SEGMENT INFORMATION - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Jan. 01, 2021	Jan. 03, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 778.8	$ 828.9
Products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	348.6	400.9
Products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	430.2	428.0
Americas
Disaggregation of Revenue [Line Items]
Revenues	349.0	402.2
EMEA
Disaggregation of Revenue [Line Items]
Revenues	267.6	272.9
APAC
Disaggregation of Revenue [Line Items]
Revenues	162.2	153.8
North America
Disaggregation of Revenue [Line Items]
Revenues	329.6	378.5
International
Disaggregation of Revenue [Line Items]
Revenues	449.2	450.4
Hardware
Disaggregation of Revenue [Line Items]
Revenues	304.1	359.0
Software
Disaggregation of Revenue [Line Items]
Revenues	144.2	148.8
Service
Disaggregation of Revenue [Line Items]
Revenues	330.5	321.1
Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	770.1	810.0
Operating Segments | Oncology Systems
Disaggregation of Revenue [Line Items]
Revenues	744.5	782.4
Operating Segments | Oncology Systems | Products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	338.5	382.0
Operating Segments | Oncology Systems | Products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	406.0	400.4
Operating Segments | Oncology Systems | Americas
Disaggregation of Revenue [Line Items]
Revenues	332.5	378.9
Operating Segments | Oncology Systems | EMEA
Disaggregation of Revenue [Line Items]
Revenues	254.2	259.8
Operating Segments | Oncology Systems | APAC
Disaggregation of Revenue [Line Items]
Revenues	157.8	143.7
Operating Segments | Oncology Systems | North America
Disaggregation of Revenue [Line Items]
Revenues	313.1	355.2
Operating Segments | Oncology Systems | International
Disaggregation of Revenue [Line Items]
Revenues	431.4	427.2
Operating Segments | Oncology Systems | Hardware
Disaggregation of Revenue [Line Items]
Revenues	279.8	320.0
Operating Segments | Oncology Systems | Software
Disaggregation of Revenue [Line Items]
Revenues	142.8	148.8
Operating Segments | Oncology Systems | Service
Disaggregation of Revenue [Line Items]
Revenues	321.9	313.6
Operating Segments | Proton Solutions
Disaggregation of Revenue [Line Items]
Revenues	25.6	27.6
Operating Segments | Proton Solutions | Products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	1.4	0.0
Operating Segments | Proton Solutions | Products and services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	24.2	27.6
Operating Segments | Proton Solutions | Americas
Disaggregation of Revenue [Line Items]
Revenues	11.3	16.9
Operating Segments | Proton Solutions | EMEA
Disaggregation of Revenue [Line Items]
Revenues	11.9	10.3
Operating Segments | Proton Solutions | APAC
Disaggregation of Revenue [Line Items]
Revenues	2.4	0.4
Operating Segments | Proton Solutions | North America
Disaggregation of Revenue [Line Items]
Revenues	11.3	16.9
Operating Segments | Proton Solutions | International
Disaggregation of Revenue [Line Items]
Revenues	14.3	10.7
Operating Segments | Proton Solutions | Hardware
Disaggregation of Revenue [Line Items]
Revenues	15.6	20.1
Operating Segments | Proton Solutions | Software
Disaggregation of Revenue [Line Items]
Revenues	1.4	0.0
Operating Segments | Proton Solutions | Service
Disaggregation of Revenue [Line Items]
Revenues	8.6	7.5
Other
Disaggregation of Revenue [Line Items]
Revenues	8.7	18.9
Other | Products transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	8.7	18.9
Other | Americas
Disaggregation of Revenue [Line Items]
Revenues	5.2	6.4
Other | EMEA
Disaggregation of Revenue [Line Items]
Revenues	1.5	2.8
Other | APAC
Disaggregation of Revenue [Line Items]
Revenues	2.0	9.7
Other | North America
Disaggregation of Revenue [Line Items]
Revenues	5.2	6.4
Other | International
Disaggregation of Revenue [Line Items]
Revenues	3.5	12.5
Other | Hardware
Disaggregation of Revenue [Line Items]
Revenues	$ 8.7	$ 18.9