Thyroid cancer following childhood low dose radiation exposure: a pooled analysis of nine cohorts

Context: The increased use of diagnostic and therapeutic procedures that involve radiation raises concerns about radiation effects, particularly in children and to the radio-sensitive thyroid gland.

Objectives: Evaluation of relative risk (RR) trends for thyroid radiation doses <0.2 gray; evidence of a threshold dose; and possible modifiers of the dose-response, e.g., sex, age at exposure, time since exposure.

Design and setting: Pooled data from nine cohort studies of childhood external radiation exposure and thyroid cancer with individualized dose estimates, ≥1,000 irradiated subjects or ≥10 thyroid cancer cases, with data limited to individuals receiving doses <0.2 gray.

Participants: Cohorts included: childhood cancer survivors (n=2); children treated for benign diseases (n=6); and children who survived the atomic bombings in Japan (n=1). There were 252 cases and 2,588,559 person-years in irradiated individuals and 142 cases and 1,865,957 person-years in non-irradiated individuals.

Intervention: There were no interventions.

Main Outcome Measure: Incident thyroid cancers.

Results: For both <0.2 gray and <0.1 gray, RRs increased with thyroid dose (P<0.01), without significant departure from linearity (P=0.77 and P=0.66, respectively). Estimates of threshold dose ranged from 0.0 to 0.03 gray, with an upper 95% confidence bound of 0.04 gray. The increasing dose-response trend persisted >45 years after exposure, was greater at younger age at exposure and younger attained age and was similar by sex and number of treatments.

Conclusions: Our analyses reaffirmed linearity of the dose-response as the most plausible relationship for ALARA ("as low as reasonably achievable") assessments for pediatric low dose radiation-associated thyroid cancer risk.