Volumetric and diffusion MRI longitudinal patterns in brain metastases after laser interstitial thermal therapy

To characterize MRI changes of brain metastases (BM) following laser interstitial thermal therapy (LITT), particularly in lesions exhibiting durable response or early progression.

Longitudinal scans from patients with LITT-treated BM were retrospectively analyzed. Treatment response was categorized as durable response, long-term disease control (i.e., stable at 1 year), stable disease < 1 year, or progression < 1 year. Volumetric and diffusion MRI changes after LITT were analyzed for each subregion (contrast-enhancing, central non-enhancing, whole lesion). Volumetric changes were modeled with bi-exponential fits in responding lesions and progressors.

295 MRI scans from 47 lesions across 42 patients (57.8 ± 14.3 years old, males:females 21:21) were analyzed. Overall, the post-LITT scan showed a lesion enlargement (p < 0.0001 for all subregions), more pronounced in the contrast-enhancing (CE) component (median = +77%, p < 0.0001), and a reduction in the apparent diffusion coefficient (ADC) (p < 0.001), especially in the central non-CE component (median = -224 × 10^-6 mm²/s, p < 0.0001), with no significant differences between responders and progressors. Based on mathematical modeling, the responding lesions shrank to half of the post-LITT size after 79.83 days (median "pseudo-half-life"), and the progressing lesions shrank for a median of 27 days (median time-to-growth) before regrowing. The estimated optimal timepoints for follow-up scans were 23 days and 125 days, yielding accuracy/specificity/sensitivity 0.82/1.0/0.55 in identifying progressing lesions (p < 0.01).

BM typically exhibit an early volume increase with diffusion restriction after LITT. Responders then show bi-exponential shrinkage with gradual diffusion increase. Progression can usually be detected only after 3-4 months, because earlier radiographic patterns may overlap with responding lesions.

Question Laser interstitial thermal therapy (LITT) is an emerging local treatment for brain metastases, but the radiographic patterns following this treatment have not been thoroughly described. Findings Responding lesions showed a typical radiographic pattern with early volumetric enlargement and diffusion restriction (not exclusive of responders), followed by a bi-exponential shrinkage and diffusion elevation. Clinical relevance Being aware of the typical radiographic changes in brain metastases responding to LITT is informative for the interpretation of follow-up images. Early volumetric and diffusion changes (< 3-4 months) do not appear to be reliable markers to predict treatment success.