Ectopic production of parathyroid hormone by carcinoma of the breast

Fine Needle Aspiration Biopsy Cytology of Argyrophilic Carcinoma (Carcinoid Tumor) of the Breast

Argyrophilic carcinoma of the breast, previously referred to as carcinoid tumor, is a rare form of ductal carcinoma that can be diagnosed by fine needle aspiration biopsy. This tumor is characterized by widespread cytoplasmic granules with affinity for reduced silver stains or argyrophilia. The tumor tends to occur in older women and behaves in a fashion similar to classic ductal carcinoma. The argyrophilia may reflect stored neurosecretory granules or, less frequently, mucinous or lactational changes. Carcinoid or other neuroendocrine syndromes are not observed.

Immunohistochemical Study of Ten Cases of Argyrophilic Carcinoma (Carcinoid) of the Breast

The significance of argyrophilia in human breast cancer is still a controversial issue. We tested immunohistochemically 10 cases of argyrophilic carcinomas of the breast and found evidence of immunoreactivity with neuroendocrine markers: chromogranin, NSE, gastrin, insulin and bombesin. Argyrophilia was demonstrated in breast cancers of the usual types and was found to be related to the secretory activity of neoplastic cells. Unfortunately, no adequate follow-up data are available to clarify the natural history of argyrophilic breast cancer. A clinical treatment different from that of conventional breast cancer is not at present justified.

Mammary Carcinoma with Argyrophilic Cells: An Immunohistochemical and Ultrastrctural Study

The immunohistochemical and ultrastructural findings of 7 primary argyrophilic tumors of the breast are described. Five cases were selected because of a growth pattern reminiscent of a « carcinoid » tumor and 2 were obtained from 70 consecutive common carcinomas. All patients were females (mean age 68.8) and none had clinical evidence of hormonal secretion. On ultrastructural examination dense-core granules were seen in addition to findings more suggestive of a common carcinoma. Immunoperoxidase stainings for ACTH, calcitonin etc. were negative in all tumors. The authors believe that organoid growth pattern, argyrophilia and dense-core neurosecretory-like granules are not sufficient elements to demonstrate the endocrine nature of a breast neoplasm.

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

A case of endocrine ductal carcinoma of the breast

A case of endocrine ductal carcinoma of the breast is presented. A 65-year-old woman was admitted with complaints of left breast mass and pain. Physical examination, mammography, ultrasonography, and computed tomography showed a mass 5 cm in diameter in the left breast suggestive of breast cancer, and incisional biopsy confirmed ductal carcinoma. Auchincloss's mastectomy was performed. The tumor, 4.0 x 3.8 cm in size, consisted of a relatively uniform proliferation of tumor cells with round nuclei and abundant eosinophilic cytoplasm. Immunohistochemically, tumors cells were positive for chromogranin A, synaptophysin, and neuron-specific enolase. Endocrine ductal carcinoma with invasion was diagnosed. No lymph node metastasis was observed, and estrogen and progesterone receptors were positive.

Treatment of skeletal disease in breast cancer with clodronate

Complications of breast cancer involving the skeleton include hypercalcaemia, bone pain and fracture. These complications arise because of progressive osteolysis which is in turn dependent on the activation of osteoclasts by tumour and host tissues. Clodronate is a powerful inhibitor of osteoclastic bone resorption which led us to evaluate its potential in metastatic breast cancer. When given intravenously it lowers serum calcium in the majority of hypercalcaemic patients. A convenient regimen is 600 mg iv as a single dose infused over several hours. We have additionally shown in a double-blind cross-over study that this regimen also has a significant effect on bone pain. This had led us to assess the longer term effects of clodronate by mouth in a prospective double-blind study of patients with established skeletal metastases. These studies are not yet complete but the agent appears to prevent hypercalcaemia and trends are emerging which indicate that the incidence of bone pain and fractures may also decrease.

Humoral hypercalcemia due to squamous cell carcinoma of renal pelvis

We describe an unusual and rare case of humoral hypercalcemia due to Stage D squamous cell carcinoma of the renal pelvis in a patient with no evidence of bony metastases. The literature on humorally mediated hypercalcemia associated with epithelial tumors of the renal pelvis is reviewed.

Neuroendocrine differentiation in breast lesions

A review of neuroendocrine features in breast carcinomas is presented and markers for neuroendocrine cells are discussed. Immunostaining for neuron specific enolase is the best screening marker for neuroendocrine cells in breast carcinomas, but immunoreactivity for hormones is not present in all neuron specific enolase (NSE) positive cases. Normal myoepithelial cells are also NSE positive. Thirty per cent of breast carcinomas are NSE positive. Biochemical demonstration of ACTH, PTH and calcitonin, and immunohistochemical demonstration of ACTH, bombesin, serotonin, prolactin, gastrin, VIP, leu-enkephalin, pancreatic polypeptide, beta-endorphin and sub P has been reported in breast carcinomas. Neuroendocrine cells have not been convincingly demonstrated in the normal breast or in benign breast lesions.

Two forms of parathyroid hormone-like adenylate cyclase-stimulating protein derived from tumors associated with humoral hypercalcemia of malignancy

Tumors associated with humoral hypercalcemia of malignancy (HHM) contain parathyroid hormone-like adenylate cyclase-stimulating proteins (hACSPs). We previously purified a 17,000 MW hACSP from an HHM-associated breast carcinoma. This report describes the characterization of hACSPs from three additional HHM-associated tumors: two typical HHM-associated tumors (squamous carcinomas) and a third unusual tumor type (pheochromocytoma). Each tumor was extracted in acid-urea/ethanol-sodium chloride, and adenylate cyclase-stimulating activity (ACSA) was examined following reverse-phase and size-exclusion HPLC and isoelectric focusing. Each tumor contained a high molecular weight form of ACSA which co-eluted with the 17,000 molecular weight breast carcinoma hACSP in each of the three separation procedures. Each also contained a lower molecular weight form(s) of ACSA (6,000-9,000 molecular weight). Both forms were inhibited by Nle8,18Tyr34bPTH(3-34) amide. The high molecular weight form was not changed to the lower molecular weight form by a reducing agent. Some HHM-associated tumors contain two forms of hACSP, one with a molecular weight of 17,000 and another with a molecular weight of 6,000-9,000, which appears to be an amino-terminal cleavage product of the larger protein.

Hypercalcemia in breast cancer. Reassessment of the mechanism

Hypercalcemia in patients with breast cancer is usually attributed to osteolytic bone metastases. Seventeen patients with biopsy-proved breast cancer and hypercalcemia were identified in a prospective, unselected manner. Biochemical and clinical evaluation included measurements of parathyroid hormone, nephrogenous cAMP, vitamin D metabolites, fasting calcium excretion, and maximal tubular phosphate reabsorption, and bone radionuclide scanning. Tumor histologic findings were also reviewed. Four of the 17 patients (23.5 percent) had no evidence of bone involvement by bone scanning or radiography. Two additional patients (a total of 35 percent) appeared to have a humoral component to their hypercalcemia as determined by the presence of elevated nephrogenous cAMP excretion. These observations suggest that humoral, tumor-derived products may play a more important role in the hypercalcemia of breast cancer than has been previously recognized.

Frequency and partial characterization of adenylate cyclase-stimulating activity in tumors associated with humoral hypercalcemia of malignancy

Parathyroid hormone-like adenylate cyclase-stimulating activity (ACSA) has previously been identified in small numbers of tumors or tumor-conditioned tissue culture medium derived from patients or animals with humoral hypercalcemia of malignancy (HHM). We examined the frequency with which this ACSA occurred in a large group of tumor extracts derived from patients with HHM (n = 20), and compared this to three control groups: normocalcemia-associated tumors (n = 20), hypercalcemic control tumors (n = 7), and normal, nonmalignant tissue samples (n = 10). Eighteen of 20 HHM-associated tumor extracts displayed ACSA whereas only 4 of 37 controls contained detectable ACSA. ACSA in one tumor was partially purified, using sequential extraction steps and reverse-phase, high-performance liquid chromatography. Highly purified ACSA (4800-fold) also contained potent in vitro bone-resorbing activity. The molecular weight as assessed by gel filtration was approximately 40,000 D. These findings provide strong support for the thesis that the humoral factor which is responsible for the syndrome of HHM is a parathyroid hormone-like adenylate cyclase-stimulating protein.

Tumor markers: value and limitations in the management of cancer patients

Sixteen tumor markers are reviewed, and measured to the ideal: produced by the tumor cell alone absent in health and in benign disease present in all patients with a given malignancy level in the blood representative of tumor mass detectable in occult disease. The only marker that approaches the ideal is human chorionic gonadotropin (HCG) in gestational trophoblastic tumors. In this malignancy, the HCG level suggests the diagnosis and stage, confirms response to therapy, and predicts relapse. The three most widely used and intensely studied tumor markers are carcinoembryonic antigen (CEA), alphafetoprotein (AFP), and HCG. CEA cannot be used in screening for cancer, but in carcinoma of the colon its elevation preoperatively increases the likelihood of advanced disease and postoperative recurrence. Postoperatively, elevated titers are often but not invariably associated with recurrent disease. AFP and HCG are useful in the management of nonseminomatous germ cell testicular tumors. Like CEA, they cannot be used for screening. They are more likely to be increased with advancing stage, and after therapy rising levels almost always mean recurrent disease. Some markers are valuable in specific circumstances, such as calcitonin in screening for familial medullary carcinoma of the thyroid. In multiple myeloma, immunoglobulins are useful in determining the tumor mass and response to therapy. In neuroblastoma, catecholamine metabolites are useful primarily in making the diagnosis. In some malignancies, the absence of effective therapy lowers the value of the marker, as for AFP in hepatoma. The remaining markers are too unreliable or too little studied to be useful in the management of an individual patient with cancer. The purpose of this paper is to provide the clinician with an understanding of the limitations of the present tumor markers that will lead to wiser use of the tests, and to provide standards to which future tumor markers should be measured.

A mouse tumor-derived osteolytic factor stimulates bone resorption by a mechanism involving local prostaglandins production in bone

Culture medium which was conditioned by tissue of a CE mouse breast tumor in vitro contained dose-dependent osteolytic activity. The osteolytic activity was not soluble in dichloromethane and ethylacetate, indicating that it was not attributable to vitamin D metabolites or prostaglandins. However, breast tumor-conditioned medium stimulated production and release of prostaglandin E2 from mouse calvaria in vitro, and the stimulation of bone resorption in vitro by breast tumor-conditioned medium was blocked by a dose of indomethacin that prevented stimulation of mouse calvarial prostaglandin E2 production and release. The resorptive activity of parathyroid hormone (PTH) was not affected by the same dose of indomethacin, suggesting that the osteolytic factor was not PTH. This was further supported by observation that mouse kidney cell cAMP production was stimulated by PTH, but not by the aqueous phase of ethylacetate-extracted breast tumor-conditioned medium. In addition to osteolytic activity, breast tumor-conditioned medium contained a dose-dependent bone cell mitogenic activity, demonstrated by the stimulation of [3H]thymidine incorporation into trichloroacetic acid-insoluble macromolecules and a corresponding increase in bone cell number in monolayer cultures of bone cells. Breast tumor-conditioned medium also contained a dose-dependent transforming growth factor-(TGF-) like activity as defined by its ability to transform anchorage-dependent growth of nontransformed cells to anchorage-independent growth. The TGF in breast tumor-conditioned medium did not compete with epidermal growth factor (EGF) for EGF receptor binding, but its transforming activity was greatly enhanced by EGF, indicating that it was a beta-type TGF. Both the osteolytic and mitogenic activities were nondialyzable, sensitive to reducing agent, and not removable by dichloromethane and ethylacetate extractions. Furthermore, the TGF activity was not removed by ethylacetate extraction. Thus, the possibility that these activities in breast tumor-conditioned medium might be mediated by the same molecule must be considered. In summary, our data suggest that the CE mouse mammary carcinoma cells produce and secrete into the culture medium an osteolytic factor which is neither PTH nor prostaglandin and which stimulates local synthesis in bone of prostaglandin E2 which in turn increases bone resorption in vitro.

Breast carcinomas with neuroendocrine differentiation

Twenty-two breast carcinomas with membrane bound granules by electron microscopy were tested for the presence of neuron specific enolase (NSE), neuropeptides and serotonin by immunohistochemistry. By light microscopy the cases studied included infiltrating ductal carcinomas, intraductal carcinomas, apocrine carcinomas, infiltrating lobular carcinomas of both classical and alveolar types, mixed lobular/colloid carcinomas, carcinoid growth pattern and one unclassified carcinoma. Ten cases showed immunoreactivity for 1 or 2 neuropeptides in scattered cells whereas all cases were positively and rather diffusely stained with anti-NSE. Immunohistochemical staining at the ultrastructural level was carried out; the presence of neuropeptides could not be confirmed. Scattered granules were marked with gold particles when antiserum against casein was used. We conclude that neither argyrophilia, nor NSE immunoreactivity nor membrane bound granules seen by electron microscopy constitute at present sufficient evidence to designate a breast carcinoma as neuroendocrine. However, our study indicates that certain breast carcinomas of several types do include cells with neuroendocrine features demonstrable convincingly by light microscopic immunohistochemistry. We have no evidence that these breast carcinomas with neuroendocrine features behave differently from their counterparts lacking such features. The intriguing speculation is that neuropeptides produced by certain breast carcinomas may act as local modulators of tumor growth and differentiation.

Differential laboratory diagnosis of hypercalcemia

It is proposed that this review will adopt the following format: establishment of hypercalcemia. This demands a discussion of the problem of normal ranges, the usage of either total calcium or ionized calcium in making this decision and where total calcium is used whether adjustment of this value for serum protein concentration should be used and if so, the formulae which have been cited to perform this. Having established hypercalcemia why is it necessary to differentiate this? This will involve reviewing those clinical situations in which differentiation of hypercalcemia has been attempted and will include an attempt to produce an up to date indication of conditions in which hypercalcemia has been described. When hypercalcemia has been established the laboratory tests which have been further used to discriminate will be divided into single tests such as N- or C- terminal parathormone, 1,25- dihydroxycholecalciferol, cyclic AMP; the combination tests which have been used including phosphate clearance, chloride vs. bicarbonate etc. proceeding to those groups which have used discriminant function to help in the decision making; dynamic testing will also be discussed particularly with reference to steroid suppression but will also include other known suppressants such as Mithramycin and Calcitonin. A final section will be included attempting to assess overall the present state of art in differentiating laboratory diagnosis of hypercalcemia and will also attempt to highlight those areas which appear to be most fruitful areas of progress in the future.

Humoral Nonparathyroid Hypercalcemia without Evidence of Bone Involvement

A case of humoral non-parathyroid hypercalcemia in a breast cancer patient is discussed. Bone metastases developed 23 months after the first clinical evidence of increased serum calcium levels and did not appear to be etiologically involved in this case of hypercalcemia. Serum levels of I-PTH and PGE2 were normal, as were urinary c-AMP levels. Furthermore, no significant response to corticosteroid therapy was observed. However, the correlation between the activity of metastatic disease and calcemia suggests that this cancer produced humoral factor(s) with calcium-mobilizing activity.

Argyrophilic cells in mammary carcinoma

Breast tumor tissues were treated by the Grimelius procedure and examined for the presence of argyrophilic cells. Carcinomas found to contain argyrophilic cells did not include classic carcinoid tumors; the group was, in fact, heterogeneous, comprising poorly differentiated ductal carcinomas, lobular carcinomas, carcinomas of uncertain origin, and colloid carcinomas. Colloid tumors were the most frequently encountered. The prominence of argyrophilic cells in colloid carcinomas raises the possibility that development into mucin-producing cells is propitious for endocrine differentiation.

Inadequacy of APUD concept in explaining production of peptide hormones by tumours

The amine precursor uptake and decarboxylation (APUD) system of cells has been claimed to derive from the embryological neural crest. This assertion has been uncritically accepted. There is much contradictory evidence, especially about the origin of the gastrointestinal and respiratory APUD cells. There is further evidence that the embryological derivation of a particular cell does not relate to the possibility of ectopic peptide hormone synthesis by malignant tumours arising from that cell type. There are many reports of APUD activity by endodermally and mesodermally derived tumours, and of "APUDomas" with endodermal microscopic features. It seems that the concept of dedifferentiation explains the observed data much more satisfactorily and that the presence of double minute chromosomes may denote gene amplification and cellular production of peptides.

A study of prostaglandin E2, parathormone, and response to indomethacin in patients with hypercalcemia of malignancy

In order to evaluate the relationship of PGE2 to hypercalcemia in cancer patients, 101 patients were screened with a radioimmunoassay for plasma prostaglandin E2 (PGE2) (NL less than 100 pg/ml). Of the 101 patients, 31 were hypercalcemia. Mean PGE2 (+/- SEM) of the 31 patients was 199 +/- 36 pg/ml. Among the 70 normocalcemic patients, mean +/- SEM PGE2 was 85 +/- 12 pg/ml (range = less than 25--225 pg/ml) (P less than 0.001). Seventeen hypercalcemic patients were initially treated with saline and furosemide, then were prospectively screened for serum parathormone (iPTH) and PGE2. Fourteen of 17 patients were then treated empirically with indomethacin (25 mg b.i.d.) for 72 hours and the PGE2 assay was repeated. Prior to therapy with indomethacin (mean +/- SEM), Ca++ = 12.2 +/- 1.5 mg/dl (NL 8.4--10.6 mg/dl), PGE2 = 87.1 +/- 36.8 pg/ml, (range = less than 25--209 pg/ml), and iPTH = 406 +/- 266 pg/ml (NL less than 400 pg/ml) (range = less than 100--825 pg/ml). PGE2 was elevated before treatment in 6/14 patients (breast, colon, renal, lung, neck tumors, and myeloma). Following treatment with indomethacin, PGE2 and calcium fell to normal levels in three patients (breast, colon, renal carcinomas). These results suggest: (1) A bimodal distribution of PGEs exists in hypercalcemic cancer patients. (2). There was some evidence of lack of whole molecule iPTH suppression in these patients. (3) Multiple stimuli of calcium mobilization may play an important etiologic role in a few hyercalcemic cancer patients and may explain the failure of indomethacin to control serum Ca++ in some patients with elevated PGE2.

Hypercalcemia in carcinoma of the breast without evidence of bone destruction: beneficial effect of hormonal therapy

A premenopausal woman with soft tissue metastases from a carcinoma of the breast developed hypercalcemia with hypophosphatemia, reduced tubular reabsorption of phosphate, elevated urinary cyclic AMP levels and normal serum PTH levels was observed. Hormonal therapy with testosterone followed by tamoxifen induced normalization of her serum calcium concomitant with the disappearance of the pleural effusion and reduction in the size of her lung metastases. The correlation between the efficacy of antitumor treatment on pleural effusion, lung metastases, and normalization of serum calcium, as well as the elevated PTH level in the pleural effusion, suggest that this breast carcinoma secreted a PTH-like substance.

The aggressive surgical management of hypercalcemia due to ectopic parathormone production

A patient with slowly progressive metastatic renal cell carcinoma had life-threatening hypercalcemia secondary to ectopic parathormone production. Aggressive surgical debulking resulted in prolonged control of the hypercalcemia. This case illustrates the value of this surgery in the control of paraneoplastic endocrine syndromes refractory to medical management in patients with slow-growing metastatic disease.

ACTH-secreting carcinoma of the breast

We report a case of ACTH-secreting carcinoma of the breast. Membrane-bound secretory granules were seen within the cancer cells electron microscopically. Cytoplasmic granules reacting immunochemically to anti-ACTH antisera were seen on light microscopy. Breast cancers have been shown to secrete calcitonin, parathyroid hormone, human chorionic gonadotropin, and norepinephrine in addition to ACTH. This suggests either the presence of neuroendocrine cells in the breast as a source of such neoplasms, as in the lung, or genomic derepression during neoplastic transformation.

Norepinephrine-producing tumors of bilateral breasts: a case report

There was a tumor in the left breast which was suspected to be carcinoid tumor at excisional biopsy. Autopsy revealed the same tumor in the right nipple. In the cells of the bilateral tumors a number of argylophil granules were diffusely demonstrated and the neurosecretory ones were verified electronmicroscopically. In the urine of the patient a moderate amount of norepinephrine was excreted. It was presumed that the norepinephrine might have been produced from the breasts. In the literature reviewed, the case of breast carcinoid has not been reported previously.

Primary hyperparathyroidism and breast cancer

Three cases demonstrating the coexistence of primary hyperparathyroidism and breast carcinoma with the disappearance of hypercalcemia following removal of parathyroid adenomas are presented. In these cases, the patients had the typical diagnostic findings of primary hyperparathyroidism. Reluctance to perform neck explorations in such patients does not appear warranted.

Hypercalcemia in small cell (oat cell) carcinoma of the lung. Coincident parathyroid adenoma in one case

Hypercalcemia is very uncommon in small cell (oat cell) carcinoma of the lung. Two cases of this neoplasm associated with symptomatic hypercalcemia are described. Despite normal skeletal roentgenograms, metastatic bone disease was demonstrated by abnormal bone scans and bone biopsies in both patients. The combination of conventional antihypercalcemia therapy, cytotoxic cancer chemotherapy, and synthetic salmon calcitonin corrected the hypercalcemia despite progression of the small cell carcinoma. One patient with elevated serum immunoreactive parathyroid hormone (PTH) had a parathyroid adenoma at autopsy. This association emphasizes that in cases of bronchogenic small cell carcinoma with hypercalcemia, conincidental primary hyperparathyroidism should be considered.

Urinary cyclic AMP in diagnosis and management of hypercalcaemia: studies of patients without primary hyperparathyroidism

Urinary adenosine -3' ,5' - cyclic monophosphate was measured in 14 patients with hypercalcaemia not caused by primary hyperparathyroidism. Increased levels were found in patients with malignant disease without bone metastases and believed to be examples of paraendocrine syndrome. Decreased levels were found in patients with metastatic carcinoma involving bone, and in patients with multiple myeloma, lymphoma and immobilisation after fracture. Results obtained during treatment for hypercalaemia are described in three patients. In two hypercalcaemic patients (one with hyperthyroidism and one with breast cancer with bone metastases) normal levels were found. This measurement is a useful substitute for assay of serum parathyroid hormone and is of value in the diagnosis of hypercalcaemia, in monitoring effects of treatment and in revealing underlying mechanisms.

Burn scar carcinoma and hypercalcemia

A case of a patient with burn scar carcinoma of an extremity associated with hypercalcemia and without bone metastasis is presented. From the data presented, it seems reasonable to assume that this squamous cell carcinoma of a burn scar secreted some type of parathyroid hormone-like substance or substances which posessed calcium-increasing biological activity resulting in hypercalcemia.

Immunochemical localization of parathyroid hormone in cancer tissue from patients with ectopic hyperparathyroidism

Immunoreactive parathyroid hormone (PTH) in nonparathyroid malignant tumors associated with hypercalcemia and hypophosphatemia in the absence of demonstrable bone metastases was determined by radioimmunoassay and immunofluorescent techniques. Six of seven tumors contained material with immunological cross-reactivity to bovine PTH by radioimmunoassay and immunofluorescence. The intensity of the immunofluorescent stain varied considerably in the different tumors. From 15 to 90% of neoplastic cells were stained specifically with fluorescein-labeled anti-PTH. In contrast, normal parathyroid glands and parathyroid adenomas showed uniform distribution of immunofluorescence in all parenchymal cells. In one malignant tumor, PTH was localized also by immunoautoradiography. In every case PTH was detected only in the cytoplasm of parenchymal cells. One patient lacked detectable PTH in his tumor, yet showed regression of the hypercalcemia to normal values after removal of large masses of neoplastic tissue and recurrence of hypercalcemia when new growth occurred.Dilutional radioimmunoassay curves of nonparathyroid malignant tumors were in most cases different from those obtained with extracts of normal parathyroid glands and parathyroid adenomas. Although both nonparathyroid neoplasmas and parathyroid extracts demonstrated immunoheterogeneity by gel filtration, greater heterogeneity was found in nonparathyroid malignant tumors. In those tumors in which immunological cross-reactivity to PTH was detected, the capability of secreting PTH may be restricted to derepressed cell clones amidst other neoplastic cells, whereas the greater heterogeneity of ectopic PTH may reflect hormone cleavage by proteolytic enzymes in the tumor that is less specific than the Pro-PTH cleaving enzyme in the parathyroids.