Which RBE for iodine 125 in clinical applications?

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiobiology

Background:

• Iodine-125 (125I) is utilized in interstitial therapy, necessitating accurate relative biological effectiveness (RBE) determination.
• Microdosimetric analysis reveals 125I's gamma spectra resemble conventional X-rays (12-140 keV), distinct from other gamma sources like Cobalt-60 (60Co) and Cesium-137 (137Cs).

Purpose of the Study:

• To discuss the selection of an appropriate RBE value for Iodine-125 (125I) in clinical interstitial therapy applications.
• To reconcile microdosimetric and radiobiological data for 125I to inform clinical RBE selection.

Main Methods:

• Review of microdosimetric studies characterizing 125I emission spectra.
• Analysis of existing radiobiological data from various biological systems and endpoints.
• Comparison of 125I RBE values with reference radiation sources (e.g., 60Co).

Main Results:

• Microdosimetry indicates 125I's spectral similarity to conventional X-rays, differing from 60Co, 137Cs, and high-energy X-rays.
• Radiobiological data show a range of RBE values for 125I (relative to 60Co) from 1 to 2.4.
• Consistent RBE values of 1.15-1.20 are observed at high doses and dose rates, while higher RBEs (up to 2) are seen at low doses/dose rates.

Conclusions:

• The RBE of 125I is dose and dose-rate dependent, aligning with theoretical and microdosimetric findings.
• Clinical application of 125I in interstitial therapy requires careful consideration of these RBE variations.
• Further research may refine 125I RBE selection for optimized therapeutic outcomes.