Quantification of local tumor response to fractionated radiation therapy for non-Hodgkin lymphoma using weekly 18F-FDG PET/CT imaging

Purpose: To quantify, in a feasibility study, metabolic and volumetric response to fractionated radiation therapy (RT) using weekly (18)F fluoro-deoxyglucose positron emission tomography (PET) imaging for 10 non-Hodgkin lymphoma (NHL) patients, and to correlate them to clinical outcome.

Methods and materials: Ten patients with chemotherapy-refractory NHL planned for radical RT were prospectively entered into a research study. PET/computed tomography (CT) scans were acquired before RT, and repeated weekly during the 3- to 4-week course of RT, and at 1 and 3 months after therapy. Gross tumor volumes were contoured on CT scans and the corresponding maximum standardized uptake values (SUV(max)) determined in the coregistered PET images. The clinical outcomes of interest were local tumor response at 3 months post-RT and local tumor status at last follow-up or time of death.

Results: (18)F fluoro-deoxyglucose uptake from inflammation was rarely observed. The responses showed a large variability between patients. SUV(max) decreased consistently with a median of -2.1% per Gy (range, -3.3 to -0.7) and the median of the volumetric response was -2.2% per Gy (range, -2.8 to +0.5). Initial SUV(max) was not correlated with local control, whereas smaller initial tumor volume was, with smaller tumors more likely to achieve local control. The responses after treatment were also correlated to local control, but not the responses during treatment.

Conclusions: Radiation does not confound the FDG uptake in the NHL tumor and normal tissues. Only smaller initial tumor volume and metabolic and volumetric response after completion of radiation therapy significantly correlated with eventual local control.