Monte Carlo investigation of I-125 interseed attenuation for standard and thinner seeds in prostate brachytherapy with phantom validation using a MOSFET

Journal Article · Fri Mar 15 04:00:00 EDT 2013 · Medical Physics

DOI:https://doi.org/10.1118/1.4793256· OSTI ID:22130555

Mason, J.; Al-Qaisieh, B.; Bownes, P. ^[1]; Henry, A. ^[2]; Thwaites, D. ^[3]

---

1. Department of Medical Physics and Engineering, St. James's Institute of Oncology, St. James's University Hospital, Leeds, LS9 7TF (United Kingdom)
2. Clinical Oncology, St. James's Institute of Oncology, St. James's University Hospital, Leeds, LS9 7TF (United Kingdom)
3. Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia and Academic Unit of Medical Physics, University of Leeds, LS2 9JT (United Kingdom)

Purpose: In permanent seed implant prostate brachytherapy the actual dose delivered to the patient may be less than that calculated by TG-43U1 due to interseed attenuation (ISA) and differences between prostate tissue composition and water. In this study the magnitude of the ISA effect is assessed in a phantom and in clinical prostate postimplant cases. Results are compared for seed models 6711 and 9011 with 0.8 and 0.5 mm diameters, respectively. Methods: A polymethyl methacrylate (PMMA) phantom was designed to perform ISA measurements in a simple eight-seed arrangement and at the center of an implant of 36 seeds. Monte Carlo (MC) simulation and experimental measurements using a MOSFET dosimeter were used to measure dose rate and the ISA effect. MC simulations of 15 CT-based postimplant prostate treatment plans were performed to compare the clinical impact of ISA on dose to prostate, urethra, rectum, and the volume enclosed by the 100% isodose, for 6711 and 9011 seed models. Results: In the phantom, ISA reduced the dose rate at the MOSFET position by 8.6%-18.3% (6711) and 7.8%-16.7% (9011) depending on the measurement configuration. MOSFET measured dose rates agreed with MC simulation predictions within the MOSFET measurement uncertainty, which ranged from 5.5% to 7.2% depending on the measurement configuration (k= 1, for the mean of four measurements). For 15 clinical implants, the mean ISA effect for 6711 was to reduce prostate D90 by 4.2 Gy (3%), prostate V100 by 0.5 cc (1.4%), urethra D10 by 11.3 Gy (4.4%), rectal D2cc by 5.5 Gy (4.6%), and the 100% isodose volume by 2.3 cc. For the 9011 seed the mean ISA effect reduced prostate D90 by 2.2 Gy (1.6%), prostate V100 by 0.3 cc (0.7%), urethra D10 by 8.0 Gy (3.2%), rectal D2cc by 3.1 Gy (2.7%), and the 100% isodose volume by 1.2 cc. Differences between the MC simulation and TG-43U1 consensus data for the 6711 seed model had a similar impact, reducing mean prostate D90 by 6 Gy (4.2%) and V100 by 0.6 cc (1.8%). Conclusions: ISA causes the delivered dose in prostate seed implant brachytherapy to be lower than the dose calculated by TG-43U1. MC simulation of phantom seed arrangements show that dose at a point can be reduced by up to 18% and this has been validated using a MOSFET dosimeter. Clinical simulations show that ISA reduces DVH parameter values, but the reduction is less for thinner seeds.

• OSTI ID: 22130555
• Journal Information: Medical Physics, Journal Name: Medical Physics Journal Issue: 3 Vol. 40; ISSN 0094-2405; ISSN MPHYA6
• Country of Publication: United States
• Language: English

Related Subjects

60 APPLIED LIFE SCIENCES
61 RADIATION PROTECTION AND DOSIMETRY
62 RADIOLOGY AND NUCLEAR MEDICINE
ATTENUATION
BRACHYTHERAPY
COMPARATIVE EVALUATIONS
COMPUTERIZED SIMULATION
COMPUTERIZED TOMOGRAPHY
DOSE RATES
DOSES
DOSIMETRY
IMPLANTS
IODINE 125
METHACRYLATES
MONTE CARLO METHOD
MOSFET
PATIENTS
PHANTOMS
PMMA
PROSTATE
RECTUM
SEEDS