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Technical Note: On the calculation of stopping-power ratio for stoichiometric calibration in proton therapy
KTH, School of Engineering Sciences (SCI), Physics. Karolinska University Hospital, Sweden.
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2015 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 42, no 9, 5252-5257 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: The quantitative effects of assumptions made in the calculation of stopping-power ratios (SPRs) are investigated, for stoichiometric CT calibration in proton therapy. The assumptions investigated include the use of the Bethe formula without correction terms, Bragg additivity, the choice of I-value for water, and the data source for elemental I-values. Methods: The predictions of the Bethe formula for SPR (no correction terms) were validated against more sophisticated calculations using the SRIM software package for 72 human tissues. A stoichiometric calibration was then performed at our hospital. SPR was calculated for the human tissues using either the assumption of simple Bragg additivity or the Seltzer-Berger rule (as used in ICRU Reports 37 and 49). In each case, the calculation was performed twice: First, by assuming the I-value of water was an experimentally based value of 78 eV (value proposed in Errata and Addenda for ICRU Report 73) and second, by recalculating the I-value theoretically. The discrepancy between predictions using ICRU elemental I-values and the commonly used tables of Janni was also investigated. Results: Errors due to neglecting the correction terms to the Bethe formula were calculated at less than 0.1% for biological tissues. Discrepancies greater than 1%, however, were estimated due to departures from simple Bragg additivity when a fixed I-value for water was imposed. When the I-value for water was calculated in a consistent manner to that for tissue, this disagreement was substantially reduced. The difference between SPR predictions when using Janni's or ICRU tables for I-values was up to 1.6%. Experimental data used for materials of relevance to proton therapy suggest that the ICRU-derived values provide somewhat more accurate results (root-mean-square-error: 0.8% versus 1.6%). Conclusions: The conclusions from this study are that (1) the Bethe formula can be safely used for SPR calculations without correction terms; (2) simple Bragg additivity can be reasonably assumed for compound materials; (3) if simple Bragg additivity is assumed, then the I-value for water should be calculated in a consistent manner to that of the tissue of interest (rather than using an experimentally derived value); (4) the ICRU Report 37 I-values may provide a better agreement with experiment than Janni's tables.

Place, publisher, year, edition, pages
2015. Vol. 42, no 9, 5252-5257 p.
Keyword [en]
Proton therapy, stopping-power, CT calibration, stoichiometric method, I-values
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:kth:diva-173977DOI: 10.1118/1.4928399ISI: 000360645000025PubMedID: 26328974Scopus ID: 2-s2.0-84939438270OAI: oai:DiVA.org:kth-173977DiVA: diva2:859170
Note

QC 20151006

Available from: 2015-10-06 Created: 2015-09-24 Last updated: 2017-12-01Bibliographically approved

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