Conversion of CT numbers into tissue parameters for Monte Carlo dose calculations: a multi-centre study

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Computational Dosimetry

Background:

• Accurate patient dose calculations in Monte Carlo treatment planning (MCTP) depend on converting computed tomography (CT) numbers to material composition and mass density.
• Conventional CT conversion schemes may not adequately represent complex tissue heterogeneities, particularly bone.

Purpose of the Study:

• To develop and validate a novel CT conversion scheme using stoichiometric CT calibration with an increased number of tissue bins.
• To compare the dosimetric accuracy of the proposed scheme against a conventional binning method in clinical patient cases.

Main Methods:

• A stoichiometric CT calibration was performed to create fourteen dosimetrically equivalent tissue bins, including ten specifically for bone.
• The proposed CT conversion scheme was validated on phantoms and subsequently compared to a standard five-bin scheme in nine clinical patient cases from a multi-centre study.
• Dose distributions (dose to medium) and dose-volume histograms were calculated and compared for both schemes.

Main Results:

• Local relative differences in dose to medium between the 14-bin and 5-bin schemes were generally less than 5%.
• While dose-volume histograms for targets and organs at risk were comparable, the 14-bin scheme yielded systematically higher doses within bony structures.
• Converting dose to medium to dose to water revealed larger discrepancies, with the 5-bin scheme's dose-to-water (D(5wat)) being up to 10% higher than the 14-bin scheme's (D(14wat)).

Conclusions:

• The study concludes that utilizing multiple bone bins in CT conversion is essential for accurate Monte Carlo (MC) calculations of patient dose distributions.
• Accurate conversion to dose to water requires a more refined tissue-equivalent binning strategy than conventional methods provide.
• The proposed 14-bin scheme offers improved accuracy for dose-to-water calculations, particularly in regions with significant bone.