Purpose: To present a novel method for monitoring radiation damage using the parameters of our 5D lung motion model. Method and Materials: The 5D breathing motion model describes breathing motion as a function of tidal volume ν and airflow f and is parameterized as: [formula omitted], where r is the position of a piece of tissue located at reference position r0. α and β are each functions of r0 and relate tissue motion with tidal volume and airflow. The continuity equation [formula omitted], where ρ is the local density, t is time, and U is the velocity field, is modified to replace tidal volume as the independent variable. At inhalation or exhalation, the f=0 and under these conditions, the continuity equation leads to [formula omitted] in the tissue reference frame. This equation provides the relationship between the divergence of α and the relative change in local tissue density. The powerful aspect of this result is that the α parameter is determined using free‐breathing scan registrations which inherently include the complex hysteresis interplay, and yet its divergence indicates the local relative density change as a function of tidal volume. The values of α were determined for repeat 5D CT scans for both irradiated and unirradiated lungs. Results: The unirradiated patient had little change in the value of α between two scan sessions. A lung cancer patient had 5D CT scans acquired after 16 Gy and 70 Gy tumor dose. The value of α changed dramatically in the irradiated regions. This result indicates that α is sensitive to radiation dose damage. Conclusion: The divergent of α is shown to be related to the relative local density variation as measured using free‐breathing 5DCT. Repeat scans indicate that variations in the α distribution, corresponding to changes in local density variations, may be sensitive to local radiation damage.