Carbon-ion pencil beam scanning for thoracic treatment – initiation report and dose metrics evaluation

Carbon-ion beam scanning has not previously been used for moving tumor treatments. We have commenced respiratory-gated carbon-ion radiotherapy (CIRT) in the thoracic and abdominal regions under free-breathing conditions as a clinical trial. This study aimed to investigate this treatment in the lungs in comparison with passive scattering CIRT. Five patients had thoracic tumors treated with carbon-ion scanned beams using respiratory gating. We analyzed the actual treatments and calculated passive scattering treatment plans based on the same planning CT. We evaluated tumor size until 3 months post treatment and each treatment plan regarding dose delivered to 95% of the clinical target volume (CTV-D95), mean lung dose, percentage of lung receiving at least 5 Gy (RBE) (Lung-V5), Lung-V10, Lung-V20, heart maximum dose (Dmax), esophagus Dmax, cord Dmax and skin Dmax. Obvious tumor deterioration was not observed up to 3 months post treatment. The dose evaluation metrics were similar item by item between respiratory-gated scanned CIRT and passive scattering CIRT. In conclusion, scanned beam CIRT provided treatments equivalent to passive scattering CIRT for thoracic tumors. Increased sample numbers and longer-term observation are needed.