Addition of magnetic resonance imaging to computed tomography-based three-dimensional conformal radiotherapy planning for postoperative treatment of astrocytomas: Changes in tumor volume and isocenter shift.

Introduction: Postoperative radiotherapy is the current gold standard treatment in astrocytomas. Computed tomography (CT)-based radiotherapy planning leads to either missing of the tumor volume or underdosing. The aim of this prospective study was to study the changes in tumor volume on addition of magnetic resonance imaging (MRI) to CT-based three-dimensional radiotherapy treatment planning of astrocytomas.

Materials And Methods: Twenty-five consecutive patients of astrocytoma (WHO grades I-IV) for postoperative three-dimensional conformal radiotherapy were included in this prospective study. Postoperative tumor volumes were contoured on CT-based images and recontoured on CT-MRI images after automated MRI co-registration on treatment planning system Eclipse 8.9.15 as per ICRU-50 report. Tumor volumes were compared with each other.

Result: The MRI-based mean and median tumor volume was 24.24 cc ± 13.489 and 18.72 cc (range 5.6-46.48 cc), respectively, while for CT it was 19.4 cc ± 11.218 and 16.24 cc (range: 5.1-38.72 cc), respectively. The mean and median isocenter shift between CT and MRI was 4.05 mm and 4.39 mm (range 0.92-6.32 mm), respectively. There is a linear relationship between MRI and CT volume with a good correlation coefficient of R (2) = 0.989, and MRI-based tumor volume was 1.208 times as compared to CT volume. Statistical analysis using paired sample t-test for the difference in CT and MRI tumor volume was highly significant (P < 0.001).

Conclusion: Addition of MRI to the CT-based three-dimensional radiation treatment planning reduces the chances of geographical miss or tumor under dosing. Thus, MRI should be an integral part of three-dimensional planning of astrocytomas.