COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Thousands		12 Months Ended
	Sep. 05, 2017 USD ($)	Dec. 31, 2020 USD ($) reload install purchase
Commitments And Contingencies [Line Items]
Number of Cobalt-60 reloads | reload		3
Number of Leksell Gamma Knife Icon System installs | install		4
Number of purchase commitments | purchase		2
Gamma Knife Perfexion System
Commitments And Contingencies [Line Items]
Long-term purchase commitment amount		$ 12,210
LINAC system
Commitments And Contingencies [Line Items]
Long-term purchase commitment amount		12,210
Service commitment, amount		$ 10,493
LINAC system | Maximum
Commitments And Contingencies [Line Items]
Service commitment, period		9 years
LINAC system | Minimum
Commitments And Contingencies [Line Items]
Service commitment, period		5 years
Mevion Medical Systems Inc | Maintenance and Support Agreement (the “Mevion Service Agreement”)
Commitments And Contingencies [Line Items]
Renewal term	5 years
Purchase agreement annual prepayment	$ 1,572
Commitment period	1 year