The study will be an effectiveness study. The study will include enrollment of a total of 20 patients with at least one PC lesion for ultrasound guided RFA to PC recurrence in the neck to assess the effectiveness in reducing patient's hypercalcemia. Patients will have surgically proven PC from prior parathyroidectomy and suspicious PC visible on ultrasound and request for treatment for uncontrolled hypercalcemia (Figure 1).

Primary Objective:

• Evaluate treatment response of RFA of recurrent PC by means of longitudinal monitoring patient serum calcium level and PTH.

Secondary Objectives:

• Evaluate the sonographic features of the recurrent PC induced by RFA.
• Evaluation of procedure-related complications
• Disease free survival until recurrence of hypercalcemia

Inclusion Criteria:

Adult patients with recurrent parathyroid carcinoma that was previously resected and persistent or recurrent hypercalciemia. Patients are not surgical candidates or strongly desire nonsurgical intervention. Eligibility for enrollment will be based upon meeting inclusion and exclusion criteria as detailed in section 4.1 and 4.2.

The study will include 20 eligible patients or lesions. Utmost care will be taken to minimize risk to patients through careful selection of patients and scheduled assessments (Table 1). Patients will be recruited through the Endocrine, Endocrine surgery, Head and Neck Surgery Clinics, as well as Neuro-Interventional Radiology Ultrasound. All patients who meet the inclusion and exclusion criteria and agree to participate in the study will be consented and then enrolled into the study. No other criteria, apart from the eligibility criteria detailed below, will be used for subject selection. We anticipate enrolling approximately 1 patient every 2 months (0.5 patients/month), for a total accrual period of up to 40 months to reach the planned sample size of 20 patients. Each participant will be followed for 12 months after RFA, so the total study duration is estimated at approximately 52 months from first patient enrollment to last patient follow-up.

Candidates for this study must meet all of the following inclusion criteria:

1. Patient is older than 18 years, is not a surgical candidate or refuses to have surgery.
2. Patient is medically fit to undergo local anesthesia with or without conscious sedation.
3. Patient is able to understand and give consent to participation in the study.
4. Confirmed parathyroid carcinoma by prior surgery and histopathology.
5. Presence of local recurrence or implant in the neck visible on ultrasound.
6. Distant oligometastasis visible on ultrasound.
7. Solid nodule without macrocalcification or internal cystic degeneration
8. Patient who is off of Cinacalcet
9. Entirety of the selected nodule is visible on ultrasound without significant extension posterior to trachea or mediastinal component (ie Type A, E, or high F).
10. Selected nodule is amenable to medial or lateral approach.
11. Normal complete blood count, blood coagulation.
12. Patient agrees to participate in the clinical study and to complete all required visits and evaluations.
13. Negative serum or urine pregnancy test for females of childbearing potential at baseline pre-procedure evaluation.

Exclusion Criteria:

Candidates will be ineligible for enrollment in the study if any of the following conditions apply:

1. Patients with cardiac arrhythmia and/or implanted cardiac device
2. Surgically naïve parathyroid lesions, parathyroid adenoma or atypical parathyroid tumor.
3. Pregnancy
4. Allergies to medications for anesthesia.
5. Cystic nodules (< 70% solid components)
6. Calcified nodules
7. Targeted nodule within 0.5 cm from the major vessels, vagus nerve, brachial plexus, and recurrent laryngeal nerve.
8. Patients with contralateral vocal cord paralysis.
9. Uncorrectable coagulopathy with PTT>1.5 X ULN or INR >1.5 or platelet count <100,000 per mm3