Which localization technique offers the best accuracy in brachytherapy?

• A. Single film technique
• B. Orthogonal films
• C. CT-based localization
• D. Magnetic resonance imaging

Answer: (C)

Accurate localization in brachytherapy hinges on obtaining a true 3D map of the implanted applicators and surrounding anatomy so the planned dwell positions and doses align with reality. CT-based localization provides three-dimensional visualization of the entire applicator geometry in relation to patient anatomy, enabling precise reconstruction of each dwell position and channel path. From the CT data, treatment planning systems can extract exact coordinates and incorporate tissue electron density, which directly affects dose calculations. This combination reduces geometric and dosimetric uncertainties and improves the match between planned and delivered dose.

Single-film and orthogonal-film methods offer only 2D projections, making it hard to reconstruct complex applicator geometry or depth relative to structures, leading to localization errors. MRI provides excellent soft-tissue contrast but can suffer from artifacts and challenges with accurately reconstructing source paths for dose calculations, especially when metallic applicators are present. In practice, CT-based localization gives the most reliable, quantitative 3D information for precise brachytherapy dose delivery.