Density override in treatment planning to mitigate the dosimetric effect induced by gastrointestinal gas in esophageal cancer radiation therapy

Figures & data

Table 1. Patient overview and relevant information.

Patient	Age	Sex	Diagnosis	Dose scheme [Gy]	Number of CBCT/fCT scans^a	Gas volume on pCT [mL]	Mean gas volume on fCTs [mL]	SD of gas volume on fCTs [mL]	Total number of times for each plan selected in the plan-selection strategy
									No DO	DO = 0.5	DO = 1
1	57	M	Distal AD	23 × 1.8	8	110	170	104	6	1	1
2	77	M	GEJ AD	23 × 1.8	11	284	505	116	11	0	0
3	63	M	GEJ AD	28 × 1.8	14	302	519	152	14	0	0
4	71	M	Distal AD	28 × 1.8	14	61	153	56	14	0	0
5	72	M	Distal AD	23 × 1.8	8	732	501	101	2	6	0
6	70	F	Middle SCC	23 × 1.8	10	56	33	20	2	6	2
7	66	M	Distal AD	23 × 1.8	23	287	61	38	0	8	15
8	62	M	Distal AD	23 × 1.8	23	425	447	162	19	4	0
9	77	M	Middle/Distal SCC	28 × 1.8	28	218	177	98	14	11	3

M: male; F: female; AD: adenocarcinoma; SCC: squamous cell carcinoma; GEJ: gastroesophageal junction; pCT: planning CT; fCT: fractional CT; CBCT: cone-beam CT; SD: standard deviation; DO: density override.

^aThe number of CBCT scans depends on which setup verification protocol was used: extensive no act level (for patients 1–6) or online protocol (for patients 7–9). Protocol change in our clinic was done during this period.

Figure 1. Example of the procedure how a fractional computed tomography (fCT) was made. (a) The original planning CT (pCT) with gas pocket delineations. (b) The gas pockets in the pCT were filled with soft-tissue density. (c) One cone-beam CT (CBCT) with gas pocket delineations. (d) Gas pockets in the CBCT were overlaid on the pCT with filled gas pockets in (b), yielding the fCT. CTV: clinical target volume; PTV: planning target volume.

Figure 2. Gas volume for all nine patients over the whole treatment course. Day 0 denotes the day that the planning computed tomography (CT) scan was acquired.

Figure 3. The minimum and maximum dose differences in the clinical target volume (CTV) between the fractional doses and the planned doses per fraction against the gas volume difference relative to the initial gas volume for both intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans with no density override (DO). The fitting line and its 95% confidence interval are denoted as black solid line and dotted lines around it, respectively. The marginal and conditional R², the slopes, and the intercepts derived from the linear mixed-effects model are also given.

Figure 4. Relationship of the minimum dose differences between no density override (DO) and DO = 0.5 and between no DO and DO = 1, for intensity modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) planning, in the fractions with decreased gas volume relative to initial gas volume. The solid line denotes the fitting line. The marginal and conditional R², the slopes, and the intercepts derived from the linear mixed-effects model are also given.

Table 2. V_95% and D_2% of planned dose and accumulated dose in the clinical target volume.

	Patient	V95% [%]							D2% [%]
		Planned dose			Single-plan-based accumulated dose			Plan-selection-based accumulated dose	Planned dose			Single-plan-based accumulated dose			Plan-selection-based accumulated dose
		No DO	DO = 0.5	DO = 1	No DO	DO = 0.5	DO = 1		No DO	DO = 0.5	DO = 1	No DO	DO = 0.5	DO = 1
IMRT	1	100.0	100.0	100.0	100.0	100.0	100.0	100.0	103.1	103.5	103.9	103.3	105.5	107.6	103.5
	2	100.0	100.0	100.0	99.7	99.7	99.8	99.7	103.9	103.2	104.5	110.2	111.5	113.8	110.2
	3	100.0	99.8	100.0	100.0	99.8	100.0	100.0	104.1	103.3	103.8	108.2	109.1	113.5	108.2
	4	100.0	100.0	100.0	99.7	99.8	99.9	99.7	103.7	103.3	103.7	107.5	107.3	108.4	107.5
	5	100.0	100.0	100.0	94.2	100.0	100.0	100.0	104.7	104.1	103.5	105.2	106.4	113.0	105.8
	6	100.0	100.0	100.0	100.0	100.0	100.0	100.0	104.3	104.1	103.8	104.4	104.1	104.0	103.6
	7	100.0	100.0	100.0	99.1	99.5	100.0	99.9	106.2	104.4	103.9	103.7	104.4	104.2	103.6
	8	100.0	100.0	100.0	99.3	99.1	99.5	99.2	106.6	104.3	104.7	106.0	108.2	111.8	105.8
	9	100.0	100.0	100.0	100.0	100.0	100.0	100.0	104.3	104.6	104.3	105.2	106.0	108.2	105.4
	Mean	100.0	100.0	100.0	99.1	99.8	99.9	99.8	104.5	103.9	104.0	106.0	106.9	109.4	106.0
VMAT	1	100.0	100.0	100.0	100.0	100.0	100.0	100.0	103.2	103.4	103.2	104.0	104.7	106.9	104.1
	2	100.0	100.0	100.0	99.8	99.7	99.8	99.8	104.4	103.7	103.2	109.9	110.0	113.2	109.9
	3	100.0	100.0	100.0	100.0	100.0	100.0	100.0	103.2	102.9	103.0	109.0	110.5	111.9	109.0
	4	100.0	100.0	100.0	99.7	99.8	99.9	99.7	104.0	103.9	104.0	105.1	105.8	106.8	105.1
	5	100.0	100.0	100.0	100.0	100.0	100.0	100.0	104.9	103.7	103.5	104.3	106.9	111.0	105.9
	6	100.0	100.0	100.0	100.0	100.0	100.0	100.0	103.3	103.4	103.5	103.3	103.4	103.5	103.3
	7	100.0	100.0	100.0	99.8	99.7	100.0	99.9	103.8	102.8	103.3	102.9	102.8	103.6	102.9
	8	100.0	100.0	100.0	99.2	99.2	99.4	99.2	104.6	103.9	103.8	105.7	106.1	110.2	105.6
	9	100.0	100.0	100.0	100.0	100.0	100.0	100.0	103.6	103.8	104.3	103.9	104.2	104.6	103.6
	Mean	100.0	100.0	100.0	99.8	99.8	99.9	99.8	103.9	103.5	103.5	105.3	106.0	106.0	105.5

DO: density override; IMRT: intensity-modulated radiation therapy; VMAT: volumetric-modulated arc therapy.