Session Item

Brachytherapy for breast cancer involves positioning several catheters through the skin into the breast tissues around the lumpectomy site. In order to get an acceptable dose distribution, the planning target volume (PTV) has to be geometrically covered by the catheters implanted.

To plan the correct positions and reduce uncertainty between virtual and final implants, we developed a virtual simulation using Surface Guided Radiotherapy (SGRT) Technology. We propose to focus on the technical aspects of this new method.

On the pre-implant CT, we delineated the PTV according to GEC ESTRO recommendations. A 3D virtual implant simulation of the catheters positions was performed using Monaco treatment planning system (Elekta AB, Stockholm) (TPS). For each virtual catheter, we created an external beam to obtain the isocenter. Markers were placed on all entry and exit points. We exported all the informations to a 3D surface patient setup system for alignment AlignRT^® (VisionRT, London) and to record and verify the system (Mosaiq).

In a radiotherapy treatment room equipped with AlignRT, we identified and marked on the skin all entry and exit points using the light field simulator of the linac. AlignRT assessed real‐time patient positioning by comparison to a reference surface (patient external contour of the CT scan).The use of surface imaging could improve the reproducibility and accuracy of patient positioning with pre scan and reduce uncertainty in catheter simulation. At the end of this operation, Radiopaque skin markers were thereafter positioned on the breast surface on the entry and exit points. A CBCT was performed and exported to the TPS. We performed a registration between the CBCT and the pre implant CT using surgical scar and clips. The position of the markers was compared to entrance and exit points. Implantation was then carried out under local anaesthetic using skin marks of the catheter inlets and outlets. Final dosimetry was performed on post-implantation CT scan.

For 4 patients, we compared coordinates form the entry and exit points defined on the pre scan and the CBCT (figure1) to evaluate the accuracy of the simulation. Mean differences observed were 0.2±0.3cm, 0.2±0.2cm and 0.3±0.4cm in left-right, supero-infero and antero-posterior (AP), respectively. A maximum difference of 1.1cm was obtained in the AP direction for external entry points. The obliqueness of the patient in this area amplified the shift

Figure 1: CBCT obtained after catheters simulation

Assessment of target volume coverage between the virtual implant and the CBCT showed a good correlation.

We concluded that 3D virtual brachytherapy using AlignRT may offer an improved technique to accurately perform interstitial implants of the breast in selected patients. Although preliminary results show excellent coverage of the target volume, additional patients will be required to establish the reproducibility of this technique and its practical limitations.