Exposure to ionising radiation increases soft tissue sarcoma (STS) risk, and risk appears to increase in line with exposure levels, though most evidence on absorbed doses between 5 and 20 Gray (Gy), and many studies define STS by anatomical site rather than morphology. Typical sources of exposure are radiotherapy, medical diagnostics (e.g. X-rays), and natural background radiation (e.g. radon).
Radiotherapy for cancer during childhood appears to have the greatest impact on STS risk, though the magnitude of the increase varies widely between studies, probably related to whether STS are defined by anatomical site or tumour morphology. Childhood cancer survivors who received radiotherapy have around 16 times higher STS risk compared to those who did not, and the risk increases with cumulative radiation dose, with those receiving the highest dose at around 50 times increased STS risk, a British cohort study showed. Receiving chemotherapy as well as radiotherapy increases subsequent STS risk even further.
Radiotherapy for cancer (other than bone sarcoma or STS) during adulthood is associated with a 77% increased risk of second STS compared to the general population, analysis of US cancer registry data shows. Cancer survivors who did not receive radiotherapy also have an increased second STS risk, but to a far lesser extent. STS risk increases with longer time since diagnosis of the first cancer (22% at 1-4 years, 120% at 15+ years), and younger age at diagnosis of the first cancer (83% at age 60-79, 432% at age 20-39); there is also some evidence that risk varies by site of the primary cancer.
Evidence for an increased risk at lower exposure levels comes from a cohort study of atomic bomb survivors with 56 years follow-up, which found risk of STS around doubled per 1Gy exposure. Studies of radiotherapy at doses lower than 5Gy have generally found no increased STS risk, but low sample size preclude firm conclusions.
The International Agency for Research on Cancer (IARC) classifies radioiodines, including Iodine-131, as possible causes of STS, based on limited evidence. Iodine-131 is a radioactive isotope which can be used to treat hyperthyroidism and some types of thyroid cancer.