Estimation of potential excess cancer incidence in pediatric 201Tl imaging

Abstract

Objective: Little information is available regarding doses of ionizing radiation from medical imaging in the growing population of children undergoing therapy for cancer who are at risk of developing second cancers. The purpose of our study was to estimate the potential excess lifetime cancer incidence and mortality associated with thallium bone imaging in pediatric patients.

Materials and methods: We retrospectively reviewed the medical records of pediatric patients treated between August 1991 and December 2003 for newly diagnosed osteosarcoma who underwent 201Tl imaging as part of the treatment protocol. According to age at diagnosis and doses of 201Tl, we estimated the excess cancer incidence and cancer mortality for boys and girls at 5 and 15 years old.

Results: The study cohort consisted of 73 patients, 32 males (median age at diagnosis, 14.8 years; age range, 8.1-20.1 years) and 41 females (median age at diagnosis, 13.3 years; age range, 6.0-20.7 years). Patients underwent a total of three 201Tl studies with a median dose of 4.4 mCi (162.8 MBq) (range, 2.2-8.4 mCi [81.4-310.8 MBq]) per study. Total median cumulative patient radiation dose for 201Tl studies was 18.6 rem (186 mSv) (range, 8.4-44.2 rem [84-442 mSv]) for males and 21.5 rem (215 mSv) (range, 7.0-43.8 rem [70-438 mSv]) for females. Estimated excess cancer incidence was 6.0 per 100 (male) and 13.0 per 100 (female) if exposed by 5 years of age; 2.0 per 100 (male) and 3.1 per 100 (female) by 15 years of age. Estimated excess cancer mortality was 3.0 per 100 for males and 5.2 per 100 for females at 5 years of age; 1.0 per 100 (male) and 1.4 per 100 (female) exposed at 15 years of age.

Conclusion: Further reduction of doses in younger patients is needed to consider 201Tl a viable option for imaging osteosarcoma.

Fig. 1. 13-year-old boy with proximal right tibial osteosarcoma. Thallium imaging was performed at diagnosis, after 9 weeks of neoadjuvant chemotherapy, and just before surgical resection for local tumor control. Histologically, tumor responded to neoadjuvant chemotherapy

A, In ²⁰¹Tl image obtained at time of diagnosis, intense uptake of ²⁰¹Tl is present in proximal right tibial diametaphyseal osteosarcoma. B, After 9 weeks of chemotherapy, ²⁰¹Tl image shows uptake of ²⁰¹Tl has diminished. C, In ²⁰¹Tl image obtained just before surgical resection, tumor avidity for ²⁰¹Tl has diminished further and mildly exceeds that of normal bone.

Fig. 2

Graph shows estimated radiation dose per mCi of ²⁰¹Tl-chloride by IV injection according to patient age at exposure.

Fig. 3. Graphs of excess incidence and mortality

A and B, Charts show estimated ²⁰¹Tl-related excess cancer incidence (I) and mortality (M) per 100 males (A) and females (B).