The aneurysmal bone cyst (ABC) is a benign but rapidly and aggressively growing tumor that occurs in adolescents and young adults. It was first described by Jaffe and Lichtenstein in 1942.[1] 80% of the patients with ABCs are younger than twenty years.[2] ABCs can appear in the whole skeleton but usually affect the metaphysis of long bones or the pelvis [3]. 70% of the cysts appear as primary lesions, 30% are secondary with preexisting osseous lesions.[4]
Patients usually present with pain and swelling in the affected region and sometimes pathological fractures can occur. If the cyst expands progressively, it forms an expansile mass and may be visible or palpable. In plain radiographs an osteolytic, multilobulated lesion can be detected; the magnetic resonance imaging shows cystic formations with typical fluid-fluid levels due to blood sedimentation.[5] Two major classifications are used for distinguishing cystic lesions of the bone: the Enneking [6] (stage 1: latent cyst; stage 2: active cyst; stage 3 aggressive cysts) and the Capanna [7] classification (type 1: central location without expansile growth; type 2 whole bony segment is affected, expansile growth; type 3: only one metaphyseal cortex is affected; type 5: periosteal lesion with peripheral expansion and cortical erosion). ABCs have to be diagnosed by biopsy and histopathological examinations.[8]
The pathophysiology of ABCs is debatable.[4] Former studies claimed that an increased vascular pressure in the venous network of the cyst leads to destruction of the bone matrix.[5, 9, 10] Recent cytogenetic and molecular studies underline the neoplastic character of ABCs due to of specific translocations of the ubiquitin-specific protease (USP) 6 gene.[11] USP6 gene arrangements are seen in 70% of primary ABCs and are lacking in secondary ABCs[12]. Histologically ABCs contain blood filled spaces surrounded by variable thick fibrous septae. Within the septae are uniform blad spindle cells, multinucleated giant cells, capillaries and varying amount of matrix findable.[4] In immunohistochemical studies CD68 and P63 were specific for ABCs, whereas only CD68 showed prognostic value for local recurrence.[13] By fluorescent in situ hybridization and next generation sequencing USP6 gene arrangements can be detected in up to 100% of the cases.[14] With these examinations primary ABCs can be differentiated from secondary ABCs and other bone affecting tumors. Other bone pathologies like a chondroblastoma, a giant cell tumor of bone, a chondromyxoid fibroma, a fibrous dysplasia or a telangiectatic osteosarcoma have to be excluded. In rare cases a malignant transformations of ABCs is reported.[4]
Different treatment modalities are described in the literature and can be divided into nonoperative, minimally invasive and operative treatment options. Among those are wide resection, marginal resection, intralesional resection such as curettage with or without adjuvants, selective arterial embolization, intralesional sclerotherapy using polidocanol or the systemic application of denosumab.[5] The optimal treatment, especially in the pelvis is still under discussion. A wide surgical resection results in best local control rate but can evoke other complications depending on the dimension of the resection and the localization.[15] Especially in the pelvis a wide resection may cause severe restrictions on the functional outcome. The most common therapy for ABCs is an intralesional curettage with the use of adjuvants. Adjuvants can be the use of a high-speed drill, phenol, hydrogen peroxide or cement augmentation with polymethylacrylat (PMMA). Novais et al. described intralesional curettage extended with a high-speed burr and bone grafting in a study with 13 patients with an ABC of the pelvis as sufficient way of therapy.[16] Even local MRI-guided percutaneous cryoablation has been used as adjuvant therapy.[17] Local recurrence rates after intralesional curettage of up to 50% are reported in the literature.[18] Transarterial embolization of ABCs can be used as adjuvant or single therapy when surgical resection is difficult or connected with high risk of complications. Henrichs et al. reported the successful use of transarterial embolization in ABCs of the sacrum as single therapy. [19] Sclerosants such as polidocanol can be used for instillation of the cyst and lead stepwise to a resolution of the cystic lesion and progressive sclerosis. Rastogi et al. reported a cure rate of 97% in a study with 72 patients with primary ABC.[20] The RANKL inhibitor denosumab can be used in cases when surgical procedures and/or embolization are insufficient or without effect.
Purpose of the study: Several case reports and only one larger historic case series about the manifestation of ABCs in the pelvis and sacrum are findable in the literature.[21–23] The present study describes 17 cases of an ABC of the pelvis treated by intralesional curettage, percutaneous instillation of polidocanol and, in selected cases, arterial embolization.