Monte Carlo dosimetry study of novel rotating MRI-compatible shielded tandems for intensity modulated cervix...

Intensity modulated brachytherapy (IMBT) with rotating metal shields enables dose modulation that can better conform to the tumor while reducing OAR doses. In this work, we investigate novel rotating shields, compatible with MRI-compatible tandems used for cervix brachytherapy. Three unique shields were evaluated using the traditional ¹⁹²Ir source. Additionally, ⁷⁵Se and ¹⁶⁹Yb isotopes were investigated as alternative sources.

Three different IMBT shields were modeled and simulated in RapidBrachyMCTPS. Each tungsten shield was designed to fit inside a 6 mm-wide MRI-compatible tandem. The active core of the source was replaced with ¹⁹²Ir, ⁷⁵Se and ¹⁶⁹Yb. Transmission factors (TFs) were calculated and defined as the dose ratio at 1 cm on opposite sides of the shielded tandem on the transverse plane. Polar and azimuthal anisotropy plots were extracted from simulations. Dose homogeneities V_200%V_100% were calculated for all radionuclide-shield combinations.

TFs are favorable for IMBT and ranged between 12.9% and 32.2% for ¹⁹²Ir, 4.0%-16.1% for ⁷⁵Se and 1.2-6.4% for ¹⁶⁹Yb for all shield designs. Average beam-widths in the polar and azimuthal directions were reduced to the range of 42°-112° and 27°-107°, respectively, for all shield-radionuclide combinations. Dose homogeneities for all the radionuclide-shield combinations were within 12% of the non-IMBT tandem.

This study has quantitatively assessed the influence of various rotating cervical cancer-specific IMBT tandem shields on dosimetry. The dynamic single-channel shields and narrow beam-widths in the polar and azimuthal direction give rise to highly anisotropic distributions. Intermediate-to-high energy radionuclides, ⁷⁵Se and ¹⁶⁹Yb substantially improve the modulation capacity of IMBT and pave the way for treating large and complex cervical cancer without interstitial needle implantation.