Hypocalcemia Following Surgical Treatment of Metastatic Anal Sac Adenocarcinoma in a Dog

Introduction

Neoplasia, specifically T-cell lymphoma and anal sac apocrine gland adenocarcinoma (ASAC), is the most common cause of hypercalcemia in dogs, which frequently involves production of parathormone-related peptide (PTHrP) by tumor cells. It has been shown that the serum calcium level may serve as a marker of disease, in that return to normocalcemia is an indication of disease remission, and hypercalcemia is suggestive of tumor progression.¹ Hypocalcemia and hypoparathyroidism have been reported following successful treatment of humoral hypercalcemia of malignancy (HHM) secondary to lymphoma.² To the authors’ knowledge, this is the first report of clinical hypocalcemia and hypoparathyroidism following ASAC removal in a dog with HHM.

Case Report

A 9 yr old neutered male mixed-breed dog weighing 17 kg was presented for evaluation of a mass associated with the right anal sac. Five months prior to presentation, the dog was evaluated by the referring veterinarian for recurrent episodes of twitching in the rear legs, hiccupping, polyuria, and polydipsia. On digital rectal examination, a firm right anal sac was palpated. No other abnormalities were detected and no additional diagnostics were performed. The clinical signs continued to progress. Four months after initial presentation to the referring veterinarian, the dog was re-evaluated, and a discrete mass was associated with the right anal sac. Additionally, enlarged medial iliac lymph nodes were noted on abdominal palpation. A complete blood cell count and biochemical profile showed an elevated total calcium of 16 mg/dL or 4.0 mmol/L (reference ranges, 8.2–12.4 mg/dL or 2.1–3.1 mmol/L, respectively), but no other significant abnormalities were found.

Upon presentation to The University of Georgia Veterinary Teaching Hospital, the dog was bright, alert, and responsive. A firm, fixed mass, measuring approximately 2 cm at its longest diameter was palpated in the region of the right anal sac. Additionally, a large mass was palpated per rectum in the caudodorsal abdomen. The remainder of the physical examination was unremarkable. An ionized calcium of 1.91 mmol/L (NOVA Stat^a reference range, 1.1–1.3 mmol/L) confirmed hypercalcemia. This and all subsequent NOVA Stat ionized calcium measurements have been summarized in Table 1. Thoracic radiographs, abdominal radiographs, and an abdominal ultrasound were performed. No abnormalities were noted on thoracic radiographs; however, several round, ill-defined, soft-tissue masses that displaced the colon ventrally were noted on abdominal radiographs. Abdominal ultrasound suggested these structures were medial iliac and caudal aortic lymph nodes, ranging in size from 2.5 cm to 6.7 cm. No additional sonographic abnormalities were noted. Cytology of these presumed lymph nodes was indicative of epithelial neoplasia consistent with ASAC; however, no lymphoid tissue was present to confirm the source of the aspirate as lymph node.

TABLE 1 Trends of Ionized Calcium with Respect to PTH, PTHrP, Calcium Supplementation, and Calcitriol

*

NOVA Stat iCA was measured in mmol/L (reference range, 1.1–1.3 mmol/L).

†

MSU iCA was measured in mmol/L (reference range, 125–1.45 mmol/L).

§

MSU intact PTH was measured in pmol/L (reference range, 3.0–17.0 pmol/L).

**

MSU PTHrP was measured in pmol/L (reference range, 0–1.0 pmol/L).

††

Prior to supplementation with calcium gluconate

§§

After supplementation with calcium gluconate

***

Calcitriol dose was reduced from q 12 hr to q 24 hr.

iCA, ionized calcium; MSU, Michigan State University; PTH, parathormone; PTHrP, parathormone-related peptide.

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Surgical removal of the anal sac mass and lymphadenectomy were performed. Aside from the enlarged medial iliac lymph nodes, abdominal exploratory was unremarkable and no surgical complications occurred. Histopathology confirmed ASAC with regional metastasis to the medial iliac lymph nodes. Postoperatively, the dog recovered in the intensive care unit and was administered Normosol-R^b supplemented with 15 mEq potassium chloride^c (50 mL/kg/24 hr), hetastarch^d (24 mL/kg/24 hr), hydromorphone^e (0.1 mg/kg IV q 4 hr), and metronidazole^f (15 mg/kg per os q 12 hr) for post-operative stress colitis. Ionized calcium measurements were performed daily following surgery to accurately assess the active fraction of serum calcium (Table 1). At 24 hr and 48 hr postoperatively, the ionized calcium decreased to 1.49 mmol/L and 1.20 mmol/L, respectively. Seventy-two hours postoperatively, the ionized calcium was below the lower end of the reference interval at 1.03 mmol/L. The dog was discharged with 500 mg calcium carbonate^g to be given orally q 12 hr, with plans to recheck his calcium in 2 days.

On the night prior to the first scheduled re-examination at the hospital, the dog was presented to the emergency service at the University of Georgia Veterinary Teaching Hospital, for inappetence and facial rubbing. He was weak, but able to stand, and muscle fasciculations were noted in his face and limbs. The remainder of the exam was unremarkable. Ionized calcium was 0.88 mmol/L. Normosol-R^b (60 mL/kg/24 hr) was administered and an infusion of 50 mg/kg of 10% calcium gluconate^h (830 mg total) was planned to be administered IV over 30 min. During the calcium gluconate infusion, however, the dog's heart rate decreased from 120 beats/min to 90 beats/min, and the infusion was discontinued due to the bradycardia. Approximately 488 mg of calcium gluconate had been given. Ionized calcium was repeated and was 0.94 mmol/L. The dog remained hospitalized at the emergency service on 0.001 ng/kg calcitriolⁱ orally q 24 hr and 625 mg calcium carbonate^g orally q 12 hr. In addition, 500 mg 10% calcium gluconate^h diluted in an equal volume of saline was administered subcutaneously q 8 hr. The bradycardia and clinical signs of hypocalcemia resolved within 24 hr of readmission to the hospital. Seventy-two hours after readmission (7 days after surgery), ionized calcium was 1.14 mmol/L. The dog was discharged from the hospital at that time. The owners were instructed to administer calcium carbonate^g (500 mg orally q 12 hr) and a maintenance dose of calcitriolⁱ (0.005 μg/kg [0.085 μg] orally q 12 hr).

Eleven days after surgery, the dog presented to the Oncology service at the Universoty of Georgia Veterinary Teaching Hospital, to discuss additional treatment recommendations. Definitive radiation therapy to both the primary tumor bed and the medial iliac lymph node bed was recommended in combination with chemotherapy. Chemotherapy options discussed included platinum agents (carboplatin or cisplatin), doxorubicin, mitoxantrone, and the oral alkylating agent melphalan.^3–8 Due to financial and quality of life concerns, the owners opted to treat the dog with chemotherapy alone, and carboplatin^j was the chosen agent. Precarboplatin baseline ionized calcium was 1.41 mmol/L. Carboplatin was initiated at 270 mg/m2 IV q 3 wk. Due to the mild elevation in ionized calcium noted on the NOVA Stat analyzer^a, calcium carbonate was discontinued; however, calcitriolⁱ was continued at 0.085 μg orally q 12 hr. Although a presurgical, baseline hypercalcemia of malignancy (HM) panel was not performed, blood was sent to Michigan State University^k (MSU) at the time of this visit. Intact parathormone (PTH) was 5.9 pmol/L (reference range, 3.0–17.0 pmol/L), PTHrP was 3.5 pmol/L (reference range, 0–1.0 pmol/L), and ionized calcium was 1.04 mmol/L (reference range, 1.25–1.45 mmol/L). These and all subsequent HM panel results have been summarized in Table 1. The sample was run in duplicate to confirm these results. Even in the absence of a presurgical baseline, these results were considered consistent with hypoparathyroidism and secondary hypocalcemia. It was decided to monitor the HM panel to aid in the assessment of the dog's disease status. Additionally, upon return of the HM panel results, the discrepancy in ionized calcium measurements between MSU's analyzer and the NOVA Stat was noted, but inexplicable.

Eighteen days after surgery, the owners reported one episode of facial twitching. They reportedly gave a single dose of 500 mg calcium cargonate, and the facial twitching resolved completely. Physical examination was unremarkable, and ionized calcium was 1.06 mmol/L (NOVA Stat). The HM panel was repeated: intact PTH was 5.4 pmol/L: PTHrP was 2.6 pmol/L; and ionized calcium was 1.09 mmol/L. Calcitriolⁱ was continued at 0.085 μg orally q 12 hr.

Ninety-five days after surgery, ionized calcium was 1.20 mmol/L (NOVA Stat). Abdominal ultrasound was repeated, revealing a mildly enlarged right medial iliac lymph node compared with the previous measurement. Cytology of this lymph node confirmed tumor recurrence. The ionized calcium was normal at this visit, but had been elevated at the previous visit, as noted in Table 1. Although chemotherapy was changed to melphalan^l and later doxorubicin^m, the cancer progressed. The owners elected to discontinue all therapy and re-examinations; however, based on phone consultations, the dog continued to have a good quality of life for several months.

The dog presented for euthanasia due to declining quality of life 425 days after surgery. A biochemical profile showed markedly increased total calcium of 19.2 mg/dL or 4.8 mmol/L (reference ranges, 9.3–11.4 mg/dL or 2.3–2.9 mmol/L, respectively) and mildly increased creatinine (1.9 mg/dL; reference range, 0.5–1.5 mg/dL). The calcium × phosophorus product was >70 mg/dL at 84.5 mg/dL, which had not been documented previously. No other significant abnormalities were noted on the biochemical profile. Blood was again sent for a HM panel to MSU. Intact PTH was 0.0 pmol/L, PTHrP was 92.5 pmol/L, and ionized calcium was 2.48 mmol/L.

At necropsy, ASAC metastases were noted in the lungs, liver, and spleen, as well as on the capsular surface of the right kidney. Hydronephrosis and hydroureter were noted in the right kidney secondary to tumor obstructing the ureter. Histologically, there was no evidence of necrosis or significant mineralization in the kidneys. Although there were a few small foci of mineral in collecting ducts and tubules, this level of mineralization was considered to be within normal limits. Masses ranging from 0.5 cm to ≥8 cm were noted in the abdominal cavity, which were histologically consistent with ASAC metastases. Parathyroid glands were dissected and considered to be within normal limits grossly. Only one of the parathyroid glands was available for histologic evaluation; however, it was judged to be within normal limits.

Discussion

ASAC is the most common tumor of the anal sacs in dogs. These tumors are locally invasive and frequently metastasize to medial iliac lymph nodes early in the course of disease. Approximately ≥50% of cases have regional nodal metastases at the time of diagnosis.^3–5 A multimodal therapeutic approach combining surgery, radiation therapy, and chemotherapy is often necessary for prolonged disease control.^3–8 Lymphadenectomy of metastatic lymph nodes is feasible at the time of primary tumor removal, and has been reported as a positive prognostic factor.^5,7,9

Normally, calcium homeostasis is regulated by parathyroid gland production of PTH and its effects on bone resorption, renal excretion, and vitamin D metabolism. The ionized form of calcium is the biologically active form, which is tightly maintained within a narrow normal range by a well-controlled negative feedback mechanism. When serum ionized calcium level is high, PTH production is inhibited, but when serum ionized calcium level is low, PTH production is stimulated. This tight regulation of calcium is extremely important to prevent clinical signs and adverse effects of hyper- and hypocalcemia.^10,11 Long-standing hypercalcemia has adverse effects on the kidneys, causing nephrogenic diabetes insipidus and decreased glomerular filtration rate and renal blood flow that result in ischemic injury to the kidneys, ultimately culminating in nephrocalcinosis.¹⁰ Clinical signs associated with hypercalcemia include polyuria, polydipsia, weakness, vomiting, constipation, and muscle twitching.¹⁰ Hypocalcemia causes neurologic and neuromuscular disturbances. Clinical signs include muscle tremors or fasciculations, seizures, and stiff gait. Additionally, facial rubbing secondary to pruritis may also occur.¹¹

Neoplasia, specifically T-cell lymphoma and ASAC, is the most common cause of hypercalcemia in dogs. Several mechanisms of paraneoplastic hypercalcemia have been described.¹⁰ HHM is most common and frequently involves production of PTHrP by tumor cells. PTHrP is structurally similar to PTH and possesses the ability to bind to PTH receptors and cause hypercalcemia by increasing osteoclast activity and decreasing renal calcium excretion.^1,10,12–14 Tumor production of PTHrP and the resultant HHM has been well-documented in cases of canine ASAC, and hypercalcemia has been reported in 27–53% of affected dogs.^{1,3,5,12–14}

Both PTH and PTHrP can be measured in serum, and this is often part of the diagnostic plan for patients presented for hypercalcemia. In healthy dogs, PTH levels should be low when calcium levels are normal or high, but PTH levels should increase when calcium is low. PTHrP levels should be low to unmeasurable in healthy dogs. In patients with primary hyperparathyroidism, PTH levels remain high in the face of either normocalcemia or hypercalcemia, and PTHrP levels should be low to unmeasurable. In patients with HHM secondary to PTHrP production, PTHrP levels remain high in the face of hypercalcemia, but PTH levels should be low to unmeasurable. PTHrP has a short half-life, so once the tumor producing PTHrP is removed or is in remission, PTHrP levels decrease, calcium levels return to the normal range, and the negative feedback mechanism resumes normal function.¹⁰ It has been shown that serum calcium levels may serve as a marker of disease in that return to normocalcemia is an indication of disease remission and hypercalcemia is suggestive of tumor progression.¹

Severe hypocalcemia and hypoparathyroidism have been reported following successful treatment of HHM secondary to lymphoma in a dog.² To the authors’ knowledge, this is the first report of clinical hypocalcemia following ASAC removal in a dog with HHM. The results of the repeated HM panels are intriguing and offer a plausible explanation of the case. Admittedly, a presurgical HM panel would be necessary to thoroughly validate the authors’ hypothesis. Nonetheless, the inappropriately low PTH in the face of hypocalcemia noted on the first postsurgical HM panel suggested long-standing suppression, and resultant atrophy, of the parathyroid glands. Once the tumor, metastatic lymph nodes, and therefore the source of the PTHrP were removed, the parathyroid glands were presumably unable to resume normal function in calcium homeostasis. Although the PTHrP remained abnormally elevated postoperatively, likely due to residual microscopic tumor, the degree of elevation did not appear to be sufficient to maintain the dog's calcium within the normal range. As a result, clinical signs of hypocalcemia developed. Furthermore, the elevated PTHrP was confirmed on two subsequent HM panels performed 1 wk and 3 wk after the initial HM panel; therefore, a spuriously elevated PTHrP seems unlikely. Approximately 2 mo after the initial panel, the HM panel was repeated. Ionized calcium was within the normal reference interval, and intact PTH was at the lower end of the reference interval. The PTHrP was again above the normal reference interval, but lower than the values obtained at the time of initial hypocalcemia. PTHrP fluctuations during this time may have been the result of change in tumor burden in response to chemotherapy; however, tumor recurrence became grossly evident in the weeks following this test. At the time of euthanasia, the HM panel was repeated. Ionized calcium was well above the normal reference interval, intact PTH was 0 pmol/L (suggesting complete suppression of the parathyroid glands), and PTHrP was markedly elevated, consistent with increased production of PTHrP by the tumor.

There was a notable difference between the NOVA Stat and HM panel ionized calcium results 11 days after surgery. It is unclear why this occurred. Based on repeatability of the HM panel results and reported facial twitching after discontinuation of the calcium carbonate at that visit, it seems likely that the NOVA Stat results were spurious.

The primary inconsistency in this case was the fact that the parathyroid gland(s) were grossly and histologically normal at necropsy. The authors’ anticipated parathyroid atrophy and hypoplasia, which would have been consistent with the intact PTH measurement of 0.00 pmol/L noted on the final HM panel; however, these changes were not found. Perhaps gross assessment of parathyroid gland size was an inaccurate predictor of function and, at least in this case, histologic assessment of only one of the four parathyroid glands inadequately represented the true clinical picture.

Conclusion

This report suggests that clinicians should provide careful monitoring of serum ionized calcium levels when treating patients with HHM. Although rare, resultant hypocalcemia can prove life threatening if severe.