Purpose: To determine the effect of dose fractionation and time delay between radiotherapyand imaging on dynamic contrast enhanced (DCE)-MRI parameters in early stage breast cancerpatients receiving neoadjuvant stereotactic ablative radiotherapy (SABR).
Methods: Early-stage breast cancer patients were scanned on a 3T-PET/MRI scanner (Siemens Biograph mMR) before and after SABR, but prior to surgery. Five patients were imaged 6-7 days after 21 Gy in 1 fraction (Group 1), five were imaged 16-19 days after 21 Gy in 1 fraction (Group 2), and five were imaged 16-18 days after 30 Gy in 3 fractions (Group 3). Tumours were automatically segmented on pre-RT images and transferred to post-RT images using deformable registration. The Toftâ€™s model was applied voxel-by-voxel providing values of Ktrans (marker of neoangiogensis) and ve (volume of extracellular-extravascular space). Mean parameter values were determined for each tumour. Changes between pre- and post-radiotherapy were assessed using a Wilcoxon signed rank test (p=0.05). The percent change in the mean for each group is reported.
Results: For Ktrans, Group 1 showed an increase of 54% (p=0.043) while Group 2 and 3 showed decreases of 17% (p=0.043) and 29% (p=0.043). For ve, there was no significant change in Group 1 (p=0.35). Groups 2, and 3, showed an increase of 24% (p=0.043), and 23% (p=0.08), respectively. Ktrans increases at one-week post-RT likely represent acute inflammation. The decrease in Ktrans 2.5 weeks post-RT is consistent with previous studies in other cancer sites showing tumour response.
Conclusion: This study is the first to quantify Ktrans and ve changes at two different early timepoints and two fractionation schemes using DCE-MRI. These results suggest a delay time of 2.5 weeks post-SABR. If these parameters are sufficiently predictive of response, it may be possible to eliminate surgery altogether, reducing the burden on patients further.