The investigators hypothesize that all-cause mortality in patients with an incidental thyroid nodule on PET-CT who did not have thyroid ultrasound (the exposure) within 3 months of the PET-CT is non-inferior within a 5% margin to those who have thyroid ultrasound at 7-years. That is, among patients with an incidental thyroid nodule on PET-CT, mortality is no more than 5% larger (in absolute difference) for those who do not have thyroid ultrasound compared to those who do. The investigators will also report mortality differences at landmark timeframes of 1-year, 3-years, 5-years, and 10-years. To estimate group differences in mortality, the investigators will conduct a non-inferiority emulated target trial utilizing clone-censor weighting to address potential immortal time bias introduced by the 3-month grace period. The investigators will adjust for demographic, potential confounder, and mortality risk adjustor factors. The investigators will stratify analyses based on baseline disease severity (estimated 5-year relative survival risk) and disease status (progression, lymph node involvement, other sites of metastases). All subjects will be accrued from the Mass General Brigham healthcare system, which includes two academic medical centers, a specialty head and neck hospital, and multiple community hospitals and numerous community clinics.

Study Hypothesis: The investigators hypothesize that all-cause mortality in patients with an incidental thyroid nodule on PET-CT who did not have thyroid ultrasound within 3 months is no worse than 5% lower than those who did have ultrasound.

Outcome Measures:

Primary Outcome: All-cause mortality. Secondary Outcomes: Numbers of thyroid cancer diagnoses, thyroid ultrasounds, thyroid biopsies, and thyroid surgeries Exploratory Outcomes: Types of thyroid cancer diagnoses

Study Population: All patients age 18-years and older with incidental thyroid nodule on PET-CT performed between 1/1/2015 and 12/31/2021.

Sites/Facilities: Mass General Brigham healthcare system including Massachusetts General Hospital, Brigham and Women's Hospital, Mass Eye and Ear, and associated community sites.

Exposure: Thyroid ultrasound evaluation within 3-months of PET-CT.

Inclusion Criteria:

1. Age >=18
2. Thyroid nodule on PET-CT performed 1/1/2015 to 12/31/2021
3. At least one clinical note in the EHR from the 36-month window prior to the PET

Exclusion Criteria:

1. Thyroid ultrasound listed in the medical record in the prior 3 years
2. Documented history of prior thyroid cancer diagnosis

Patients who had a PET-CT at Mass General Brigham 1/1/2015 to 12/31/2021

All deidentified data used in the study analysis.

Study protocol with study analysis plan submitted at time of clinicaltrials.gov registration. IPD and analytic code will be shared upon manuscript publication.

General public via the Harvard Dataverse repository.