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Cyberknife CT
Research project

Cyberknife Accelerated Partial Breast Irradiation

Status: Ongoing project

Improving the accuracy of the Cyberknife Accelerated Partial Breast Irradiation (CK-APBI) technique for postoperative radiotherapy for early-stage breast cancer

What we do

About our project


Accelerated Partial Breast Irradiation (APBI) is a new treatment option for early-stage breast cancer patients after breast-conserving surgery. The advantages of APBI are a shorter treatment course and a smaller irradiated volume, with less risk of long-term toxicities. In radiotherapy, there is a safety margin used for geometric uncertainties of treatment delivery. The geometric uncertainty of APBI delivered with the Cyberknife is unknown. The Cyberknife uses fiducials for real-time tracking of the target during treatment delivery. It is unknown what the optimal type of fiducial is, with the smallest uncertainty and highest patient comfort.

Aim of the project 

The first objective is to quantify the geometric uncertainty of CK-APBI and propose a validated margin to compensate for this uncertainty. The second objective is to compare different types of fiducials for real-time tracking with respect to geometric uncertainty and patient comfort. The three types of fiducials are interstitial gold markers, surgical clips and superficial skin gold markers.


During lumpectomy, the surgeon placed 4-5 surgical clips as a standard procedure. Three interstitial gold markers will be inserted outside the tumorbed in the preparation phase of CK-APBI. One to three gold markers will be taped on the skin of the breast.

A planning CT scan will be made. At each of 5 treatment fractions, an in-room CT scan will be acquired before treatment. During treatment, 2 orthogonal planar kV images will be acquired at regular intervals. On all these images, the different types of fiducials will be identified and their geometric uncertainty will be assessed.

Patient inclusion

Patients meeting all of the following criteria are eligible for trial participation:

  • Female
  • Planned treatment with CK-APBI at Erasmus MC
  • At least 3 surgical clips in the tumor bed, being large titanium (1 cm) or tantalum type
  • Signed informed consent
  • No prior irradiation to the chest

Our research focus

Context of the CK-APBI project

The prognosis of early-stage breast cancer patients is excellent with modern treatment techniques. The benefit of adjuvant radiotherapy is partly offset by treatment induced mortality. The mortality mainly results from cardiovascular toxicity and secondary lung cancers. The CK-APBI project is part of a larger project aiming to reduce radiation induced toxicity. 

Prior studies in this project

Our first study showed that APBI results in a lower secondary cancer risk than whole breast irradiation. Our second study showed that prioritizing lung sparing during treatment planning for external beam APBI reduced the secondary lung cancer risk on average five-fold, with only a small increase in breast fibrosis risk. The mean heart dose was also reduced with lung sparing. A non-coplanar beam setup, like CK, results in lower toxicity risks than a coplanar beam setup. 

Possible impact on patient care

The results of the CK-APBI study could lead to a smaller margin used in clinical practice, which would result in lower doses to non-target tissues. It could also result in the omission of the invasive procedure of interstitial marker placement, if either surgical clips or skin markers proof to be good alternatives.


  • Prof. J.P. Pignol, Dalhousie University, Halifax, Canada
  • Accuray Inc., Sunnyvale, USA

Our team