Development of a Quantitative Method Based on the Hill-Shading Technique for Assessing Morphological Changes in the Bone During Image-Guided Radiotherapy for Bone Metastasis

Abstract

We hypothesized that positioning corrections matching the image in image-guided radiotherapy (IGRT) for bone metastasis contain information on temporal structural changes in irradiated bone metastatic lesions during the treatment period. To extract and quantify these changes, a hill-shading technique was used to emphasize the characteristics of the structure. Spatial frequency components of the bone lesions were described based on hillshading transformations of the images, and a quantification method was suggested. The matching images of 11 patients who received IGRT for lytic bone metastases were evaluated retrospectively. The hill-shading technique was applied to images of both metastatic lesions and normal bone outside the irradiation field. Outlined bone microstructures were analyzed by a two-dimensional power spectrum using fast Fourier transformation, and the frequency components were quantified. Correlations between the frequency components and cumulative radiation doses were analyzed between the irradiated metastatic bone and normal bone outside the irradiation field. The high-frequency components of the metastatic bone lesion images decreased by a mean of 7% (minimum:−0.2%, maximum: −13.2%) following cumulative irradiation doses of 20–30 Gy versus 0–10 Gy. In the normal bone outside the irradiation field, high-frequency components increased by a mean of 0.07% (minimum:−2.0%, maximum: +4.4%) following irradiation doses of 20–30 Gy versus 0–10 Gy. High-frequency components were significantly different between the normal bone and bone metastases following radiotherapy (p <0.0001). Hill-shading transformation of matching images obtained during the treatment period enabled adaptation and evaluation of irradiation-induced changes in the microstructure of bone metastatic lesions.