A new concept for non-invasive renal tumour ablation using real-time MRI-guided radiation therapy.

OBJECTIVE To present a new concept for non-invasive renal tumour ablation using real-time magnetic resonance imaging (MRI)-guided radiation therapy. All currently available treatment techniques for localized renal cell carcinoma (RCC) have to be performed in a laparoscopic or percutaneous way. MATERIALS AND METHODS A technical prototype MRI-accelerator which performs real-time 1.5 T MRI imaging during the irradiation has been constructed. We performed a technical feasibility study on real-time MRI-guided arc therapy using repeated breath-holds for renal tumour ablation by (i) investigating renal mobility during breath-holding, (ii) performing dose calculation and (iii) measuring the radiation delivery time on a phantom. The renal mobility during free breathing and end-expiration breath-holding during 15 s was investigated for three patients with renal tumour appearance. Conventional MRI screening data of four patients was used for arc therapy dose calculation. Tumour and normal tissues were delineated and a tumour margin of 3 mm was applied. The radiation delivery time of a 25-Gy arc therapy plan was measured on a phantom. RESULTS Renal mobility during free breathing varied from 10 to 25 mm, whereas breath-holding resulted in nearly non-moving kidneys (0 to 2 mm) for all patients. Arc therapy dose calculation resulted in an adequate tumour coverage. The radiation delivery time of the arc therapy plan was about 10 min. This means that 20 to 40 repeated breath-holds of 15 to 30 s will be needed for a single session treatment. A higher maximum dose rate would reduce the number of breath-holds needed and improve patient comfort. A phase I study will be started to proof the clinical feasibility. CONCLUSION Real-time MRI-guided radiation therapy using an MRI-accelerator might become a valuable non-invasive alternative to the current RCC treatment options.