Aim: To test and evaluate direct sagittal-plane tumor delineation for MRI-based image-guided brachytherapy (IGBT) planning for patients with cervical cancer.
Materials and methods: An image registration method based on the sagittal source MR images was developed and employed in ten patients with an indwelling ring/tandem applicator. The gross tumor volume (GTV) was delineated separately on the sagittal (GTV-S) and axial images (GTV-A). GTV conformity indices and dose-volume histogram analyses were compared among GTV-S and GTV-A (paired t-test).
Results: Image quality and delineation in the sagittal images was graded superior to the axial images. The ratio of common volume of the axial and sagittal volumes to that of the axial volume was 0.77 +/- (standard deviation) 0.2. The GTV-S mean volume (19.6 +/- 13.8 mL) was significantly larger than the GTV-A mean volume (10.3 +/- 7.3 mL, p=0.003). The GTV-S mean D99 (5.2 +/- 2.5 Gy) was significantly lower than the GTV-A mean D99 (6.9 +/- 2.7 Gy, p=0.013). The GTV-S mean D90 (6.8 +/- 2.8 Gy) was significantly lower than the GTV-A mean D90 (8.5 +/- 3.1 Gy, p=0.016).
Conclusions: Registration of the sagittal source MRI and contouring the GTV directly on the sagittal images is feasible and practical for IGBT. Consistently larger sagittal GTVs may be explained by the better visualization and more continuous tumor topology in the sagittal plane, compared to the discrete oblique sectioning of the uterus/tumor and partial volume loss in the axial plane.