[Combined internal-external radiotherapy (CIERT) in a cell model].

Area of Science:

• Radiobiology
• Medical Physics

Background:

• Combined internal-external radiotherapy (CIERT) necessitates unified biologic radiation effect assessment.
• The biological effective dose (BED) concept aids in evaluating total radiation dose.
• A cell model was used to assess BED validity for different radiation types and dose rates.

Purpose of the Study:

• To evaluate the biological effective dose (BED) concept in a cell model for combined internal-external radiotherapy (CIERT).
• To compare cell survival predictions from the BED model with experimental results from X-ray and Tc-99m irradiation.

Main Methods:

• FRTL-5 thyroid cells were irradiated with X-ray and Tc-99m pertechnetate, alone and in combination.
• Cellular uptake of Tc-99m was controlled, and dose calculations were based on measured uptake.
• Cell survival was assessed using the colony forming assay, with BED model predictions compared to experimental data.

Main Results:

• Radiobiologic parameters for X-ray irradiation were determined (α = 0.22 ± 0.02 Gy⁻¹, β = 0.021 ± 0.001 Gy⁻²), with a repair half-life of 1.51 ± 0.21 h.
• The BED model accurately explained dose response curves for X-ray and Tc-99m irradiation.
• When Tc-99m was taken up intracellularly, cell survival was lower than predicted by the BED model, indicating supra-additive effects.

Conclusions:

• The BED formalism is valid for varying dose rates and radiation qualities.
• Supra-additive effects observed with intracellular Tc-99m may stem from Auger and conversion electrons.
• Further research is needed to fully understand the mechanisms behind supra-additive effects in CIERT.