Conversion of Greyscale Intensity Values from CBCT Images Acquired on Elekta XVI to HU for Treatment Planning Dose Calculations

Abstract

Purpose: To develop and evaluate a method to convert the greyscale intensity values from CBCT images acquired on an Elekta XVI into HU for treatment planning dose calculations.

Material: The method presented in this work is based on the automatic detection of the air and soft tissue peaks from the greyscale intensity histograms of the acquired CBCT images. The CIRS model 062 electron density phantom was scanned on a Siemens Somatom Sensation Open, and the resulting linear relationship between the electron densities and HU was used as reference. For each CBCT image, a linear conversion was determined by matching the greyscale intensity values detected for the air and soft tissue peaks to the reference values. The method was evaluated by performing treatment planning dose calculations on an Alderson head phantom, Alderson pelvis phantom and a thorax patient. For the Alderson head and pelvis phantoms, one 6 MV, 10x10 cm² field, 1 Gy to the isocenter was used. And for the thorax patient, one 6 MV, 10x20 cm² field, 1 Gy to the dose maximum was used.

Oncentra MasterPlan was used for dose calculation and the DICOM RT dose results were exported to MapCHECK for gamma analysis.

Results: The gamma analysis criteria were set to 3% and 3 mm and absolute dose was selected for comparison between the two DICOM RT dose results. With these criteria the pass rate was 93% or better for the isocenter plane and selected planes located between ±40 mm from the isocenter.

Conclusion: Our results show that the presented method for automatic conversion of CBCT greyscale images to HU works as expected. This makes it possible to use CBCT images acquired on an Elekta XVI for dose calculations, for instance after anatomical changes during the course of treatment.