Other diagnostic tests may be needed to confirm a diagnosis of mediastinal lymphoma, such as thoracic ultrasonography with
fine-needle aspiration or Tru-Cut biopsies of the suspect mass. Cytologic or histologic confirmation may differentiate lymphoma
from thymoma, another tumor occasionally associated with paraneoplastic hypercalcemia in dogs and cats (Figures F & G). Abdominal radiography, abdominal ultrasonography, and bone marrow aspiration and cytology are recommended in cases of hypercalcemia
of unknown origin and suspected lymphoid tumors. Humoral hypercalcemia of malignancy with normal PTHrP concentrations may
be identified in some lymphoma cases and may occur because PTHrP can be sporadically released by the tumor or because some
lymphoma cells possess 1 alpha-hydroxylase activities that lead to a syndrome of dysregulated 1,25-dihydroxycholecalciferol
(calcitriol) formation, although this mechanism of paraneoplastic hypercalcemia has been rarely reported in companion animals.9,15-19
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