ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll	$ 278	$ 1,071
Accrued patient treatment costs	2,507	899
Accrued clinical research costs	1,658	1,562
Accrued manufacturing costs	252	41
Accrued professional services	289	279
Accrued other	355	313
Total accrued expenses and other current liabilities	$ 5,339	$ 4,165