ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll	$ 320	$ 1,071
Accrued patient treatment costs	2,086	899
Accrued clinical research costs	479	1,562
Accrued manufacturing costs	328	41
Accrued professional services	305	279
Accrued other	331	313
Total accrued expenses and other current liabilities	$ 3,849	$ 4,165