Dosimetric intercomparison of permanent Ho-166 seed's implants and HDR Ir-192 brachytherapy in breast cancer

Abstract

Aim

To provide a comparative dosimetric analysis of permanent implants of Ho¹⁶⁶-seeds and temporary HDR Ir¹⁹²-brachytherapy through computational simulation.

Background

Brachytherapy with Ir¹⁹²-HDR or LDR based on temporary wires or permanent radioactive seed implants can be used as dose reinforcement for breast radiation therapy. Permanent breast implants have not been a practical clinical routine; although, I¹²⁵ and Pd¹⁰³-seeds have already been reported. Biodegradable Ho¹⁶⁶-ceramic-seeds have been addressed recently.

Material and methods

Simulations of implants of nine Ho¹⁶⁶-seeds and equivalent with HDR Ir¹⁹²-brachytherapy were elaborated in MCNP5, shaped in a computational multivoxel simulator which reproduced a female thorax phantom. Spatial dose rate distributions and dose–volume histograms were generated. Protocol's analysis involving exposure time, seed's activities and dose were performed.

Results

Permanent Ho¹⁶⁶-seed implants presented a maximum dose rate per unit of contained activity (MDR) of 1.1601[[ce:hsp sp="0.25"/]]μGy[[ce:hsp sp="0.25"/]]h⁻¹[[ce:hsp sp="0.25"/]]Bq⁻¹; and, a normalized MDR in standard points (8[[ce:hsp sp="0.25"/]]mm, equidistant to 03-seeds – SP₁, 10[[ce:hsp sp="0.25"/]]mm – SP₂) of 1.0% (SP₁) and 0.5% (SP₂), respectively. Ir¹⁹²-brachytherapy presented MDR of 4.3945[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]10⁻³[[ce:hsp sp="0.25"/]]μGy[[ce:hsp sp="0.25"/]]h⁻¹[[ce:hsp sp="0.25"/]]Bq⁻¹; and, 30% (SP₁), and 20% (SP₂). Therefore, seed's implant activities of 333[[ce:hsp sp="0.25"/]]MBq (Ho¹⁶⁶) and 259[[ce:hsp sp="0.25"/]]GBq (Ir¹⁹²) produced prescribed doses of 58[[ce:hsp sp="0.25"/]]Gy (SP₁; 5d) and 56[[ce:hsp sp="0.25"/]]Gy (SP₁, 5 fractions, 6[[ce:hsp sp="0.25"/]]min), respectively.

Conclusions

Breast Ho¹⁶⁶-implants of 37–111[[ce:hsp sp="0.25"/]]MBq are attractive due to the high dose rate near 6–10[[ce:hsp sp="0.25"/]]mm from seeds, equivalent to Ir¹⁹²-brachytherapy of 259[[ce:hsp sp="0.25"/]]GBq (3 fractions, 6[[ce:hsp sp="0.25"/]]min) providing similar dose in standard points at a week; however, with spatial dose distribution better confined. The seed positioning can be adjusted for controlling the breast tumor, in stages I and II, in flat and deep tumors, without any breast volumetric limitation.