Simultaneous integrated boost (SIB) to dominant intra-prostatic lesions during extreme hypofractionation for prostate cancer: the impact of rectal spacers

Table 1 Dose-volume objectives and constraints for the SPORT trial adopted in this study, minor and major variations, definitions and the achieved median (range) dose levels and plan quality metrics, and NTCPs

Target volumes	Objectives/constraints	Minor variation	Major variation	Pre-spacer	Post-spacer
CTVpb	V_Px [%] ≥ 95%	95% > V_Px ≥ 90%		95.72 (90.86–99.97)	97.88 (92.44–99.98)*
	medianD [Gy]			47.77 (46.16–52.50)	47.77 (46.14–52.40)
	D_2% ≤ 55 Gy			48.93 (46.20–52.60)	48.55 (47.00–52.60)
Tpsv	V_40Gy [%] ≥ 95%	V_40Gy ≥ 90%	V_40Gy < 90%	96.98 (93.12–99.31)	98.40 (95.89–99.81)*
PTVpsv	V_36.25 Gy [%] ≥ 95%	V_36.25 Gy ≥ 90%	V_36.25 Gy < 90%	95.41 (91.28–97.02)	95.93 (94.60–99.79)
	D_98% [Gy] ≥ 34.4 Gy			35.37 (34.50–35.90)	35.60 (35.00–37.20)*
	D_max [%] < 120% of boost			106.12 (104.67–109.34)	106.28 (105.04–111.38)
	D_2% [%] ≤ 107% of boost			103.19 (101.00–104.67)	103.70 (102.20–106.02)

Organs at risk	Objectives/constraints	Minor variation	Major variation	Pre-spacer	Post-spacer
Rectum	V_18.1 Gy [%] < 50%			29.42 (18.16–42.35)	23.22 (3.43–34.99)**
	V_29Gy [%] < 20%			12.59 (5.69–19.40)	5.48 (0.01–13.30)**
	V_36Gy [cc] < 1 cc	< 2 cc	> 2 cc	1.60 (0.51–2.00)	0.42 (0.00–1.00)**
EUD [Gy]	EUD [Gy]			59.42 (55.37–65.11)	52.24 (24.30–57.40)**
NTCP [%]	NTCP [%]			4.05 (1.75–11.46)	0.86 (0.00–2.72)**
Bladder	V_18.1 Gy [%] < 40%			12.67 (5.35–22.33)	11.29 (7.37–21.83)
	V_37Gy [cc] < 10 cc	10 cc ≤ V_37Gy < 20 cc	V_37Gy ≥ 20 cc	6.42 (0.72–9.33)	5.22 (0.43–8.39)
Urethra	V_42Gy [%] < 50%			19.12 (0.03–49.81)	19.59 (0.00–47.06)
	V_45Gy [cc] < 0.01 cc			0.00 (0.00–0.00)	0.00 (0.00–0.00)
Urethra_PRV	V_42Gy [%] < 50%			27.82 (9.76–47.16)	25.30 (9.52–44.04)
	V_45Gy [cc] < 0.1 cc			0.04 (0.00–0.10)	0.01 (0.00–0.08)
Plan	CI			1.15 (1.04–1.22)	1.18 (1.11–1.31)
	R50%			3.53 (3.15–3.87)	3.58 (3.37–4.11)

V_xGy = volume receiving × Gy, EUD = equivalent uniform dose, RTOG conformity index \(\left( {{\text{CI}} = \frac{{{\text{Volume}}\;{\text{ of}}\;{\text{ 95}}\%\;{\text{isodose}}}}{{{\text{PTV }}\;{\text{volume }}}}} \right)\), \({\text{R}}_{50} = \frac{{{\text{Vol}}_{{50\% \;{\text{pres}}}} }}{{{\text{PTV}}\;{\text{ volume}}}}\), where \({\text{Vol}}_{{{\text{50}}\%\;{\text{pres}}}}\) is the tissue volumes receiving at least 50% of the PTV prescription dose. All patients’ pre- and post-spacer plans were generated using matched boost dose levels* indicates a p-value ≤ 0.05 and ** indicates a p-value ≤ 0.01. Bladder and rectum volume in pre- and post- spacer scans are provided in Additional file 1: Table ST3.

ISSN: 1748-717X