Determination of the relative precision of atoms in a macromolecular structure.

Several real-space indices and temperature factors are compared with respect to their correlation with atomic positional error and their ability to indicate atoms and residues with the worst of subtle errors. The best index, rED, is a correlation coefficient between model and map electron densities, similar to one proposed earlier, but incorporating two improvements. Firstly, resolution is accounted for explicitly by calculating the model electron density by Fourier transformation of resolution-truncated scattering factors. Secondly, the deviation between model and map electron densities is assigned to neighboring atoms according to their contribution to the electron density of each grid point. With maps of various qualities, rED is the single index with best correlation to atomic error with grouped or individual atoms, and it is the most reliable indicator of poor residues. With poorer omit maps, imprecision of individual atoms is best diagnosed by a combination of low rED or high B factor. With the improved methods, 60-70% of the least precise atoms can detected in a fully refined structure. Similarly, 40-80% of the least precise atoms of an unrefined model can be detected by comparison with an isomorphous replacement map. This is useful in assessing and improving the quality of a model, but not sufficient to confidently validate all atoms of a structure at sub-atomic resolution.

Electrons↗