Sci-Thurs PM: Delivery-07: Evaluation of prospects to use daily megavoltage CT studies for adaptive radiotherapy.

PURPOSE: To evaluate gross tumor volume (GTV) changes for non-small cell lung cancer (NSCLC) patients using daily megavoltage CT (MVCT) studies acquired before each treatment fraction on helical tomotherapy, and to relate the potential benefit of adaptive image-guided radiotherapy to changes in GTV. METHODS: 17 patients were prescribed 30 fractions of radiotherapy on helical tomotherapy for NSCLC at London Regional Cancer Program from December 2005 to March 2007. The GTV was contoured on the daily MVCT studies of each patient. Adapted plans were created using merged MVCT-kVCT image sets to investigate the advantages of replanning for patients with differing GTV regression characteristics. RESULTS: The average GTV change observed over 30 fractions was -38%, ranging from -12 to -87%. No significant correlation was observed between GTV change and patient's physical or tumor features. The pattern of GTV changes of the 17 patients could be broadly divided into 3 groups with distinctive potential for benefit from adaptive planning. CONCLUSIONS: GTV changes are difficult to predict quantitatively based on patient or tumor characteristics. If changes do occur, there are points in time during the treatment course when it may be appropriate to adapt the plan to improve sparing of normal tissues. If the GTV decreases by greater than 30% at any point in the first twenty fractions of treatment, adaptive planning is appropriate to further improve the therapeutic ratio.