Monte Carlo aided room scatter studies in the primary air kerma strength standardization of a remote afterloading¹⁹²Ir HDR source

Abstract

Corrections for room scatter, [ksc(d, H)]RM, were derived using Monte Carlo methods for rooms of various sizes, including the brachytherapy treatment room, in which primary air kerma strength standardization of a remote afterloading microSelectron 192Ir HDR source was carried out, using a 60 cm3 spherical graphite ion chamber. It was observed that for a given source-to-floor height, H, the air kerma rate due to room scatter was found to be decreasing with increase in source-to-detector distance, d, whereas it is assumed to be constant in the experimental determination (multiple-distance method) of the air kerma strength (AKS). Irrespective of the room size considered in the present study and when H = 100 cm, the assumption of 'constant room scatter' for d = 50, 75 and 100 cm resulted in overestimation of the AKS by about 1%. However, when d was in the range 13-25 cm, it was only 0.20%. The room scatter correction for the shadow cone method was also computed and the result showed that no additional correction was required on account of forward scatter from the cone. Further, at larger d (50-100 cm), the number of low-energy photons (40-200keV) relative to the total number of photons was found to be very significant, which may be useful to revisit the Spencer-Attix cavity theory.

Comparison of air‐kerma strength determinations for HDR 192Ir sources

“…Selvam et al 19 presented Monte Carlo calculations of room scatter from an HDR 192 Ir source and determined that air-kerma strength could be overestimated by 1% if constant scatter is assumed and large measurement distances are used (>50 cm). To determine whether the assumption of constant room scatter was valid for measurements at the UWMRRC, a series of Monte Carlo simulations were performed using MCNP5.…”

“…The algorithm used with the seven-distance technique introduced by Goetsch et al 2 assumes that the room scatter component of the ionization chamber signal is constant over all seven measurement distances. Selvam et al 19 presented Monte Carlo calculations of room scatter from an HDR 192 Ir source and determined that air-kerma strength could be overestimated by 1% if constant scatter is assumed and large measurement distances are used (>50 cm). To determine whether the assumption of constant room scatter was valid for measurements at the UWMRRC, a series of Monte Carlo simulations were performed using MCNP5.…”

Computation of relative dose distribution and effective transmission around a shielded vaginal cylinder with Ir192 HDR source using MCNP4B

“…MCNP4B gives the users the added flexibility to define their own geometrical regions from the surfaces of analytical geometry and then to combine them with Boolean operators. 16 Pointwise cross-section data are used. For photons, the code takes account of incoherent and coherent scattering, the possibility of fluorescent emission after photoelectric absorption, absorption in pair production with local emission of annihilation radiation, and bremsstrahlung.…”

“…The density of stainless steel for both capsules as well as cables was assumed as 8.02 g cm −3 as per the specification of AISI 304. 16 FIG. 2.…”

Determination of contributions of scatter and distance error to the source strength of 192Ir HDR brachytherapy source

“…The in-air measurement of source strength using cylindrical ionization chamber requires the determination of scatter radiation from walls and floor of the room and also the offset in positioning of the chamber. Few studies [4][5][6][7][8][9][10] have been reported for determining the room scatter contribution from an 192 Ir HDR brachytherapy source. Goestch et al [4] described a method for calibrating 192 Ir HDR brachytherapy afterloading systems.…”