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Purpose: Radiation therapy can potentially cause a second malignancy. There is clinical evidence that those occur typically within the beam path in the medium/high dose region. The purpose of this study was to assess the risk for developing a radiation induced tumor within the treated volume and to compare this risk for proton therapy and IMRT., Methods: Fully contoured age and gender specific whole body phantoms (4-year and 14-year old) were uploaded into a treatment planning system and typical tumor volumes were contoured based on patients treated for optic glioma and vertebral body Ewing's sarcoma. Lifetime attributable risks (LARs) for developing a second malignancy were calculated using a risk model incorporating factors for cell kill, mutations, repopulation, and inhomogeneous organ doses., Results: For standard fractionation schemes, the LAR for developing a second malignancy from radiation therapy alone were found to be up to 2.7% for a 4-year old optic glioma patient treated with IMRT considering a soft tissue carcinoma risk model only. Sarcoma risks were found to be below 1% in all cases. For the 14-year old, risks were found to be about a factor of 2 lower. For the Ewing's sarcoma cases the risks based on the sarcoma model were typically higher than the carcinoma risks, i.e. up to 1.3% LAR for soft tissue sarcoma. Generally, the risk from proton therapy turned out to be lower by a factor of 2 to 10. However, comparison of a 3-field and 4-field proton plan shows that the distribution of the dose, i.e., the particular treatment plan, plays a role as well., Conclusions: In general, proton therapy can significantly reduce the risk for developing an in-field second malignancy. Risk analysis based on our formalism could be applied within treatment planning programs to guide treatment plans for pediatric patients. Federal Share of program income earned by Massachusetts General Hospital on NIH/NCI C06 CA059267., (© 2012 American Association of Physicists in Medicine.)