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Objective: We evaluated the changes in treatment response over time after single 131I-rituximab radioimmunotherapy (RIT) according to non-Hodgkin lymphoma (NHL) types.Methods: Fifteen aggressive and 21 indolent lymphoma cases undergoing RIT were evaluated. All patients underwent 18F-FDG-PET-CT before and 5 days, 1, and 3 months after RIT. The maximum standardized uptake value (SUV) and the sum of the products of the longest perpendicular diameters of tumours (SPD) were evaluated. Treatment responses were evaluated 1 and 3 months after RIT RESULTS: In aggressive lymphoma, SUV decreased at 5 days after RIT but increased after that. SPD decreased at 1 month but significantly increased at 3 months. Complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD) at 1 month after RIT were changed to PD at 3 months after RIT. In indolent lymphoma, the SUV decreased continuously until 1 month after RIT. The SPD significantly decreased at 1 month and tended to further decrease to 3 months. CR, PR, SD, and PD at 1 month after RIT were achieved in 0, 8, 13, and 0 cases, respectively. Among the 13 SD cases, one changed to CR, three changed to PR, and nine had not changed at 3 months after RIT.Conclusions: The treatment response to single RIT differed depending on NHL type. These findings suggest a need to establish an optimal treatment regimen based on NHL aggressiveness. [ABSTRACT FROM AUTHOR]