Six degrees-of-freedom prostate and lung tumor motion measurements using kilovoltage intrafraction monitoring.

PURPOSE: Tumor positional uncertainty has been identified as a major issue that deteriorates the efficacy of radiation therapy. Tumor rotational movement, which is not well understood, can result in significant geometric and dosimetric inaccuracies. The objective of this study was to measure 6 degrees-of-freedom (6 DoF) prostate and lung tumor motion, focusing on the more novel rotation, using kilovoltage intrafraction monitoring (KIM).

METHODS AND MATERIALS: Continuous kilovoltage (kV) projections of tumors with gold fiducial markers were acquired during radiation therapy for 267 fractions from 10 prostate cancer patients and immediately before or after radiation therapy for 50 fractions from 3 lung cancer patients. The 6 DoF motion measurements were determined from the individual 3-dimensional (3D) marker positions, after using methods to reject spurious and smooth noisy data, using an iterative closest point algorithm.

RESULTS: There were large variations in the magnitude of the tumor rotation among different fractions and patients. Various rotational patterns were observed. The average prostate rotation angles around the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) axes were 1.0 ± 5.0°, 0.6 ± 3.3°, and 0.3 ± 2.0°, respectively. For 35% of the time, the prostate rotated more than 5° about the LR axis, indicating the need for intrafractional adaptation during radiation delivery. For lung patients, the average LR, SI, and AP rotation angles were 0.8 ± 4.2°, -0.8 ± 4.5°, and 1.7 ± 3.1°, respectively. For about 30% of the time, the lung tumors rotated more than 5° around the SI axis. Respiration-induced rotation was detected in 2 of the 3 lung patients.

CONCLUSIONS: The prostate and lung tumors were found to undergo rotations of more than 5° for about a third of the time. The lung tumor data represent the first 6 DoF tumor motion measured by kV images. The 6 DoF KIM method can enable rotational and translational adaptive radiation therapy and potentially reduce treatment margins.