Influence of source batch SK dispersion on dosimetry for prostate cancer treatment with permanent implants. Issue 8 (29th July 2015)

• Record Type: Journal Article
• Authors: Nuñez‐Cumplido, E.; Perez‐Calatayud, J.; Casares‐Magaz, O.; Hernandez‐Armas, J.
• Is Part Of: Medical physics. Volume 42:Issue 8(2015)Part 1
• Page Start: 4933
• Page End: 4940
• Publication Date: 2015-07-29
• Subjects: biological organs -- brachytherapy -- cancer -- dosimetry -- Gaussian distribution -- iodine -- palladium -- radioisotopes; Dose‐volume analysis -- Therapeutic applications, including brachytherapy; Radiation therapy -- Scintigraphy; prostate -- brachytherapy -- dosimetry -- batch -- class -- 125I -- 103Pd; Dosimetry -- Isotopes -- Anatomy -- Statistical distribution -- Brachytherapy -- Statistical analysis -- Manufacturing -- Cancer -- Radioactive sources -- Medical treatment planning; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4926848
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: In clinical practice, specific air kerma strength ( SK ) value is used in treatment planning system (TPS) permanent brachytherapy implant calculations with 125 I and 103 Pd 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). Methods: 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 cm 3 ) were considered and two typical commercial sources of different radionuclides were used. Using a conventional TPS, clinically accepted calculations were made for 125 I 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 ( V 100 and D 90 ) 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 V 100 and D 90 variations ranged between 0.2% and 1.78% for both sources. Variations for the overdosage parameters V 150 and V 200 compared to dose coverage parameters were observed and, in general, variations were larger for parameters related to 125 I sources than 103 Pd sources. For OAR dosimetry, variations with respect to the reference D 0.1 cm 3 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 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 V 150, rectum V 100, and rectum D 0.1 cm 3 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 125 I than for 103 Pd . This is because for 103 Pd, a greater number of sources were used to obtain a valid dose distribution than for 125 I, resulting in a lower variation for each SK value for each source (because the variations become averaged out statistically speaking).