1136 - Osteoarthritis (OA): Calculating Lifetime Attributable Oncogenic Risk from Low-Dose Radiotherapy (LDRT)

Purpose/Objective(s): LDRT is an effective analgesic modality for OA refractory to standard treatments. However, clinical adoption is often hindered by concerns regarding stochastic oncogenic risk. A risk calculator was developed to provide customizable, BEIR VII-based risk estimation for patients with OA, adjustable for age, sex, height, and joint-specific scatter proximity.

Materials/Methods: Utilizing a linear no-threshold model, we calculated LAR for a standard 6-fraction 3.0 Gy regimen. BEIR VII incidence coefficients were used for ages 40-80 years, and extrapolated via linear regression for 85-100. Anthropometric variability was addressed by stratifying joints into three height-based cohorts (<160 cm, 160-185 cm, >185 cm). Site-specific weighting factors were assigned based on ICRP 103 tissue sensitivities, with distal extremities (hand/foot) distinct from intermediate (knee/elbow) and axial (spine/hip/shoulder) joints to account for red bone marrow volumes exposed and truncal scatter volume.

Results: Radiation-induced malignancy risk demonstrated a steep inverse correlation with age and anatomical distance from the trunk. For the foot, LAR at age 70 was negligible (male: 0.07%, female: 0.09%). In the geriatric population (age 80+), LAR for all sites, including high-weighting axial joints, dropped significantly (eg, male spine at 80: 1.0%) as the typical time to develop malignancy exceeds the remaining life expectancy. Conversely, the highest risk profile was identified in short-stature (<160 cm) females age 40 receiving axial treatment (spine LAR: 6.0%). By age 100, statistical risk for all cohorts converged toward zero, reflecting a "stochastic floor."

Conclusion: Anthropometric and age-based stratification reveals that LDRT for OA carries a risk profile frequently lower than the acute morbidity/mortality rates associated with invasive arthroplasty or chronic NSAID use. This granular data supports practice patterns favoring LDRT for distal and geriatric indications. The developed calculator provides a convenient, robust tool for clinicians to discuss case-specific risks with patients and referring providers, facilitating evidence-based informed consent.