Assessment of Organ Dosimetry for Planning Repeat Treatments of High-Dose 131I-MIBG Therapy: 123I-MIBG Versus Posttherapy 131I-MIBG Imaging

Abstract

Purpose: To evaluate detailed organ-based radiation-absorbed dose for planning double high-dose treatment with I-MIBG.

Methods: In a prospective study, 33 patients with high-risk refractory or recurrent neuroblastoma were treated with high-dose I-MIBG. Organ dosimetry was estimated from the first I-MIBG posttherapy imaging and from subsequent I-MIBG imaging prior to the planned second administration. Three serial whole-body scans were performed per patient 2 to 6 days after I-MIBG therapy (666 MBq/kg or 18 mCi/kg) and approximately 0.5, 24, and 48 hours after the diagnostic I-MIBG dose (370 MBq/kg or 10 mCi/1.73 m). Organ radiation doses were calculated using OLINDA. I-MIBG scan dosimetry estimations were used to predict doses for the second I-MIBG therapy and compared with I-MIBG posttherapy estimates.

Results: Mean ± SD whole-body doses from I-MIBG and I-MIBG scans were 0.162 ± 112 and 0.141 ± 0.068 mGy/MBq, respectively. I-MIBG and I-MIBG organ doses were variable-generally higher for I-MIBG-projected doses than those projected using posttherapy I-MIBG scans. Mean ± SD doses to liver, heart wall, and lungs were 0.487 ± 0.28, 0.225 ± 0.20, and 0.40 ± 0.26, respectively, for I-MIBG and 0.885 ± 0.56, 0.618 ± 0.37, and 0.458 ± 0.56, respectively, for I-MIBG. Mean ratio of I-MIBG to I-MIBG estimated radiation dose was 1.81 ± 1.95 for the liver, 2.75 ± 1.84 for the heart, and 1.13 ± 0.93 for the lungs. No unexpected toxicities were noted based on I-MIBG-projected doses and cumulative dose limits of 30, 20, and 15 Gy to liver, kidneys, and lungs, respectively.

Conclusions: For repeat I-MIBG treatment planning, both I-MIBG and I-MIBG imaging yielded variable organ doses. However, I-MIBG-based dosimetry yielded a more conservative estimate of maximum allowable activity and would be suitable for planning and limiting organ toxicity with repeat high-dose therapies.

Figure 1

Study schema.

Figure 2

¹³¹I-MIBG post-therapy scans: anterior and posterior whole-body images performed at 48 h (A), 72 h (B), and 120 h (C) post-injection. A reference standard was included for imaging at all time points (arrows). Images show physiologic uptake in liver, spleen, GI tract, kidneys, and bladder. Uptake is also seen in the left pelvic lesion (arrow head). ¹²³I-MIBG scans: anterior and posterior whole-body images performed at 1.5 h (D), 24 h (E), and 48 h (F) post-injection. A reference standard was included for imaging at all time points (arrows). Physiologic uptake is seen in the heart, liver, spleen, GI tract, and bladder.

Figure 3

Time-activity clearance curves for the whole body and organs for ¹³¹I -MIBG (A) and ¹²³I-MIBG (B). Blood clearance time-activity curve for ¹³¹I-MIBG (C).

Figure 4

¹²³I-MIBG pre-therapy (A) and post-therapy (B) scans: anterior and posterior whole-body images performed at 24 h post-injection. Images show multiple abnormal foci of uptake in the axial and appendicular skeleton that are resolved (arrows) or decreased (arrow heads) in the post-therapy scan.