Psammoma body

**Definition:** A psammoma body (Greek psammos = sand) is a spherical, concentrically laminated, dystrophically calcified deposit that appears basophilic (purple) on H&E. It is also called a "calcospherite."

**Pathophysiology:** Formation begins with a nidus, typically a necrotic or degenerating cluster of tumor cells or a small infarcted papillary tip, often a twisted or whorled cell cluster. Dystrophic calcification then occurs in concentric layers (calcium phosphate/hydroxyapatite) around the nidus. Serum calcium is normal because this is dystrophic, not metastatic, calcification. The calcified layers produce the characteristic "onion-skin" lamellated appearance.

**High-yield associations (mnemonic: PSaMMoma):** - **P**apillary carcinoma of the thyroid. Also look for Orphan Annie-eye nuclei (optically clear nuclei), nuclear grooves, and pseudoinclusions. Psammoma bodies here suggest papillary architecture and a high likelihood of lymphatic spread. - **S**erous cystadenocarcinoma of the ovary (and serous endometrial carcinoma). Psammoma bodies are common in the serous subtype. - **M**eningioma. Psammoma bodies are seen with whorled spindle cell patterns, and the tumor is often NF2-associated (chromosome 22). It is typically a benign, extra-axial, dural-based tumor. - **M**esothelioma (asbestos-associated; pleural, with Calretinin positivity).

**Other associations (less common):** Papillary renal cell carcinoma, somatostatinoma of the pancreas, prolactinoma (pituitary adenoma, rarely), and some pancreatic neuroendocrine tumors. They can also be seen in benign conditions (for example, cervical Pap smears and chronic inflammatory processes) and in pineal gland "brain sand."

**Clinical significance:** - Visible on imaging as small calcifications (for example, in thyroid ultrasound as punctate microcalcifications, which raise suspicion for papillary carcinoma, and on CT of meningiomas). - In ovarian and endometrial tissue, psammoma bodies are a clue to serous histology. - Their presence in a cytology or biopsy specimen from an unusual site (for example, a lymph node) can suggest metastasis from a papillary thyroid or ovarian serous primary.

**Exam pearls:** - Psammoma bodies are a form of dystrophic calcification (normal serum calcium), distinct from metastatic calcification (hypercalcemia). - "Whorled pattern + psammoma bodies" = meningioma. - "Orphan Annie nuclei + psammoma bodies" = papillary thyroid carcinoma. - "Ovarian mass + elevated CA-125 + psammoma bodies" = serous cystadenocarcinoma.