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Stopping‐power ratios for electron beams used in total skin electron therapy
Author(s) -
Ding George X.
Publication year - 2021
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.15121
Subject(s) - dosimetry , stopping power , electron , cathode ray , monte carlo method , imaging phantom , nuclear medicine , beam (structure) , linear particle accelerator , physics , materials science , absorbed dose , percentage depth dose curve , ionization chamber , atomic physics , optics , ionization , mathematics , nuclear physics , medicine , statistics , ion , quantum mechanics , detector
Purpose The electron beams for total skin electron therapy (TSET) are often degraded by a scatter plate in addition to extended distances. For electron dosimetry, both the AAPM TG‐51 and IAEA TRS‐398 recommend the use of two formulas developed by Burns et al [Med. Phys. 23, 489–501 (1996)] to estimate the water‐to‐air stopping‐power ratios (SPRs). Both formulas are based on a fit to SPRs calculated for standard electron beams. This study aims to find: (1) if the formulas are applicable to beams used in TSET and (2) the impact of the ICRU report 90 recommendations on the SPRs for these beams. Methods The EGSnrc Monte Carlo code system is used to generate 6 MeV high dose rate total skin electron (HDTSe) beams used in TSET. The simulated beams are used to calculate dose distributions and SPRs as a function of depth in a water phantom. The fitted SPRs using the empirical formulas are compared with MC‐calculated SPRs. Results The electron beam quality specifier, the depth in water at which the absorbed dose falls to 50% of its maximum value, R 50 , decreases approximately 1 mm for each additional 100‐cm extended distance ranging from 2.24 cm at SSD = 100 to 1.72 cm at SSD = 700 cm. For beams passing through a scatter plate, R 50 is 1.76 cm (1.14) at SSD = 300 and 1.48 cm (0.85 cm) at SSD = 600 cm with an Acrylic plate thickness of 3 mm (9 mm), respectively. The discrepancy between fitted and MC‐calculated SPRs at d ref as a function of R 50 is <0.8%, and in many cases <0.4%. The difference between fitted and MC‐calculated SPRs as a function of depth and R 50 is within 1% at depths <0.8R 50 for beams with R 50 ≥ 1.14 cm. The ICRU‐90 recommendations decrease SPRs by 0.3%–0.4% compared to the use of data recommended in ICRU‐37. Conclusion The formulas used by the major protocols are accurate enough for clinical beams used in TSET and the error caused using the formulas is <1% to estimate SPRs as a function of depth and R 50 for depths <0.8R 50 for beams used in TSET with R 50 ≥ 1.14 cm. The impact of the ICRU‐90 recommendations shows a decrease of SPRs by a fraction of a percent for beams used in TSET.