Inclusion Criteria:

          -  Must have histologically confirmed Stage IB, II, or IIIA NSCLC.

          -  Must have an activating RET gene fusion in tumor based on polymerase chain reaction
             (PCR) or next generation sequencing (NGS).

          -  Must have received definitive locoregional therapy with curative intent (surgery or
             radiotherapy) for Stage IB, II, or IIIA NSCLC.

             -- Must have undergone the available anti-cancer therapy (including chemotherapy or
             durvalumab) or not be suitable for it, based on the investigator's discretion.

          -  Must have completely recovered from definitive therapy (surgery or radiotherapy) as
             well as adjuvant therapy at the time of randomization.

          -  Maximum time allowed between definitive therapy completion and randomization must be:

               -  10 weeks if no chemotherapy was administered

               -  26 weeks if adjuvant chemotherapy was administered

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.

          -  Adequate hematologic, hepatic, and renal function.

          -  Willingness of men and women of reproductive potential to observe conventional and
             highly effective birth control for the duration of the study and for at least 2 weeks
             after last dose of study drug.

        Exclusion Criteria:

          -  Additional oncogenic drivers in NSCLC, if known.

          -  Evidence of small cell lung cancer.

          -  Clinical or radiologic evidence of disease recurrence or progression following
             definitive therapy.

          -  Known or suspected interstitial fibrosis or interstitial lung disease or history of
             (noninfectious) pneumonitis that required steroids.

          -  Clinically significant active cardiovascular disease or history of myocardial
             infarction within six months prior to planned start of selpercatinib or prolongation
             of the QT interval corrected for heart rate using Fridericia's formula (QTcF) greater
             than 470 milliseconds.

          -  Have known uncontrolled human immunodeficiency virus (HIV)-1/2 infection.

          -  Have known active hepatitis B or C.

          -  Active uncontrolled systemic bacterial, viral, or fungal infection or serious ongoing
             intercurrent illness, such as hypertension or diabetes, despite optimal treatment.

          -  Major surgery within 4 weeks prior to planned start of selpercatinib.

          -  Clinically significant active malabsorption syndrome or other condition likely to
             affect gastrointestinal absorption of the study drug.

          -  Other malignancy unless nonmelanoma skin cancer, carcinoma in situ of the cervix or
             other in situ cancers or a malignancy diagnosed greater than or equal to two years
             previously and not currently active.

          -  Pregnancy or lactation.

          -  Prior treatment with a selective RET inhibitor (e.g. selpercatinib or pralsetinib).