The effect of time-delayed contrast-enhanced scanning in determining the gross tumor target volume of large-volume brain metastases

Background and purpose: To assess the variation of large-volume brain metastases (BMs) boundaries and shapes using enhanced magnetic resonance (MR) scanning with different delay times and to provide a basis for determining the gross tumor target volume (GTV) for radiotherapy of BMs.

Materials and methods: We prospectively enrolled 155 patients initially diagnosed with BMs (561 lesions > 1 cm). Contrast-enhanced (CE) T1-weighted imaging scans were performed 1, 3, 5, 10, 18, and 20 min after gadolinium-based contrast agent injection and GTVs were determined as GTV_-1min, GTV_-3min, GTV_-5min, GTV_-10min, GTV_-18min, and GTV_-20min, respectively, which were subsequently fused in different phases. Fusion of the six GTVs was defined as GTV_-total, which was set as the reference GTV. The volume, shape, and signal intensity of the GTVs and brain white matter (BWM) were compared at different delay times.

Results: GTV_-3min, GTV_-5min, GTV_-10min, GTV_-18min, and GTV_-20min volumes increased by 2.2 %, 3.8 %, 6.5 %, 9.5 %, and 10.6 %, respectively (P < 0.05) compared with GTV_-1min. Compared with GTV_-total, GTV_-1min, GTV_-3min, GTV_-5min, GTV_-10min, GTV_-18min, and GTV_-20min volumes reduced by 25.4 %, 22.1 %, 18.7 %, 15.0 %, 11.2 %, and 10.3 %, respectively (P < 0.05). Compared with GTV_-total, 29 (51.8 %) fused GTVs had a volume reduction rate < 5 %, 45 (80.4 %) had a Dice similarity coefficient > 0.95, and all contained GTV_-10min, GTV_-18min or GTV_-20min. The signal intensity ratio between the GTV and BWM peaked at 5 min (0.351 ± 0.24).

Conclusion: Enhanced MR scans with different delay times show significant differences in the boundaries and shapes of large-volume BMs, and time-delayed multi-phase CE scanning should be used in GTV determination, with time phases ≥ 10 min being mandatory.