Using Fluorodeoxyglucose Positron Emission Tomography to Assess Tumor Volume During Radiotherapy for Non–Small-Cell Lung Cancer and Its Potential Impact on Adaptive Dose Escalation and Normal Tissue Sparing

Purpose To quantify changes in fluorodeoxyglucose (FDG)-avid tumor volume on positron emission tomography/computed tomography (PET/CT) during the course of radiation therapy and examine its potential use in adaptive radiotherapy for tumor dose escalation or normal tissue sparing in patients with non–small-cell lung cancer (NSCLC). Methods and Materials As part of a pilot study, patients with Stage I–III NSCLC underwent FDG-PET/CT before radiotherapy (RT) and in mid-RT (after 40–50 Gy). Gross tumor volumes were contoured on CT and PET scans obtained before and during RT. Three-dimensional conformal RT plans were generated for each patient, first using only pretreatment CT scans. Mid-RT PET volumes were then used to design boost fields. Results Fourteen patients with FDG-avid tumors were assessed. Two patients had a complete metabolic response, and 2 patients had slightly increased FDG uptake in the adjacent lung tissue. Mid-RT PET scans were useful in the 10 remaining patients. Mean decreases in CT and PET tumor volumes were 26% (range, +15% to −75%) and 44% (range, +10% to −100%), respectively. Designing boosts based on mid-RT PET allowed for a meaningful dose escalation of 30–102 Gy (mean, 58 Gy) or a reduction in normal tissue complication probability (NTCP) of 0.4–3% (mean, 2%) in 5 of 6 patients with smaller yet residual tumor volumes. Conclusions Tumor metabolic activity and volume can change significantly after 40–50 Gy of RT. Using mid-RT PET volumes, tumor dose can be significantly escalated or NTCP reduced. Clinical studies evaluating patient outcome after PET-based adaptive RT are ongoing.