The lesion is subperiosteal and has a cap of hyaline cartilage where the cartilage cells are sometimes regularly aligned or irregular and contain minute foci of ossification (Fig. 9.6). As age advances, the mass progressively ossifies.

These tumours are benign and usually cease to grow after skeletal maturation. Removal is therefore curative.

The earlier name ‘giant cell reparative granuloma’ derives from the unconvincing idea (also applied to solitary bone cysts) that it was a reaction to trauma which caused intramedullary haemorrhage. The cause is unknown though they are generally considered hyperplastic or reactive rather than neoplastic. Identical lesions occur in Noonan syndrome, the genetic cause of which is known to be mutation of the PTPN11 gene and a chromosomal translocation t(X;4) has been identified in a long bone lesion.

Giant cell granuloma is usually seen in young people under 20 and in females twice as frequently as males. The mandible, anterior to the first molars, where the teeth have had deciduous predecessors, is the usual site. There is frequently only a painless swelling, but growth is sometimes rapid and the mass can, rarely, erode through the bone, particularly of the alveolar ridge, to produce a purplish soft-tissue swelling (Fig. 9.7).

Radiographs show a rounded cyst-like radiolucent area, often faintly loculated or with a soap-bubble appearance (Fig. 9.8). Roots of teeth can be displaced or occasionally resorbed.

Giant cell granuloma forms a lobulated mass of proliferating vascular connective tissue packed with giant cells (osteoclasts) (Figs 9.9 and 9.10). Signs of bleeding into the mass and deposits of haemosiderin are frequently seen. Fibroblastic proliferation or prominent osteoid and bone formation are common. There are no changes in blood chemistry.

Curettage of giant cell granulomas is usually adequate and wide excision unnecessary; small fragments that may be left behind may cause little trouble, rarely require further treatment and may resolve spontaneously. Recurrence follows ­incomplete removal and a further limited operation may become necessary.

Key features are summarised in Box 9.2.

Haemangiomas of bone are essentially similar to those in soft tissues. They are usually cavernous (Fig. 9.11), but there is also an arteriovenous type (fast-flow angioma) which has large feeder arteries, tends to expand rapidly and is likely to bleed severely if opened.

A haemangioma may not be suspected if its vascularity is not obvious. Opening it or extracting a related tooth may therefore release torrents of blood, occasionally with fatal results. However, once the diagnosis has been made, wide en bloc resection is the only practical treatment. If there are identifiable feeder vessels selective arterial embolisation makes surgery considerably safer and may be all that is required.

A molecular genetic study has shown chromosomal deletions at 1p together with translocations t(11;22) in a small number of cases. What genes are affected by these chromosomal anomalies is unknown. However, these changes are distinct from other neuroectodermal tumours, such as neuroblastoma or Ewing’s sarcoma, demonstrating that this lesion is a distinct entity.

Melanotic neuroectodermal tumour consists of slit-like or larger spaces lined by cuboidal, melanin-containing cells in a fibrous stroma. Small round cells, which are non-pigmented, form clusters in the stroma or lie in the alveolar spaces (Figs 9.13 and 9.14). Some degree of atypia and infrequent mitotic figures are often seen. The round cells resemble neuroblasts and may be associated with neurofibrillary material indicating neural differentiation.

Ultrastructurally, pigmented cells show both epithelial and melanocytic features. Cells with melanosomes in various stages of maturation, and cells with neuroblastic features, including neurosecretory granules and cytoplasmic processes may be seen.

Frequent mitotic figures and loss of differentiation of the melanocyte-like cells suggest more aggressive behaviour. In a tumour that recurred and later disseminated widely after treatment, only the melanocyte-like cells remained at autopsy.

Radiography, CT scans and MRI are not diagnostic in themselves, but the appearance of a destructive maxillary tumour in the maxilla in an infant, together with submucosal pigmentation and high levels of urinary vanillylmandelic acid are distinctive. Serum levels of epinephrine, norepinephrine, vanillylmandelic acid and neuron-specific enolase may be high but typically return to normal after treatment. The clinical and radiographic features of melanotic neuroectodermal tumour of infancy are so distinctive that preliminary biopsy which may provoke the tumour into greater activity should not be necessary.

These tumours are frequently described as benign, but up to 15% may recur and a very few have metastasised and disseminated widely. Histology is an unreliable guide to behaviour, as even those tumours which show histologically malignant features may do well. Nevertheless, a high mitotic index, and possibly CD99 expression, may indicate more aggressive behaviour.

Surgery is the treatment of choice, but the extent of the resection is controversial and can range from local excision and curettage to total maxillectomy. The tumour is non-encapsulated but usually separates easily from the bone at operation. Conservative treatment is usually curative but, even when excision is incomplete, recurrence is rare. Regional lymph node dissection may be necessary only for rare cases where there has been lymphatic invasion. Radiotherapy and chemotherapy (undesirable in an infant) have been shown to be ineffective in controlling this tumour.

Key features are summarised in Box 9.4.