Table 3.

Mean (standard deviation) values for the dose–volume metrics of the four plan types

Plan types	D2% (Gy)	D98% (Gy)	D50% (Gy)	HI	CI	$V_{33.2}^{\text{PTV}}$ (%)	$V_{35}^{\text{PTV}}$ (%)	$V_{32}^{\text{r}}$ (%)	$V_{28}^{\text{r}}$ (%)	$V_{18}^{\text{r}}$ (%)	$V_{32}^{\text{rw}}$ (%)	$V_{28}^{\text{rw}}$ (%)	$V_{18}^{\text{rw}}$ (%)
Hom-NoSpc	36.2 (0.2)	32.8 (0.2)	35.0 (0.1)	0.095 (0.007)	0.99 (0.01)	96.2 (0.9)	49.9 (3.8)	4.1 (1.7)	8.6 (3.1)	23.9 (7.8)	8.2 (3.5)	13.7 (5.1)	25.5 (7.6)
Hom-Spc	36.2 (0.2)	33.0 (0.1)	35.0 (0.0)	0.090 (0.007)	1.01 (0.02)	97.2 (0.5)	51.5 (2.7)	0.9 (1.2)	2.5 (2.8)	10.9 (6.7)	1.6 (2.4)	3.7 (4.0)	13.8 (6.7)
Het-NoSpc	39.1 (0.2)	33.2 (0.3)	37.5 (0.1)	0.155 (0.009)	1.12 (0.02)	98.1 (0.6)	87.4 (3.2)	4.6 (1.7)	9.0 (2.9)	23.9 (6.1)	9.5 (3.8)	14.7 (4.9)	26.3 (6.6)
Het-Spc	38.9 (0.2)	33.7 (0.4)	37.6 (0.1)	0.140 (0.013)	1.14 (0.03)	99.0 (0.6)	91.9 (2.7)	1.2 (1.6)	2.8 (3.0)	11.4 (6.9)	1.8 (2.5)	4.3 (4.4)	14.4 (6.7)

CI, $V_{33.2}^{\text{Body}}/\text{PTV}$, conformity index; D_n%, minimum dose to n% of the PTV structure; Het, plans with D_2% ≤ 40.2 Gy; HI, (D_98% − D_2%)/D_50%, homogeneity index; Hom, plans with D_2% ≤ 37.5 Gy; NoSpc, plans computed on CT scanned before spacer insertion; PTV, planning target volume; Spc, plans computed on CT scanned after spacer insertion; $V_{\text{m}}^{\text{PTV}}$, percentage of PTV structure receiving ≥m (Gy); $V_{\text{m}}^{\text{r}}$, percentage of rectal structure receiving ≥m (Gy); $V_{\text{m}}^{\text{rw}}$, percentage of rectal-wall structure receiving ≥m (Gy).