Dose specification for Ir-192 high dose rate brachytherapy in terms of dose-to-water-in-medium and dose-to-medium-in-medium

Author(s)

Fonseca, Gabriel Paiva ^{[1, 2]} ; Tedgren, Asa Carlsson ^{[3, 4]} ; Reniers, Brigitte ^{[1, 5]} ; Nilsson, Josef ^[3] ; Persson, Maria ^[3] ; Yoriyaz, Helio ^[2] ; Verhaegen, Frank ^{[6, 1]}

Affiliation

^[1] Maastricht Univ, Med Ctr, GROW Sch Oncol & Dev Biol, Dept Radiat Oncol MAASTRO, NL-6201 BN Maastricht - Netherlands

^[2] IPEN CNEN SP, Sao Paulo - Brazil

^[3] Karolinska Univ Hosp, Dept Med Phys, Sect Radiotherapy Phys & Engn, Stockholm - Sweden

^[4] Linkoping Univ, Dept Med & Hlth Sci IMH, Med Radiat Phys, Linkoping - Sweden

^[5] Hasselt Univ, CMK, Res Grp NuTeC, B-3590 Diepenbeek - Belgium

^[6] McGill Univ, Dept Oncol, Med Phys Unit, Montreal, PQ H3G 1A4 - Canada

• Document type: Journal article
• Source: Physics in Medicine and Biology; v. 60, n. 11, p. 4565-4579, JUN 7 2015.
• Web of Science Citations: 6

Abstract

Dose calculation in high dose rate brachytherapy with Ir-192 is usually based on the TG-43U1 protocol where all media are considered to be water. Several dose calculation algorithms have been developed that are capable of handling heterogeneities with two possibilities to report dose: dose-to-medium-inmedium (D-m,D-m) and dose-to-water-in-medium (D-w,D-m). The relation between D-m,D-m and D-w,D-m for Ir-192 is the main goal of this study, in particular the dependence of D-w,D-m on the dose calculation approach using either large cavity theory (LCT) or small cavity theory (SCT). A head and neck case was selected due to the presence of media with a large range of atomic numbers relevant to tissues and mass densities such as air, soft tissues and bone interfaces. This case was simulated using a Monte Carlo (MC) code to score: D-m,D-m, D-w,D-m (LCT), mean photon energy and photon fluence. D-w,D-m (SCT) was derived from MC simulations using the ratio between the unrestricted collisional stopping power of the actual medium and water. Differences between D-m,D-m and D-w,D-m (SCT or LCT) can be negligible (<1%) for some tissues e.g. muscle and significant for other tissues with differences of up to 14% for bone. Using SCT or LCT approaches leads to differences between D-w,D-m (SCT) and D-w,D-m (LCT) up to 29% for bone and 36% for teeth. The mean photon energy distribution ranges from 222 keV up to 356 keV. However, results obtained using mean photon energies are not equivalent to the ones obtained using the full, local photon spectrum. This work concludes that it is essential that brachytherapy studies clearly report the dose quantity. It further shows that while differences between D-m,D-m and D-w,D-m (SCT) mainly depend on tissue type, differences between D-m,D-m and D-w,D-m (LCT) are, in addition, significantly dependent on the local photon energy fluence spectrum which varies with distance to implanted sources. (AU)

• FAPESP's process: 11/22778-8 - 3D dosimetry based on medical images and Monte Carlo codes to use in brachytherapy
• Grantee: Hélio Yoriyaz
• Support Opportunities: Regular Research Grants
• FAPESP's process: 11/23765-7 - Dose-volume relationship in planning systems for 3D brachytherapy using mcnp5 code, brachyvision and oncentra
• Grantee: Gabriel Paiva Fonseca
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate
• FAPESP's process: 11/01913-4 - Monte Carlo modelling of the patient and treatment delivery complexities for high dose rate brachytherapy
• Grantee: Gabriel Paiva Fonseca
• Support Opportunities: Scholarships in Brazil - Doctorate