Purpose: To assess differences in delineated target volumes of liver metastases using contrast-enhanced CT and different MRI sequences for radiation treatment planning.
Patients and methods: 25 patients with 43 colorectal liver metastases were recruited. Tumor margins were defined by two experienced radiologists. The resulting D90 was assessed and the CT-based 3-D dose distribution merged with the according MRI dataset by employing image fusion. A theoretical D90 as a result of MRI-based treatment planning was assessed for various MRI sequences individually.
Results: In venous phase contrast-enhanced CT, the mean tumor volume was 20 ml; T1-weighted (T1w) MRI, 27 ml; contrast-enhanced T1w 42 ml; T2w 65 ml. The difference between the target volumes as assessed by either CT or MRI was 181% for T1w images, 178% for contrast-enhanced T1w, and 246% for T2w sequences. All differences were statistically significant (p < 0.05). The analysis of the dose-volume histograms revealed statistically significant differences (i.e., for the D90) for the different target volumes specified by CT and MRI: mean D90 on CT, 18 Gy; plain T1w, 16 Gy; contrast-enhanced T1w, 15.5 Gy; T2w, 12 Gy. Hence, delineation of a larger target volume in T2w MRI compared to contrast-enhanced CT resulted in a smaller D90. The mean differences of tumor volumes assessed by CT and plain T1w were significantly higher in the group of patients showing local tumor recurrences as compared to patients with long-term local tumor control (p = 0.002).
Conclusion: For treatment planning of liver metastases, the use of either plain T1w or T2w sequences is recommended to delineate the clinical target volume as completely as possible and not to miss potential tumor cell congregations in the surroundings as in CT.