Poster — Thur Eve — 23: Artificial Electron Disequilibrium Due to Inaccurate Cone‐Beam CT Data for Adaptive Lung Radiation Therapy

Cone‐beam computed tomography (CBCT) is becoming a clinically useful imaging modality for image‐guided adaptive radiation therapy. The Varian On‐Board Imaging system (Varian Medical Systems Inc., Palo Alto California) uses a 125 kVp source, emitting a conical x‐ray beam, and a flat‐panel amorphous silicon detector. Unfortunately, CBCT images are prone to artifacts such as those caused by acceptance of x‐ray scatter from the patient at the detector plane, intra‐fraction motion, and x‐ray spectral “beam‐hardening”. Previous studies indicate that large dose inaccuracies occur when using CBCT lung images for adaptive dose computations, compared to other treatment sites with less tissue heterogeneity. We have compared dose distributions calculated using CBCT and 4‐dimensional (4D) time‐averaged CT (Philips Inc., Cleveland, OH) lung images of the same patient. Using 6MV fields, an under‐dosage of 55Gy was predicted for the CBCT planned target volume, compared to 60Gy predicted by the 4DCT based plan. CT number profiles from CBCT and 4DCT lung images revealed many undervalued pixels in the CBCT data, some corresponding to vacuum (−1000HU)! Monte Carlo simulations of dose deposition, using a water and lung slab geometry, were used to study the effects of ultra‐low density on the 3D dose distribution. It was found that a specific transition‐density induces lateral electron disequilibrium, and causes an undervaluation of dose in mid‐lung, along the central axis of the beam. Thus, CBCT images containing depressed CT number values in lung caused an artificial electron disequilibrium problem, which can be misinterpreted in adaptive treatment re‐planning.