Ct/Mri Fusion Significantly Reduces The Risk Of Geographic Miss When Ablating The Ovaries By Radiotherapy

Ovarian ablation is well recognized as a highly effective therapy for pre-menopausal women with early stage and metastatic breast cancer. The efficacy of radiotherapy for ovarian ablation has been reported in the range of 75 - 97% using various treatment techniques and fractionation schemes. Standard radiotherapy field borders are defined with reference to bony landmarks in the absence of more sophisticated localization techniques. We employed CT/MRI fusion to localize the ovaries in women undergoing ovarian ablation and compared target volume coverage with that provided by standard field borders. The local ethics committee approved this retrospective study. Planning CT and MRI images were obtained in the treatment position. The ovaries were outlined on fused CT/MRI images and standard anterior/posterior fields were extended to cover the contoured volumes. All patients were treated with 18MV photons; seven patients received 15 Gy in 5 fractions, one patient received 20 Gy in 5 fractions. Standard fields were defined as follows: Superior border - Inferior edge of sacroiliac joint, inferior border - mid-obturator foramen, lateral border - 1cm lateral to pelvic brim. Dosimetric parameters were compared using the Wilcoxon signed ranks test. Eight women with a median age of 48 years (34 to 50) were included. All were either menstruating or peri-menopausal with 7 having distant metastases. Both ovaries were identified on MRI in all cases. The median cranial-caudal dimension of the ovaries was 2.4 cm (0.8 to 6.0) and the median volume was 10.5 cm3 (0.8 to 24.4). Fields designed based on CT/MRI images encompassed both ovaries in all cases with the volume of ovary receiving 95% of the prescribed dose (V95) = 100%. Treated volumes were significantly larger than standard fields with p = 0.012. Using standard radiation fields, complete geographic miss occurred for four ovaries in three patients (37.5%). The median distance of the ovaries from the field edge was: 0.5 cm (-3.9 to 6.7) for the superior field border; 2.6 cm (-0.9 to 3.3) for the lateral field borders; and 6.4 cm (3.3 to 9.3) for the inferior field border (- indicating a coordinate location outside of the standard field borders). The V95 was <80% for 12 ovaries (75%) and <50% for 7 ovaries (44%). CT/MRI planning techniques provided significantly better V95 for right and left ovary with p = 0.017 and 0.028 respectively. CT/MRI fusion provides significantly better target volume delineation and coverage when ablating the ovaries using radiotherapy. Geographical misses due to failure to localize the ovaries may partially account for the varying efficacy of radiotherapy induced ovarian ablation previously reported.