Comparison of second cancer risk due to out-of-field doses from 6-MV IMRT and proton therapy based on 6 pediatric patient treatment plans

Background and purposeThis study compared 6-MV IMRT and proton therapy in terms of organ specific second cancer lifetime attributable risks (LARs) caused by scattered and secondary out-of-field radiation.Materials and methodsBased on simulated organ doses, excess relative and excess absolute risk models were applied to assess organ-specific LARs. Two treatment sites (cranium and central spine) were considered involving six treatment volumes and six patient ages (9-month, 4-year, 8-year, 11-year, 14-year, and adult).ResultsThe LARs for thyroid cancer from a 6 cm diameter field treating a brain lesion in a 4-year old patient were estimated to be 1.1% and 0.3% in passive proton therapy and IMRT, respectively. However, estimated LARs for bladder cancer, more than 25 cm from the field edge for the same patient and treatment field, were estimated to be 0.2% and 0.02% from IMRT and proton therapy, respectively. Risks for proton beam scanning was found to be an order of magnitude smaller compared to passive proton therapy.ConclusionIn terms of out-of-field risks, IMRT offers advantage close to the primary field and an increasing advantage for passive proton therapy is noticed with increasing distance to the field. Scanning proton beam therapy shows the lowest risks.