Impact of interseed attenuation and tissue composition for permanent prostate implants

Abstract

The purpose is to evaluate the impact of interseed attenuation and prostate composition for prostate treatment plans with 125I permanent seed implants using the Monte Carlo (MC) method. The effect of seed density (number of seeds per prostate unit volume) is specifically investigated. The study focuses on treatment plans that were generated for clinical cases. For each plan, four different dose calculation techniques are compared: TG-43 based calculation, superposition MC, full MC with water prostate, and full MC with realistic prostate tissue. The prostate tissue description is from the ICRP report 23 (W. S. Snyer, M. J. Cook, E. S. Nasset, L. R. Karkhausen, G. P. Howells, and I. H. Tipton, "Report of the task group on reference man," Technical Report 23, International Commission on Radiological Protection, 1974). According to the comparisons, the seed density has an influence on interseed attenuation. A plan with a typical low seed density (42 0.6 mCi seeds in a 26 cm3 prostate) suffers a 1.2% drop in the CTV D90 value due to interseed attenuation. A drop of 3.0% is calculated for a higher seed density (75 0.3 mCi seeds, same prostate). The influence of the prostate composition is similar for all seed densities and prostate sizes. The difference between MC simulations in water and MC simulations in prostate tissue is between 4.4% and 4.8% for the D90 parameter. Overall, the effect on D90 is ranging from 5.8% to 12.8% when comparing clinically approved TG-43 and MC simulations in prostate tissue. The impact varies from one patient to the other and depends on the prostate size and the number of seeds. This effect can reach a significant level when reporting correlations between clinical effect and deposited dose.

Effect of chemical composition and density of the pelvic structure in intracavitary brachytherapy dosimetry

“…Whole phantom differences of 8% and 8.3% on average dose rates were found between the media, for 192 Ir and 137 Cs, respectively. These results are in good agreement with those reported by Carrier et al (2006). Regarding the specific voxels x¼ À6, À 4, À 2, 2, 4 and 6 cm and z¼0, the average dose rate difference was 9.2% between the two media.…”

Quantifying the effect of seed orientation in postplanning dosimetry of low‐dose‐rate prostate brachytherapy

“…Finally, urethra V125 is investigated as it has been correlated with toxicity in the recent Radiation Oncology Therapy Group-0321 (Refs. [36][37][38] results.…”

Sensitivity of low energy brachytherapy Monte Carlo dose calculations to uncertainties in human tissue composition

“…Since the photoelectric effect is the dominant interaction mode for the low energy photons emitted by isotopes such as 103 Pd, 125 I, and 131 Cs, or by electronic brachytherapy ͑EBS͒ sources, tissue heterogeneities and interseed attenuation influence dosimetry. [9][10][11][12][13][14][15][16] Postsurgical clips and prostate calcifications may also have an influence. Furthermore, the continuous low energy spectrum of EBS sources is subject to a non-negligible spectral hardening effect.…”