Incidence, risk factors, and dose-volume relationship of radiation-induced rib fracture after carbon ion radiotherapy for lung cancer

Figures & data

Table I. Patient characteristics.

Characteristic
Age  (median)	47 – 83 y.o. (81.5)
Sex
Male	12 patients
Female	6 patients
T stage
1a	5 patients
1b	8 patients
2a	5 patients
Rib-tumor distance
≧ 2.0 cm	4 patients
< 2.0 cm	14 patients

Footnote: Rib-tumor distance was defined as the minimum distance from the rib to the edge of gross tumor volume.

Table II. Comparison of the probability of radiation-induced rib fracture (RIRF) for each patient characteristics.

Characteristic		number of cases	2 - year cumulative incidence of RIRF (%)	p value
Age	≧ 75 y.o.	n = 8	45	0.67
	< 75 y.o.	n = 10	55
	Sex	Male	n = 12	33	0.11
Female		n = 6	75
Prescribed dose		52.8 Gy(RBE)	n = 13	41	0.03
	60.0 Gy(RBE)	n = 5	73

Footnote: RIRF, radiation induced rib fracture; Gy (RBE), the dose of carbon ion radiotherapy calculated by multiplying the carbon absorbed dose (Gy) with the relative biological effectiveness.

Table III. Comparison of dosimetric parameters between fractured ribs and non-fractured ribs.

Parameter	Fractured ribs	Non-fractured ribs	p value
V60 (cm^3)	1.0 ± 0.5	0.4 ± 0.2	<0.001
V50 (cm^3)	1.9 ± 0.5	0.7 ± 0.2	0.03
V40 (cm^3)	3.1 ± 0.6	1.6 ± 0.3	0.04
V30 (cm^3)	4.9 ± 0.6	2.6 ± 0.9	0.04
V20 (cm^3)	6.2 ± 0.6	4.3 ± 0.4	0.13
Dmax (Gy(RBE))	53.7 ± 1.8	41.7 ± 2.0	<0.001

Footnote: Vx, volume of rib receiving > x Gy (RBE); D_max, maximum point dose of the ribs; , the minimum dose in the most irradiated rib volumes of X - cm³.

Table IV. Optimal cut-off value determined by ROC analysis and calculated area under the curve.

	Opitmal cut-off value	AUC	Incidence of RIRF above/ below the cut-off value
	18.9 Gy (RBE)	0.68	56/10%
	31.3 Gy (RBE)	0.75	49/8%
	38.2 Gy (RBE)	0.78	53/4%
	43.6 Gy (RBE)	0.76	52/7%
Dmax	49.7 Gy (RBE)	0.74	54/8%
V60	0.1 cm^3	0.67	68/21%
V40	0.9 cm^3	0.76	52/7%
V30	4.7 cm^3	0.74	59/16%

Footnote: ROC, receiver operating characteristic curve; AUC, area under the curve.

Figure 1. Thick line showed cumulative incidence of radiation-induced rib fracture per patients. Among 18 patients, seven patients (39%) developed rib fracture. Dashed line and thin line showed comparison of the cumulative incidence between two groups [, ≥38.2 Gy (RBE) or less] divided by optimal cut-off value derived from ROC analysis (p <0.05).