Purpose: Cone-beam CT and MRI based adaptive platform have recently been developed enabling adaptive radiotherapy (ART). Stereotactic partial breast irradiation (sPBI) is well suited for daily ART due to the day-to-day variation of the breast tissue. In this work we compared the dosimetry of treatment plans designed on two adaptive systems Ethos and Unity, and a non-ART Cyberknife.
Methods: Treatment plans were generated for 16 patients (eight left and right) in each system using our department criteria for a prescription of 30Gy in 5Fx. The plans were then compared in an independent system and dosimetric parameters recorded for the ipsilateral lung volume receiving 9Gy (V9Gy), contralateral lung and heart (left-sided only) V1.5Gy, normal breast V30Gy and V15Gy, gradient-index (GI) and Paddick conformity-index (CI).
Results: PTVs had a mean volume of 156cc (44-369cc). Cyberknife provided the best GI, CI, normal breast V15Gy and ipsilateral lung V9Gy followed by Ethos and Unity, all showing statistical significance (p<0.05). With CI of 0.91, 0.89 and 0.85 for Cyberknife, Ethos and Unity, respectively. Similarly, GI was 2.8, 3.1 and 3.6 with a similar increase in normal breast dose. Ipsilateral lung V9Gy was 4.1%, 5.4% and 6.6% for Cyberknife, Ethos and Unity, respectively. Negligible difference was observed for the heart and contralateral lung between Ethos and CyberKnife, but Unity delivered 1.8-5.8 and 4.7-11.9 times more dose, respectively, than other two modalities and was significant. Treatment delivery time was largely similar between the three technologies.
Conclusion: The above differences can largely be attributed to the capabilities of the different technologies to deliver radiation with CyberKnife providing non-coplanar beams, and Ethos having a movable isocenter. While Unity system offered superior image quality, the plan quality was inferior to the other two. This data will be continuously expanded to aid the physicians’ decision on the optimal modality for sPBI.