A quantification strategy for missing bone mass in case of osteolytic bone lesions Andrea Fränzle a) Department of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany Maren Bretschi and Tobias Bäuerle Department of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany Kristina Giske Department of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany Jens Hillengass Department of Internal Medicine V, University of Heidelberg, Im Neuenheimer Feld 410, 69120 Heidelberg, Germany Rolf Bendl Medical Informatics, Heilbronn University, Max-Planck-Strasse 39, 74081 Heilbronn, Germany and Department of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany (Received 14 June 2013; revised 4 October 2013; accepted for publication 16 October 2013; published 11 November 2013) Purpose: Most of the patients who died of breast cancer have developed bone metastases. To un- derstand the pathogenesis of bone metastases and to analyze treatment response of different bone remodeling therapies, preclinical animal models are examined. In breast cancer, bone metastases are often bone destructive. To assess treatment response of bone remodeling therapies, the volumes of these lesions have to be determined during the therapy process. The manual delineation of missing structures, especially if large parts are missing, is very time-consuming and not reproducible. Re- producibility is highly important to have comparable results during the therapy process. Therefore, a computerized approach is needed. Also for the preclinical research, a reproducible measurement of the lesions is essential. Here, the authors present an automated segmentation method for the mea- surement of missing bone mass in a preclinical rat model with bone metastases in the hind leg bones based on 3D CT scans. Methods: The affected bone structure is compared to a healthy model. Since in this preclinical rat trial the metastasis only occurs on the right hind legs, which is assured by using vessel clips, the authors use the left body side as a healthy model. The left femur is segmented with a statistical shape model which is initialised using the automatically segmented medullary cavity. The left tibia and fibula are segmented using volume growing starting at the tibia medullary cavity and stopping at the femur boundary. Masked images of both segmentations are mirrored along the median plane and transferred manually to the position of the affected bone by rigid registration. Affected bone and healthy model are compared based on their gray values. If the gray value of a voxel indicates bone mass in the healthy model and no bone in the affected bone, this voxel is considered to be osteolytic. Results: The lesion segmentations complete the missing bone structures in a reasonable way. The mean ratio v r /v m of the reconstructed bone volume v r and the healthy model bone volume v m is 1.07, which indicates a good reconstruction of the modified bone. Conclusions: The qualitative and quantitative comparison of manual and semi-automated segmenta- tion results have shown that comparing a modified bone structure with a healthy model can be used to identify and measure missing bone mass in a reproducible way. © 2013 American Association of Physicists in Medicine.[http://dx.doi.org/10.1118/1.4828843] Key words: bone metastasis, osteolytic lesions, segmentation, image analysis, preclinical animal model 1. INTRODUCTION Breast cancer is the most common invasive cancer in women. Metastasis is the major cause of deaths due to malignant tumor disease and approximately 70% of all patients who died of breast cancer develop tumors in bone as their disease progression. 1 Bone metastases are frequently associated with complications such as hypercalcemia due to osteolysis, spinal 123501-1 Med. Phys. 40 (12), December 2013 © 2013 Am. Assoc. Phys. Med. 123501-1 0094-2405/2013/40(12)/123501/8/$30.00