Increasing the number of arcs improves head and neck volumetric modulated arc therapy plans

Figures & data

Figure 1. Histograms showing the effect of using two, four, six or eight arcs on boost and elective inhomogeneity and mean dose to the composite salivary and swallowing structures for all patients.

Table I. The effect of arc number on boost and elective planning target volume (PTV) dose inhomogeneity (IH_B/HI_B and IH_E/HI_E respectively) and organ-at-risk (OAR) mean dose.

Number of Arcs (#)	Two arcs	Four arcs	Six arcs	Eight arcs
PTVB V95%^†† (%)	99.3 ± 0.7	99.6 ± 0.5	99.7 ± 0.3	99.7 ± 0.4
PTVB V107% (%)	0.9 ± 1.2	0.4 ± 0.9	0.2 ± 0.5	0.1 ± 0.3
IHB^# (%)	1.6 ± 1.6	0.8 ± 1.0	0.5 ± 0.6	0.4 ± 0.5
HIB^¶ (%)	9.2 ± 1.5	8.4 ± 1.4*	7.7 ± 1.1*^†	7.5 ± 1.1*^†
PTVE V95%^†† (%)	98.6 ± 0.5	99.0 ± 0.4*	99.2 ± 0.4*^†	99.2 ± 0.4*^†
PTVE V107% (%)	10.8 ± 3.4	8.1 ± 3.0*	6.5 ± 2.4*^†	5.5 ± 1.9*^†
IHE^# (%)	12.2 ± 3.2	9.1 ± 3.1*	7.4 ± 2.5*^†	6.2 ± 1.9*^†
HIE^¶ (%)	14.0 ± 1.2	12.7 ± 1.6*	11.8 ± 1.3*^†	11.4 ± 1.0*^†
Monitor units	473 ± 34	543 ± 40	595 ± 48	619 ± 39
Contralateral parotid m(10/10 pts)	24.1 ± 7.5	22.2 ± 7.4*	21.4 ± 6.9*	21.0 ± 6.8*
Ipsilateral parotid (10/10 pts)	30.5 ± 6.7	28.2 ± 6.2*	26.8 ± 5.6*	26.4 ± 5.6*
Contralateral submandibular (9/10 pts)	38.2 ± 7.2	35.6 ± 7.7*	34.3 ± 7.3*	34.5 ± 7.3*^†
Composite salivary structure	28.2 ± 6.5	26.0 ± 6.3*	24.9 ± 5.8*	24.5 ± 5.6*
Composite swallowing structure	30.0 ± 8.0	27.5 ± 8.1*	26.6 ± 8.5*^†	26.0 ± 8.3*^†
OARcomp dose^** (Gy)	29.1 ± 4.0	26.8 ± 4.5*	25.7 ± 4.8*^†	25.3 ± 4.7*^†
Body - PTV V5Gy (cm^3)	5258 ± 1355	5310 ± 1357	5318 ± 1347*	5346 ± 1345*^†
Body - PTV V30Gy (cm^3)	1882 ± 519	1917 ± 506	1943 ± 512*	1969 ± 514*^†
Body - PTV V50Gy (cm^3)	338 ± 130	335 ± 124	325 ± 117	332 ± 117

*significant differences compared to two-arc plans; ^†significant differences compared to four-arc plans; ^††PTV_B/PTV_E V95%, boost/elective planning target volume (PTV) receiving ≥ 95% prescribed dose; ^#IH_B/IH_E, PTV_B/PTV_E dose inhomogeneity; ^¶HI_B/HI_E, boost/elective homogeneity index; ^**OAR_comp Dose, average of the mean doses to the composite salivary and swallowing structures.