SPECT predictive of lymphoma RIT success

Advanced low-grade non-Hodgkin lymphoma (NHL) typically leads to relapse and death in patients who undergo conventional chemo and radiation therapies, but the emergence of radioimmunotherapy (RIT) has provided an encouraging alternative. Now researchers are finding quantitative SPECT data about dose uptake can be used to predict survival after therapy, according to a study published May 19 in the Journal of Nuclear Medicine.

RIT with radiolabeled monoclonal antibodies, including I-131 tositumomab and Y-90 ibritumomab tiuxetan, has led to response rates of about 95 percent as a frontline treatment and 60-83 percent in patients who have already been treated previously with conventional therapy.

A team of researchers including Yuni K. Dewaraja, PhD, from the department of radiology at University of Michigan in Ann Arbor, evaluated dosimetry data and particularly stratified curves of dose absorption within tumors to gauge patients’ progression-free survival (PFS).

“The most important finding of the present study is the clearly demonstrated separation of PFS curves when stratified by mean tumor-absorbed dose,” wrote Dewaraja et al. “This result and the robust correlation shown between predicted and delivered tumor-absorbed doses demonstrate the potential for tumor dosimetry–driven treatment planning in radioimmunotherapy of NHL.”

The researchers identified dosimetric and other factors predicting PFS in 39 relapsed or refractory NHL patients who had undergone I-131 ositumomab RIT namely tumor-absorbed dose measurements for 130 tumors using SPECT/CT and the dose planning method Monte Carlo code. Researchers also assessed the biological effects of non-uniformity in absorbed dose. Histology, presence of disease and prior treatment history were also taken into account. Tumor-level outcome was evaluated based on reduction of volume in follow-up CT. Patient outcomes were evaluated as overall and complete response and PFS with advanced molecular imaging including PET/CT. 

Results of the study showed that the mean tumor-absorbed dose was 275 cGy and a strong correlation was found between tracer predicted dose and resulting therapy. Univariate tumor-level analysis showed that dosimetric factors correlated with tumor shrinkage, overall and complete response to therapy as well as PFS, whereas non-dosimetric factors did not display strong correlations with patient response.  

“A higher mean tumor-absorbed dose was significantly predictive of improved PFS after I-131 tositumomab radioimmunotherapy,” concluded the authors. “Hence tumor-absorbed dose, which can be estimated before therapy, can potentially be used to design radioimmunotherapy protocols to improve efficacy.”