Investigation of real tissue water equivalent path lengths using an efficient dose extinction method

Area of Science:

• Medical Physics
• Radiation Oncology
• Biomedical Engineering

Background:

• Accurate conversion of CT Hounsfield Units (HU) to relative stopping power (RSP) is critical for proton therapy dose calculations.
• Current validation methods rely on tissue substitutes, which may not fully represent biological tissue variations.
• Direct validation using real tissue samples can improve the accuracy and population-specific applicability of CT to RSP conversion.

Purpose of the Study:

• To evaluate the accuracy of CT HU to RSP conversion using a novel dose extinction method on animal tissue samples.
• To assess potential variations in RSP conversion across different biological tissue samples and populations.
• To validate the accuracy of the dose extinction method itself using known tissue substitutes and calibration blocks.

Main Methods:

• A novel dose extinction method utilizing a broad proton beam and a 2D ion-chamber detector was employed.
• Water Equivalent Path Lengths (WEPL) were measured on animal tissue samples by analyzing transmission dose profiles.
• Measurements were validated against tissue substitutes and Lucite blocks with known WEPLs; comparisons were made with CT-derived RSP using the Stoichiometric calibration method.

Main Results:

• The dose extinction method achieved high accuracy for tissue substitutes (±0.5% deviation/error) and calibration blocks (±0.5% deviation/error).
• Measurements on biological tissue samples showed deviations within ±0.3%, with minimal differences (<1%) observed for tissues from different sources.
• Differences between measured WEPLs and CT-calculated values were within 1%, with exceptions noted for some bony tissues.

Conclusions:

• The dose extinction system provides an efficient and accurate method for measuring WEPL, enabling robust validation of CT HU to RSP conversions.
• Direct validation with real tissue samples confirms the accuracy of CT to RSP conversion and reveals minimal inter-sample variability for most tissues.
• This approach supports the development of more precise proton therapy planning by addressing population-specific variations in tissue properties.