I-131 Tositumomab myeloablative radioimmunotherapy for non-Hodgkin's lymphoma: Radiation dose to the testes

Objectives Radioimmunotherapy (RIT) using high-dose I-131-tositumomab (anti-CD20) and stem cell support has been used to treat relapsed non-Hodgkin’s lymphoma (NHL). Objective was to study testicular radiation absorbed dose and its effects on testosterone level.

Methods We reviewed 67 male patients with NHL treated with myeloablative I-131 tositumomab as part of research protocols. Individualized organ dosimetry was performed using serial planar imaging after infusing I-131 tositumomab antibody (370 MBq). Radiation absorbed doses (cGy/mCi) were determined for vital normal organs using standard MIRD methods.

Results Absorbed doses (cGy/mCi, mean ± s.d.) were: 3.82 ±1.06 for liver, 4.40 ± 1.83 for lungs, 5.60 ± 3.94 for spleen, and 4.35±2.20 for testes. Therapeutic dose was 21.8 ± 6.7 (range 9.3- 50.8) GBq. Estimated absorbed dose for testes was 25.4±14.6 Gy, with a median of 22.0 Gy and coefficient of variation of 0.58. Twenty-eight patients (42%) received testicular doses ≥ 25 Gy. Pretreatment and one-year posttreatment plasma testosterone levels were measured in 25/67 patients. I-131-tositumomab therapy induced non-significant (p >0.05) testosterone suppression (from 4.4±2.0 to 4.2±3.0 ng/mL, 95%) in patients receiving <25 Gy, but significant (p< 0.05) suppression (from 4.7±1.6 to 3.3±2.7 ng/mL, 70%) in those receiving ≥ 25 Gy.

Conclusions In 42% of patients the testicular dose was ≥ 25 Gy. Higher dose was associated with suppression of testosterone levels, but this may provide an advantage in treating a highly aggressive NHL, in which the testes may be sanctuary sites.

Research Support Supported by P01 CA44991, K23 CA 85479 and The LRF and LLS grants and GSK