PROPERTY AND EQUIPMENT - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) unit deposit	Dec. 31, 2019 USD ($)
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 30,418	$ 41,480
Loss on write down of impaired assets and associated removal costs	(8,264)	$ 0
Non-US
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 3,195
Gamma Knife Unit
Property, Plant and Equipment [Line Items]
Number of impaired asset units | unit	6
Impairment of long-lived assets to be disposed of	$ 3,051
Number of removed asset units | unit	4
Asset retirement obligation liability	$ 1,350
Increase (decrease) in accounts payable	(80)
Tangible asset impairment charges	4,401
Proton Beam Radiation Therapy (“PBRT”)
Property, Plant and Equipment [Line Items]
Tangible asset impairment charges	$ 3,863
Deposits towards purchase of proton beam systems
Property, Plant and Equipment [Line Items]
Number of deposits | deposit	2
Two MEVION S250i Proton Beam Radiation Therapy Systems
Property, Plant and Equipment [Line Items]
Accumulated capitalized interest costs	$ 1,613
Long term purchase commitment deposits for assets	$ 2,250
Units removed from service in prior years | Gamma Knife Unit
Property, Plant and Equipment [Line Items]
Number of impaired asset units | unit	2
Units removed from service in current year | Gamma Knife Unit
Property, Plant and Equipment [Line Items]
Number of impaired asset units | unit	1
Units that will be removed from service in future years | Gamma Knife Unit
Property, Plant and Equipment [Line Items]
Number of impaired asset units | unit	3