3317 - Calculation of Dose Perturbations Caused by 2 mm of Intraprostatic Calcification for Permanent Prostate Seed Implant Using Pd103 in Excel/Python

A. F. Youssef¹, D. Alxander¹, and A. Youssef²; ¹Guthrie/Lourdes Memorial Hospital, Binghamton, NY, ²New Giza Medical School, Giza, Egypt

Purpose/Objective(s): Prostate seed implant is one of the standard treatments used in treating prostate cancer. Using low energy namely as in Pd¹⁰³ seeds with average energy of 21 Kev can cause significant dose changes when it interacts with intraprostatic calcifications. At low energies the photoelectric effect which is dependent on the atomic number of the tissue (Z³), is responsible for more than 50% of the interactions with tissues and about 93% of the absorbed dose. Intraprostatic calcification with high atomic number (20) can affect the dose absorption. Intraprostatic calcification is a common finding in older patients and can be present in various parts of the prostate. This study is an attempt to calculate the dose perturbations caused by intraprostatic calcification in prostate seed implants. Materials/

Methods: An excel sheet was made to represent one slice of prostate with 0.5 cm thickness. Each cell was made to represent 0.5 cm of the prostate with a volume of 0.125 cm³. 11 seeds of Pd103 were placed as per protocol along the periphery and core of the prostate. The dose contribution of each seed was calculated according to the TG43 equation to every cell in that plan. A comparison plan was made using commercial software. An assumption was made that a 2 mm intraprostatic calcification was present in 5 different positions, 2 around the core and 3 at the periphery. The interaction of the 2 mm calcifications on dose changes were calculated using the TG43 equation in excel/python.
Results: Our dose calculations matched the commercial software with 92% accuracy. 2mm of calcification placed behind one seed decreased the dose by 87%. The maximum attenuation in the multiple seed configuration ranged from 8% to 69% depending on the position of calcifications in relation to the seeds.
Conclusion: Intraprostatic calcification can cause significant dose changes in prostate seed implant using low energy seed isotopes. The effects can result in lower dose coverage around the prostate periphery or increased dose absorbed close to the urethra resulting in unexpected side effects or underdosing of tumor.