Time to resolution of iodine-123 metaiodobenzylguanidine ( I-MIBG) avidity and local control outcomes for high-risk neuroblastoma following radiation therapy.

Introduction: I-MIBG scan is used in neuroblastoma (NB) to monitor treatment response. Time to resolution of I-MIBG avidity after radiation therapy (RT) is unknown. We sought to determine time to resolution of I-MIBG avidity after RT and local failure (LF) rate.

Methods: We performed a retrospective review of children with high-risk NB who underwent I-MIBG scans pre- and post-RT from 2003 to 2019. Time from RT to resolution of I-MIBG activity was analysed. LF and cumulative incidence of local progression (CILP) after RT stratified by site, presence of residual disease and use of boost RT were determined.

Results: Forty-two patients with median age 3.9 years (1.9-4.7 years) were included, with median follow-up time 3.9 years (1.4-6.9). Eighty-six lesions were treated with RT to median dose of 21.6 Gy. Eighteen of 86 lesions were evaluable for time to resolution of MIBG avidity after RT, with median resolution time of 78 days (36-208). No LF occurred among 26 patients who received RT to primary sites after GTR, versus 4/12 (25%) patients treated with residual primary disease. 2-year CILP was 19% (12% primary disease 25% metastatic disease (P = 0.18)). 2-year CILP for non-residual primary, residual primary, non-residual metastatic and residual metastatic lesions was 0%, 42%, 11% and 30% respectively (P = 0.01) and for boosted and non-boosted residual lesions was 29% and 35% (P = 0.44).

Conclusion: Median time to MIBG resolution after RT was 78 days. Primary lesions without residual disease had excellent local control. LF rate was higher after RT for residual disease, with no benefit for boost RT.