The dosimetric effect of prostatic calcifications on Low Dose Rate (LDR) brachytherapy

Low Dose Rate brachytherapy is a widely used procedure in Australia and in the world to cure prostate cancer. Commercial Treatment Planning System (TPS) used in every day clinical practice approximates all tissues to water, neglecting the existence of calcifications. However, their presence in the prostate may perturb the dose because of their higher photoelectric effect cross section in comparison to water. This study addresses quantitatively the effect of prostatic calcification in the clinical outcome of brachytherapy treatment, through of experimental measurements and Monte Carlo simulations. Four pathological calcification samples were characterised with µ-PIXE to determine their heavy elemental composition. Spectral analysis detected eight elements with atomic number greater than 13. It was discovered that calcium, phosphorus and zinc were the most predominant heavy elements in the calcification composition. The dosimetric effect of the four calcification samples was studied experimentally. Each calcification was embedded in a PMMA phantom and located between a closely placed I-125 brachytherapy seed and a MOSkinTMdetector. Dose rate reductions between 3.41% and 40.12% were observed. Geant4 based simulations were used to evaluate quantitatively the dosimetric effect of calcifications. Four real patient brachytherapy treatments were modelled in the simulation experimental set-up, in terms of distribution of brachytherapy seeds and calcifications in the prostate. Different calcification compositions were modelled. Dose reductions were observed to be around 7-20% and 20-30% for small and average sized calculi, respectively. Single large calcifications and closely placed calculi caused dose reductions between 30-60%. It was also found that calculi smaller than 0.5mm in diameter have a negligible dosimetric impact. The simulation study showed a varying reduction on dosimetric parameters when a hydroxyapatite composition was adopted to model the patients’ calcifications. It was calculated that the parameter D90 was reduced by 2-3% regardless of calcification surface area and volume. The parameters V100, V150 and V200 were reduced by as much as 2.5% and on average by 1%. These reductions were also found to relate to the surface area and volume of calcifications. This work shows that calcifications can have a statistically significant impact on the clinical outcome of brachytherapy treatment. Such impact depends strongly on specific factors in the patient anatomy and treatment. These factors include the brachytherapy seeds distribution, the number, size, composition and spatial distribution of calcifications in the prostate