Daily megavoltage ct imaging for lung cancer during a course of external beam radiotherapy with a helical tomotherapy device

Abstract

Our aim was to study the process of daily megavoltage (MV) CT imaging for a pulmonary malignancy. We report our initial experience with a helical tomotherapy device. The initial 20 MVCT images obtained during a course of external beam radiotherapy with the Hi*Art system (Tomotherapy Inc.) were analyzed. The treatment was designed to treat a lesion in the upper right lung to a total of 68 Gy at 2 Gy per fraction. The current study is limited to the first 20 of a total planned 34 fractions. The treatment was designed with the clinical target volume (CTV) defined on exhale and inhale simulation kilovoltage (KV) CT studies. The images were fused with a free breathing CT, since the treatment was to be delivered with free shallow breathing. The fusion was based on the registration of the spinal cord. The maximum extent of the tumor among the three CT scans was used to determine the CTV. A 10 mm margin was used to determine the planning target volume. Daily MVCT images were acquired with the Hi*Art system with the patient in the treatment position. The spinal cord was used to align the daily MVCT images to the simulation KVCT image. Each day, the set-up errors were recorded and corrected by the radiation therapists by adjusting the position of the treatment couch. The treatment was subsequently delivered with the Hi*Art system. Tumor dimensions were later measured on the daily MVCT images in the superior/inferior, anterior/posterior, and lateral dimensions. The tumor volume was calculated using the tumor dimensions as the volume constituting an ellipsoid structure. Figure 1 displays a sample of the treated tumor’s simulation KVCT images, and a sample of the tumor’s MVCT images acquired at the time of the 20th treatment. These corresponding images were obtained after adjusting for set-up errors by aligning on the spinal cord. A clear response is observed with shrinkage of the tumor. The size of the tumor documented on a daily basis clearly decreased over the 20 fractions. Fig. 2 documents the reduction in the tumor volume to approximately half of its original size in 20 fractions. Megavoltage imaging can be useful in two ways in the process of treating of a pulmonary malignancy with radiotherapy. It can be used to adjust patient position by daily aligning the patient’s observed internal anatomy to the expected patient position, thereby eliminating set-up errors. In addition, it can be used to measure previously unobserved tumor response during a radiotherapy course. Tumor shrinkage within the first 20 days of therapy was documented with daily megavoltage imaging.