Cyberknife SBRT of breast cancer – dosimetry and treatment planning

Objective: To present treatment planning, irradiation and dosimetrical characteristics of accelerated partial breast irradiation (APBI) performed with Cyberknife (CK) M6. Material and methods: Before CT imaging, four gold markers were implanted in the cavity wall which are detected by X-ray imaging during irradiation, and light emitting diodes placed on the chest wall were used for real time tracking. Irradiation was performed with non-coplanar beams using MLC with step-and shoot IMRT technique. The dose prescription was 4 x 6.25 Gy (25 Gy total dose). 2 mm margin was added to CTV to create PTV. To evaluate forty treatment plans, dose-volume parameters were used for ipsilateral non-target breast, contralateral breast, heart (left and right sided lesion separately), ipsi- and contralateral lung, skin and ribs. Dosimetry was compared to multicatheter interstitial breast implants. Results: On average, 40 beams with 57 segments were used. The mean volume of tumour bed, CTV and PTV were 7.6 cm3, 52.2 cm3 and 73.0 cm3, respectively. V100, V75 and V50 for non-target breast were 0.72%, 4.52% and 10.37%, respectively. D0.04 cm3 for contralateral breast was 0.48 Gy. Mean dose and D0.04 cm3 for heart was 0.81 Gy and 5.15 Gy (left sided lesion), and 0.32 Gy and 2.02 Gy (right sided lesion). For ipsilateral lung, the mean dose and D10 were 1.24 Gy and 3.10 Gy, and for contralateral lung, 0.11 Gy and 0.25 Gy. D0.01cm3 and D0.04 cm3 for skin were 2.33 Gy and 2.24 Gy, and 2.21 Gy and 2.14 Gy for ribs. For comparison with BT, V100 for non-target breast was lower with CK. Dose parameters for ipsilateral lung, skin and ribs were lower for BT, but the heart was better protected with CK. Conclusion: Regarding dosimetry, CK SBRT is an appropriate and feasible technique for delivering APBI. In many respects, it provides BT-like dose distributions.