Bone marrow reconversion mimicking pelvis metastases in a patient with rectal cancer: a pitfall on magnetic resonance images.

Abstract

The bone marrow is one of the largest organs in the body. There are two main types of bone marrow in human body: red and yellow bone marrow in the human body. Their proportions are determined by the overall distribution of fat and water, which ultimately determines the magnetic resonance imaging (MRI) signal intensity. At birth, red bone marrow (40% fat, 40% water and 20% protein) is present throughout the entire skeleton. It gradually converts into yellow bone marrow (80% fat, 15% water and 5% protein) (1). The conversion begins in the extremities and progresses toward the axial skeleton, and it can always be completed by approximately 25 years of age (1-3). When the existing hematopoietic marrow cannot meet the demand for hematopoiesis, the body shifts marrow distribution with replacement of yellow marrow by red marrow, which is called bone marrow reconversion (BMR) (4). However, the occurrence of BMR can be misleading, and its interpretation of imaging can be challenging. Regarding the patients with malignancies, it is occasionally difficult to differentiate BMR from bone metastasis, raising the risk of delayed or missed diagnoses. BMR located in some vertebrae of patients with malignant tumor mimicking bone metastases has been reported by sporadic literatures (5-8). However, little literature reports BMR located in the pelvis. Herein, we report a case of BMR in a patient with rectal cancer which was misdiagnosed as bone metastases by a radiologist using original pelvic MRI.