Impact of bowel and rectum air on target dose with robustly optimized intensity-modulated proton therapy plans

Figures & data

Table 1. Information on investigated cases.

Case index	Site	Beams	Prescription dose (Gy [RBE]) on CTV (GTV)	Adaptive plan
1	Prostate	90,180,270	2 × 28 (2 × 36)	No
2	Prostate	145,180,205	(2 × 36)	No
3	Prostate	90,180,270	2 × 28 (2 × 36)	No
4	Prostate	90,180,270	2 × 28 (2 × 36)	No
5	Prostate	90,180,270	2 × 28 (2 × 36)	No
6	Prostate	90,180,270	2 × 28 (2 × 36)	No
7	Prostate	90,345	(2 × 36)	No
8	Prostate	90,180,270	2 × 28 (2 × 36)	No
9	Prostate	90,180,270	2 × 28 (2 × 36)	No
10	Prostate	90,180,270	2 × 28 (2 × 36)	Yes
11	Prostate	90,270	(2 × 36)	No
12	Gyn	0,75,285 (HFP) 25,55_15,80 (boost)	1.8 × 25 (1.8 × 33)	Yes
13	Gyn	0,70,180 (SIB)	1.8 × 25 (1.8 × 33)	Yes
14	Gyn	90,150,180, 210,270	1.8 × 25 (1.8 × 33)	Yes
15	Colon	100,150,205	2 × 15	No
16	Colon	90,160,220	2 × 22	No
17	Embryonal rhabdomyo-sarcoma	0,120,230,270 (init) 0,90,180,270 (boost)	1.8 × 20 (1.8 × 28)	Yes

Patient orientation was head-first supine, except for case 12, which was head-first prone (HFP). Case 2 had implants in both hips, and case 7 had an implant in the right hip. In the other 9 prostate cases the boost plans used lateral beams only. Cases 2, 7 and 11 were prostate-only cases. For those non-prostate cases, initial and boost beam data are listed separately. Gy [RBE]: Gray [relative biological effectiveness–weighted]; init: initial treatment; SIB: simultaneous integrated boost.

Figure 1. ΔVair extracted from the pCT and QACT in: (a) rectum; (b) small bowel; and (c) large bowel for the 17 cases. Total ΔVair is displayed in (d). Cases 1–11 are prostate cancer (prostate bed–only in cases 7 and 11); cases 12–14 are gynecologic cancer; cases 15 and 16 are colon cancer; and case 17 is embryonal rhabdomyosarcoma. For each case, air volume in the pCT is displayed in blue before the mean and SD of ΔVair in the QACTs.

Figure 2. (a) $\Delta$D99% and (b) $\Delta$D95% in the initial plan in all study cases; (c) $\Delta$D99%; and (d) ΔD95% in the boost plan in the prostate cases. In (a) and (b), results from optimization with and without virtual CTs are displayed. “Mean_MO” and “Mean_noMO” stand for the mean value of the dose changes from the plans with and without the virtual CTs in the optimization, respectively. In (c) and (d), virtual CTs were used.

Figure 3. Registered pCT and QACT (left column), dose distribution in the pCT (middle column), and difference in doses in the pCT and QACT (right column) for cases 12 (top row), 17 (middle row), and 14 (bottom row). The CTV contour in the pCT is in solid green, and the dashed green line on the QACT was obtained by DIR. Brown = contour of air in pCT; red = contour of air in QACT.

Table 2. Correlation coefficients and t-test p values between $\Delta$D99%,$\mathrm{ }\Delta$D95%, and ΔVair in rectum, small and large bowel, and total change of air volume.

Sites	Air volume change in	Rectum (p)	Small bowel (p)	Large bowel (p)	Total
Prostate (case 1-11)	D99 change in CTV	0.62 (.01)	−0.21 (.28)	0.35 (<.01)	0.65 (<0.01)
	D95 change in CTV	0.52 (<.01)	−0.05 (.15)	0.14 (<.01)	0.43 (<0.01)
Others (case12-17)	D99 change in CTV	0.94 (.32)	0.49 (.14)	0.42 (.03)	0.61 (0.05)
	D95 change in CTV	0.64 (.14)	0.95 (.07)	0.77 (.02)	0.94 (0.04)