Stopping‐power ratios for electron beams used in total skin electron therapy

Abstract

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, R50, 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, R50 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 dref as a function of R50 is <0.8%, and in many cases <0.4%. The difference between fitted and MC‐calculated SPRs as a function of depth and R50 is within 1% at depths <0.8R50 for beams with R50 ≥ 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 R50 for depths <0.8R50 for beams used in TSET with R50 ≥ 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.

Skin dose distributions between Stanford and rotational techniques in total skin electron therapy (TSET)

“…Although for rotational technique similar mean dose coverages at the 1st 5 mm skin are seen, the dose coverages at the 2nd, and the 3rd 5 mm skin are significantly reduced for patient treated at the extended distance 500 cm. This is because the depth, R 50 , in water at which the absorbed dose falls to 50% of its maximum value, decreases approximately 1 mm for each additional 100 cm extended distances 17 . The thickness of the acrylic scatter plate needs to be reduced in order to achieve the same skin depth coverage.…”

“…This is because the depth,R 50 ,in water at which the absorbed dose falls to 50% of its maximum value, decreases approximately 1 mm for each additional 100 cm extended distances. 17 The thickness of the acrylic scatter plate needs to be reduced in order to achieve the same skin depth coverage. Therefore, when patients are treated at an extended distance of 700 cm, there is no need to have an acrylic scatter plate in the patient treatment.…”

“…The Monte Carlo EGSnrc system 12–14 was used to generate the incident beams, 6 MeV high dose rate total skin electron (HDTSe) beams, from Varian Clinac and TrueBeam accelerators, which are currently used in our institutions for TSET. The EGSnrc 12,13 system has been successfully used to simulate electron beams used in TSET in recent years 11,15–17 …”

“…The EGSnrc 12,13 system has been successfully used to simulate electron beams used in TSET in recent years. 11,[15][16][17] The generation of the incident electron beams and the validation of the beam simulation for the 6 MeV HDTSe beam from Varian TrueBeam used in this study have been reported in a previous study. 11 The simu-lated beams were stored in phase-space files for patient dose calculations, and the default EGSnrc 13 simulation parameters were used in this study.…”

Technical note: Bremsstrahlung dose in the electron beam at extended distances in total skin electron therapy

“…13,14 There are very limited measured bremsstrahlung data for extended distances greater than 300 cm. Investigation by Chen et al 9 showed that bremsstrahlung dose is only ∼1% even at SSD > 500 cm for a 6 MeV beam from Varian 21EX-S and compared with the data of Das et al 12 The discrepancy was assumed to be caused by different 6 MeV beams generated from a Varian 21 EX-S and Saturne-I machines reported by Das et al 12 The results from recent Monte Carlo studies of 6 MeV beams by Ding et al 15,16 did not find a significant increase in bremsstrahlung doses even at extended distances of up to 700 cm. These results challenge the accuracy of the common belief that the bremsstrahlung dose is up to 5% at an extended distance of 500 cm.…”