MR imaging assessment of irregular shrinkage of tumor morphology and volume in cervical cancer during radiation therapy

To study the clinical significance of the morphological and volume changes in cervical cancer during an ongoing course of radiation therapy (RT) using MR imaging. Methods: Serial MR imaging examinations were performed in 60 advanced cervical cancer patients. MR imaging was obtained at the start of RT, at 20-25 Gy (2-2.5 weeks of RT), at 45-50 Gy (4-5 weeks of RT), and 1-2 month post-RT. Tumor morphology was classified qualitatively as well-defined (round/oval with a well-demarcated smooth margin) vs. lobulated vs. irregular and tumor volume was assessed in each serial MR examination independently by ROI volumetry and diameter volumetry. ROI volumetry was traced on the computer workstation with a trackball in each sagittal T2-weighted image and calculated by the summation of all tumor areas in each slice and multiplication by the slice profile. Diameter volumetry was to measure the largest three orthogonal tumor diameters in each orthogonal measurement plane and calculate as an ellipsoid formula (V=d1 x d2 x d3 x p/6). Serial tumor volume was compared between the two measurement methods. Results: The proportion of lobulated and irregular tumors increased early during RT and declined lately post-RT (68% pre-RT, 80% at 2-2.5 weeks of RT, 72% at 4-5 weeks of RT, 33% post-RT). Accordingly, ROI volumetry and diameter volumetry correlated well pre-RT (r1=0.89) and post-RT (r4=0.80), but poorly during RT (r2 = 0.17 at 2-2.5 weeks of RT, r3 = 0.69 at 4-5 weeks of RT). Conclusions: Cervical cancers regress in a non-uniform fashion during RT and undergo increasingly irregular shrinkage. Measurement with ROI volumetry techniques, which can optimally measure irregular volumes, provides better assessment of radiation response during treatment than diameter volumetry.