Cysts in the peritoneal cavity elicit little pericyst reaction because they grow without compressing surrounding host parenchyma, which is the main source of pericyst tissue (Fig.3.16). Brain cysts tend to be discovered before they develop daughters and matrix. Because they evoke little host reaction, brain lesions have an insubstantial pericyst (Fig. 3.17). Orbital hydatids cause unilateral exophthalmos and, if untreated, slowly compress the globe and enlarge the bony orbit, causing unilateral blindness (Fig. 3.18). Bone cysts occur most often in the pelvis, vertebral column, long bones, and skull and cause destruction without evoking bone reaction. A true pericyst develops only if cortical erosion allows involvement of soft tissue. This accounts for the often bizarre appearance of bony hydatids and possibly for their unusual propensity to cross joints and involve adjacent bones (Fig. 3.19).

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