Effects of ROI Placement on PET-Based Assessment of Tumor Response to Therapy

Purpose . Quantitative PET response assessment during therapy requires regions of interest (ROI). Commonly, a fixed-size ROI is placed at the maximum uptake point in the pretreatment study. For intratreatment, the ROI is placed either at the maximum uptake point (ROI peak ) or at the same location as the pretreatment ROI (ROI same ). We have evaluated the effects of the ROI placement on response assessment. Methods . PET scans of 15 head and neck cancer patients were used to evaluate the effects of the two ROI methods on response assessment. Results . The average intratreatment ROI peak uptake was 13.4% higher than the ROI same uptake (range −14% to 38%). The average relative change in ROI peak uptake was 7.9% lower than ROI same uptake (range −5% to 36%), resulting in ambiguous tumour classification in 19% of the tumours. Conclusion . Quantitative PET response assessment using a fixed-size ROI is sensitive the ROI placement. The difference between ROI peak and ROI same could be substantial resulting in ambiguous response assessment. Although the fixed-size ROI is simple to implement, it is also prone to the limitations and should be used with caution. Clinical trial data are necessary to establish reliable thresholds for fixed-size ROI techniques and to evaluate their efficacy for response assessment.