Influence of source batch SK dispersion on dosimetry for prostate cancer treatment with permanent implants.

Purpose: In clinical practice, specific air kerma strength (S_K) value is used in treatment planning system (TPS) permanent brachytherapy implant calculations with ¹²⁵I and ¹⁰³Pd sources; in fact, commercial TPS provide only one S_K input value for all implanted sources and the certified shipment average is typically used. However, the value for S_K is dispersed: this dispersion is not only due to the manufacturing process and variation between different source batches but also due to the classification of sources into different classes according to their S_K values. The purpose of this work is to examine the impact of S_K dispersion on typical implant parameters that are used to evaluate the dose volume histogram (DVH) for both planning target volume (PTV) and organs at risk (OARs). Methods: The authors have developed a new algorithm to compute dose distributions with different S_K values for each source. Three different prostate volumes (20, 30, and 40 cm3) were considered and two typical commercial sources of different radionuclides were used. Using a conventional TPS, clinically accepted calculations were made for ¹²⁵I sources; for the palladium, typical implants were simulated. To assess the many different possible S_K values for each source belonging to a class, the authors assigned an S_K value to each source in a randomized process 1000 times for each source and volume. All the dose distributions generated for each set of simulations were assessed through the DVH distributions comparing with dose distributions obtained using a uniform S_K value for all the implanted sources. The authors analyzed several dose coverage (V₁₀₀ and D₉₀) and overdosage parameters for prostate and PTV and also the limiting and overdosage parameters for OARs, urethra and rectum. Results: The parameters analyzed followed a Gaussian distribution for the entire set of computed dosimetries. PTV and prostate V₁₀₀ and D₉₀ variations ranged between 0.2% and 1.78% for both sources. Variations for the overdosage parameters V₁₅₀ and V₂₀₀ compared to dose coverage parameters were observed and, in general, variations were larger for parameters related to 125I sources than ¹⁰³Pd sources. For OAR dosimetry, variations with respect to the reference D_0.1cm³ were observed for rectum values, ranging from 2% to 3%, compared with urethra values, which ranged from 1% to 2%. Conclusions: Dose coverage for prostate and PTV was practically unaffected by S_K dispersion, as was the maximum dose deposited in the urethra due to the implant technique geometry. However, the authors observed larger variations for the PTV V₁₅₀, rectum V₁₀₀, and rectum D_0.1cm³ values. The variations in rectum parameters were caused by the specific location of sources with S_K value that differed from the average in the vicinity. Finally, on comparing the two sources, variations were larger for ¹²⁵I than for ¹⁰³Pd. This is because for ¹⁰³Pd, a greater number of sources were used to obtain a valid dose distribution than for ¹²⁵I, resulting in a lower variation for each S_K value for each source (because the variations become averaged out statistically speaking). [ABSTRACT FROM AUTHOR]