ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll	$ 166	$ 320
Accrued patient treatment costs	1,393	2,086
Accrued clinical research costs	663	479
Accrued manufacturing costs	388	328
Accrued professional services	417	305
Accrued other	320	331
Total accrued expenses and other current liabilities	$ 3,347	$ 3,849