Reducing monitor units for robotic radiosurgery by optimized use of multiple collimators.

Advances in image guidance and dose delivery techniques, and increased use of hypofractionation, have led to prolonged radiotherapy fraction duration. This is also the case with robotic radiosurgery, as extensive on-line image guidance procedures, many beams, and usually high fraction doses are used for tumor irradiation. At this institution, early stage non-small-cell lung cancer patients are treated with image guided tumor tracking for respiratory motion compensation. Approximately 130 circular beams and up to approximately 39 000 monitor units (MUs) are used for delivery of a total treatment dose of 60 Gy. The large number of MUs leads to long treatment times and the radiation leakage increases with the number of MUs. Generally, per patient, a single (small) cone is used. To substantially reduce the number of MUs, the authors have developed a new planning strategy for combined use of a small and a large cone. The large cone aims at dose delivery around the PTV center, while the small cone shapes the dose around the (irregular) PTV edges. The authors systematically investigated relationships between the number of MUs, the plan quality, the selected cone diameters, and the beam-direction setup. Plan quality was assessed with the conformity index, mean lung dose (MLD), V20 of the lungs, and by visual inspection. The reduction in MUs was determined by comparing two-cone plans with corresponding one-cone plans that yielded equal MLD, i.e., equal predicted lung toxicity. With the proposed two-cone approach, the required number of MUs reduced by on average 31% (range 4%-56%). The beam-on time per treatment fraction reduced by on average 8 min (range 1-15.2 min). All plans obeyed the clinically applied constraints and were considered clinically acceptable by an involved physician.

Endovascular coils as lung tumour markers in real-time tumour tracking stereotactic radiotherapy: preliminary results.

To evaluate the use of endovascular coils as markers for respiratory motion correction during high-dose stereotactic radiotherapy with the CyberKnife, an image-guided linear accelerator mounted on a robotic arm. Endovascular platinum embolisation coils were used to mark intrapulmonary lesions. The coils were placed in subsegmental pulmonary artery branches in close proximity to the target tumour. This procedure was attempted in 25 patients who were considered unsuitable candidates for standard transthoracic percutaneous insertion. Vascular coils (n = 87) were successfully inserted in 23 of 25 patients. Only minor complications were observed: haemoptysis during the procedure (one patient), development of pleural pain and fever on the day of procedure (one patient), and development of small infiltrative changes distal to the vascular coil (five patients). Fifty-seven coils (66% of total inserted number) could be used as tumour markers for delivery of biologically highly effective radiation doses with automated tracking during CyberKnife radiotherapy. Endovascular markers are safe and allow high-dose radiotherapy of lung tumours with CyberKnife, also in patients who are unsuitable candidates for standard transthoracic percutaneous marker insertion.