A study was conducted to investigate secondary malignant neoplasm (SMN) occurrence following radiotherapy (RT) for cancer in children and young adults, to examine the spatial distribution of SMNs in relation to the irradiated field, and to evaluate a possible role of bystander effects in SMN distribution. Forty-two SMNs were identified among 7257 subjects diagnosed with cancer while living in Yorkshire, UK. Thirty-two of these occurred in patients receiving RT. Distances between SMN locations and RT field edge were estimated along with dose at SMN site. Expected radiation-induced SMN frequency in remote tissues receiving less than 0.1 Gy was predicted using risk estimates based on atomic bombing data. After a median follow-up period of 7.58 years, patients treated with RT were at a nearly five-fold increased risk of developing a subsequent primary neoplasm than the general population in the 0-29 years age range. The most common type of secondary malignancy associated with RT was of the central nervous system (28%), followed by sarcoma (25%) and leukemia (19%). Considering only solid SMNs developing 5 years or more from treatment, the spatial distribution showed a strong pattern of proximity to the irradiated field, with 68% occurring in-field or within 8 cm of the field edge. The SMN frequency in distant tissues receiving doses of less than 0.1 Gy was low but compatible with local absorbed dose.