Assessment of set-up errors in the radiotherapy of patients with head and neck cancer: standard vs. individual head support

Figure 1

Figure 2

Inter-fraction displacements recorded on units A and B at different periods

	2014 Standard head support			2015 Individual support			2017 Individual support
	VRT	LNG	LAT	VRT	LNG	LAT	VRT	LNG	LAT
UNIT A
Average displacement – M [mm]	- 0.86	0.05	0.05	- 0.47*	- 0.24*	- 0.05	- 0.16*	0.88	0.18
Systematic error – Σ [mm]	0.66	1.04	0.88	0.91	0.79	0.95	0.82	0.59	0.83
Random error – σ [mm]	1.49	1.21	1.37	1.56	1.28	1.38	1.28	1.14	1.2
UNIT B
Average displacement – M [mm]	- 0.60	0.46	- 0.43	- 0.02*	0.51	0.25	- 0.18*	0.46	- 0.02
Systematic error – Σ [mm]	1.09	0.80	0.82	0.92	0.74	0.94	0.93	0.69	0.88
Random error – σ [mm]	1.77	1.83	1.47	1.77	1.22	1.41	1.71	1.56	1.44

Clinical target volume - planning target volume (CTV-PTV) safety margins for units A and B at different periods (calculated according to van Herk8)

	CTV-PTV safety margin [mm]
	VRT	LNG	LAT
UNIT A
2014 – standard head support	2.7	3.4	3.2
2015 – individual head support	3.4	2.9	3.3
2017 – individual head support	2.9	2.3	2.9
UNIT B
2014 – standard head support	4.0	3.3	3.1
2015 – individual head support	3.5	2.7	3.3
2017 – individual head support	3.5	2.8	3.2

Number of IC displacements and of gross errors at treatment units in relation to time for both units_ In the brackets, the most prevalent direction of applied movements is indicated

	Isocenter movements
	VRT	LNG	LAT	Gross errors
	Regression errors
	Unit A / unit B (direction)	Unit A / unit B (direction)	Unit A / unit B (direction)
2014 – standard head support	11 / 15 (P)	5 / 15 (I)	4 / 5 (L)	2 / 5
2015 – individual head support	12 / 13 (P)	3 / 6 (I)	9 / 8 (R)	0 / 1
2017 – individual head support	5 / 14 (P)	7 / 10 (I)	6 / 7 (L)	0 / 1