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

Abstract

The present work is primarily focused on the estimation of relative dose distribution and effective transmission around a shielded vaginal cylinder with an 192Ir source using the Monte Carlo technique. The MCNP4B code was used to evaluate the dose distribution around a tungsten shielded vaginal cylinder as a function of thickness and angular shielding. The dose distribution and effective transmission of 192Ir by 0.8 cm thickness tungsten were also compared with that for gold and lead. Dose distributions were evaluated for different distances starting from 1.35 cm to 10.15 cm from the center of the cylinder. Dose distributions were also evaluated sequentially from 0 degrees to 180 degrees for every 5 degrees interval. Studies show that all the shielding material at 0.8 cm thickness contribute tolerable doses to normal tissues and also protect the critical organs such as the rectum and bladder. However, the computed dose values are in good agreement with the reported experimental values. It was also inferred that the higher the shielding angles, the more the protection of the surrounding tissues. Among the three shielding materials, gold has been observed to have the highest attenuation and hence contribute lowest transmission in the shielded region. Depending upon the shielding angle and thickness, it is possible to predict the dose distribution using the MCNP4B code. In order to deliver the higher dose to the unshielded region, lead may be considered as the shielding material and further it is highly economic over other materials.

Direction modulated brachytherapy (DMBT) tandem applicator for cervical cancer treatment: Choosing the optimal shielding material

“…10,16,17 With the current high dose rate (HDR) radiation sources (e.g., 192 Ir), OAR sparing is limited with conventional applicators as the dose distributions are always (quasi-)isotropic around the source. Moreover, even with an intent to deliberately create an anisotropic dose distribution through partial shielding (e.g., a single-channel shielded vaginal applicator with fixed collimation angles of 90°, 180°, or 270°1 6,18 ), such large angles may not be optimal per an early theoretical study on intensity-modulated brachytherapy (IMBT) conducted by Ebert 19 who suggested that optimal plans are generated with collimation angles in the range of 22.5°-45°. The integrated tungsten alloy shields positioned at the two ends of an ovoid applicator can reduce the radiation dose to the bladder and rectum.…”

The evolution of brachytherapy treatment planning

“…These studies identified limitations of the current protocol for clinical practice. Others have looked at dose comparison in clinical geometries such as a shielded vaginal cylinder 51,57 or shielded rectal applicator. 56 In these cases, the dose differences were dramatic.…”

Establishment of air kerma reference standard for low dose rate Cs‐137 brachytherapy sources

“…In *F5 tally, the radius of the sphere of exclusion of 1/8 to 1/2 mean free paths (a fixed value of 0.1 cm was used as exclusion distance) was maintained for particles of average energy at the sphere. ( 36 ) The air kerma was estimated from the computed photon energy fluence at a distance of 1 m from the source using the mass energy absorption coefficient $(0.0293\hspace{0.17em}{\text{cm}}^2/\mathrm{g})$. It may be noted that the energy fluence computed by the *F5 tally of MCNP includes the effect of attenuation and scattering from the source matrix and all surrounding media.…”