Skeletal Oncology

Leonard Rosenthall and Ralph Wong Introduction

Radioisotopes play an important role in the diagnosis of benign and malignant bone lesions. Bone scanning is commonly used to detect metastases from malignant tumors because it is more sensitive than x-rays and allows the whole body to be surveyed. The general principles of skeletal imaging are reviewed in Chapter 7. This Chapter will concentrate on aspects specific to bone tumors.

Primary Benign Bone tumors

Benign and malignant primary bone tumors can be quite difficult to distinguish with radiophosphate imaging. In general, benign lesions tend to have lower accretions than malignant lesions, but in the individual case it is neither reliable nor predictive of the histopathological outcome.

Adamantinoma

Adamantinoma is a rare tumor that occurs most commonly in the mid-diaphysis of the tibia. In rare cases it can behave like a malignant tumor with metastases to the lung. It appears as a large radiolucent lesion on the radiograph, and shows moderate uptake of radiophosphate.

Aneurysmal Bone Cyst

Aneurysmal bone cyst (ABC) is a distinctive non-neoplastic solitary lesion of bone consisting of a cystic cavity filled with nonendothelial-lined spaces containing blood. Radiographically, there is a striking "blow-out" distention of part of the contour of the affected bone, and it may simulate a malignant lesion. More than half of these lesions occur in the metaphyseal regions of the long bones or vertebrae, and up to one-third are linked to benign or malignant processes such as nonossifying fibroma, chondroblastoma and giant cell tumor of bone. Due to their location and potentially rapid, aggressive growth, they can be difficult to distinguish from a primary malignancy. Despite the presence of blood filled cavities, the perfusion segment of the three-phase radiophosphate bone study may be negative. More than half of the delayed images depict a doughnut pattern of uptake (central photopenia encompassed by a band of activity).

Bone island

A bone island is a common lesion that consists of a nest of compacted trabeculae of mature lamellar bone. It may enlarge after puberty, but it does not have malignant potential. Delayed bone scans are usually normal but may show a mild increased uptake (Fig. 1).

Nuclear Medicine, edited by William D. Leslie and I. David Greenberg. ©2003 Landes Bioscience.

Figure 1. Bone island. Upper: Radiograph of a large sclerotic lesion in the intertrochanteric region. Lower: Radiophosphate bone scan shows no evidence of increased uptake. This confirms that the lesion is metabolically inactive.

Chondroblastoma

Chondroblastoma is an uncommon primary bone tumor of chondrogenic origin that is seen in younger individuals before epiphyseal closure. It usually has its origin from the region of the growth plate, most commonly in the proximal humerus, but it can arise in flat bones. They are composed of dense concentrations of rounded or polyhedral chondroblasts. Histological features are similar to chondromyxoid fibroma and the two may be related. Malignant transformation rarely can occur. Radiophosphate concentration in these lesions can be intense.

Desmoplastic Fibroma

Desmoplastic fibroma is an extremely rare benign intraosseous fibrous tumor occurring mostly in the long bones. Three quarters of the cases are seen below the age of 30 years, with peak incidence in the second decade of life. The tumor is usually located centrally in the metaphysis where it symmetrically expands the bone and thins the cortex. Radiographically it appears as an osteolytic lesion with well circumscribed margins. Nonspecific increased uptake of radiophosphate can be seen.

Enchondroma

Enchondroma is a common benign lesion that appears in the medullary portion of bone and is composed chiefly of mature hyaline cartilage. The solitary enchondroma has a predilection for the phalanges and metacarpals of the hand. It can also occur in large long and flat bones (pelvic and shoulder girdle) where they have a potential for malignant transformation, and may be difficult to distinguish from a low grade chondrosarcoma. The bone scan may demonstrate normal or mildly increased uptake of radiophosphate (Fig. 2). A lesion which changes from normal to high uptake over time, especially when associated with pain, is suspect for malignant conversion.

Multiple enchondromas, or enchondromatosis, is an anomaly usually disclosed in infancy. When one side of the body exhibits greater involvement than the other, the condition is called Ollier's disease. The combination of enchondromatosis and soft tissue hemangiomata is referred to as Maffucci's syndrome. About 8% of the cases of Maffucci's syndrome degenerate into chondrosarcomas.

Epidermoid Cyst

Epidermoid cysts are sharply delineated, squamous cell lined and filled with desquamated keratinized debris. They occur in the calvarium, but traumatic epidermoid cysts may develop in the phalanges. The scintigraphic presentation in the calvarium is usually normal, but if the lesion is large enough a photopenic focus may be appreciated.

Fibrous Defects

Fibrous defects are focal collections of nonspecific fibrous tissue located predominantly in the cortical bone, but they may also be found in cancellous bone. Sites of predilection are the long bones, primarily the femur and tibia. Several classifications have been proffered for these fibrous lesions, but a simplified version divides them into fibrous cortical defects and nonossifying fibromas. The former occurs in about 30% of children over 2 years of age, and radiographically it presents

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