ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, shares in Thousands, $ in Thousands		12 Months Ended
	Jun. 12, 2020 USD ($)	Dec. 31, 2020 USD ($) segment unit customer site $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Accounting Policies [Line Items]
Advertising expense		$ 237	$ 144
Interest costs capitalized		119	110
Original cost of operating lease contracts		30,418	41,480
Accumulated depreciation		45,739	55,461
Loss on write down of impaired assets and associated removal costs		$ (8,264)	0
Term of contract		10 years
Revenues		$ 17,837	$ 20,605
Percentage of tax benefit realized upon settlement		50.00%
Antidilutive securities excluded from computation of earnings per share (in shares) | shares		406	387
Number of reportable segments | segment		2
GKCE acquisition
Accounting Policies [Line Items]
Goodwill	$ 1,265
GKCE acquisition | Purchased intangible assets
Accounting Policies [Line Items]
Purchased intangible assets	$ 78
Revenues
Accounting Policies [Line Items]
Revenues		$ 17,837	$ 20,605
Customer One | Revenues
Accounting Policies [Line Items]
Concentration risk, percentage		35.00%	30.00%
Customer One | Accounts Receivable
Accounting Policies [Line Items]
Concentration risk, percentage		11.00%	12.00%
Customer Two | Accounts Receivable
Accounting Policies [Line Items]
Concentration risk, percentage		11.00%	15.00%
Customer Three | Accounts Receivable
Accounting Policies [Line Items]
Concentration risk, percentage		11.00%	30.00%
Customer Four | Accounts Receivable
Accounting Policies [Line Items]
Concentration risk, percentage		20.00%
Restricted Stock Units (RSUs)
Accounting Policies [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares) | shares		13	3
Accounting Standards Update 2016-02
Accounting Policies [Line Items]
Revenue recognized, including assessed tax		$ 16,204	$ 19,396
Accounting Standards Update 2014-09
Accounting Policies [Line Items]
Revenues		$ 1,633	1,209
Proton Beam Radiation Therapy (“PBRT”)
Accounting Policies [Line Items]
Property equipment, useful life		20 years
Gamma Knife And Radiation Therapy
Accounting Policies [Line Items]
Original cost of operating lease contracts		$ 75,241	92,135
Accumulated depreciation		45,416	$ 55,148
Gamma Knife Center Ecuador S.A. (“GKCE”)
Accounting Policies [Line Items]
Accounts receivable		$ 467
Gamma Knife Unit
Accounting Policies [Line Items]
Number of impaired asset units | unit		6
Number of customers with intent to terminate contract | customer		4
Number of sites | site		4
Maximum
Accounting Policies [Line Items]
Stock options exercise price (in dollars per share) | $ / shares		$ 3.90	$ 3.90
Maximum | Medical and Office equipment
Accounting Policies [Line Items]
Property equipment, useful life		10 years
Minimum
Accounting Policies [Line Items]
Stock options exercise price (in dollars per share) | $ / shares		$ 2.25	$ 2.68
Minimum | Medical and Office equipment
Accounting Policies [Line Items]
Property equipment, useful life		3 years