Preliminary report of the 6-minute walk test as a predictor of radiation-induced pulmonary toxicity

Purpose: To assess the 6-minute walk test (6MWT) as a predictor of radiation therapy-induced lung injury (RTLI). Methods and Materials: The 6MWT is a simple, economical, and reproducible test that measures both how far a person can walk in 6 min and any associated changes in vital signs. As part of a prospective trial to study RTLI, a pre-RT 6MWT was performed in 41 patients. The predictive capacities of pre-RT 6MWT, forced expiratory volume in 1 s (FEV1), and single-breath diffusing capacity for carbon monoxide (DLCO) for the development of RTLI were assessed with receiver operating curve (ROC) techniques. To evaluate the 6MWT, alone or with mean lung dose (MLD) of radiation, as a predictor of RTLI, the rates of RTLI in patient subgroups defined by 6MWT results were compared by using Fisher's exact test. Results: Thirty-one patients with <3 months' follow-up were evaluable. The median baseline 6MWT result was 1400 ft. Of 31 patients, 7 developed Grade <2 RTLI. Of 15 patients with an MLD >18 Gy (the median), 5 developed RTLI, compared with 2 of 16 with MLD ≤18 Gy (p = 0.22). Among those with an MLD ≤18 Gy, the RTLI rates were 0 of 8 and 2 of 8 for 6MWT results <1400 ft or <1400 ft, respectively, p = 0.46. The ROC area under the curve for individual metrics was as follows: FEV1 0.66, MLD 0.70, DLCO 0.61, and 6MWT 0.47. Combining FEV1 with 6MWT increased the ROC to 0.71, suggesting that the ratio might be a better predictor than the individual values. Patients with a high 6MWT/FEV1 ratio had a lower rate of RTLI than those with a relatively low ratio. Conclusions: The 6MWT might provide prognostic information beyond pulmonary function tests and dosimetric parameters in predicting RTLI. Additional work is needed to better assess the utility of these functional metrics.