Medical Physics Department, University Hospital of the Canary Island, La Cuesta – Ofra, 38003 La Laguna, Spain and Medical Physics Department, Aarhus University Hospital, Nørrebrogade 44, 8000 Aarhus, Denmark
In clinical practice, specific air kerma strength (SK) value is used in treatment planning system (TPS) permanent brachytherapy implant calculations with 125I and 103Pd sources; in fact, commercial TPS provide only one SK input value for all implanted sources and the certified shipment average is typically used. However, the value for SK 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 SK values. The purpose of this work is to examine the impact of SK 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).
The authors have developed a new algorithm to compute dose distributions with different SK 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 SK values for each source belonging to a class, the authors assigned an SK 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 SK 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.
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 were observed for rectum values, ranging from 2% to 3%, compared with urethra values, which ranged from 1% to 2%.
Dose coverage for prostate and PTV was practically unaffected by SK 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 values. The variations in rectum parameters were caused by the specific location of sources with SK value that differed from the average in the vicinity. Finally, on comparing the two sources, variations were larger for than for . This is because for , a greater number of sources were used to obtain a valid dose distribution than for , resulting in a lower variation for each SK value for each source (because the variations become averaged out statistically speaking).