Dosimetric effect of uncorrected rotations in lung SBRT with stereotactic imaging guidance.

PURPOSE: To evaluate the dosimetric impact of uncorrected rotations on the planning target volume (PTV) coverage for early stage non-small cell lung cancer patients treated with stereotactic body radiotherapy using Brainlab ExacTrac image guidance. METHODS: Twenty-two patients were retrospectively selected. Two scenarios of uncorrected rotations were simulated with magnitude of 1°, 2°, 3° and 5°: (1) rotation around the treatment isocenter; and (2) roll and yaw rotations around a setup isocenter. The D95 of PTV from recalculated dose on the rotated CT was compared to that from the clinical plan. A logistic regression model was used to predict the probability of dose differences between recalculated and original plans that are less than 2% based on the rotation angle, PTV volume, and distance between the treatment and setup isocenter. RESULTS: Logistic regression model showed the uncorrected isocentric rotations of up to 2.5° in all directions have negligible dosimetric impact. For non-isocentric rotations, a rotational error of 2° may cause significant under-dose of the PTV. Statistically significant (p<0.05) parameters in the logistic regression model were angle for isocentric rotations, angle and distance for non-isocentric roll rotations, and angle, distance and the PTV volume for non-isocentric yaw rotations. CONCLUSIONS: The severity of the dose deviations due to uncorrected rotations depends on the type and magnitude of the rotation, the volume of the PTV, and the distance between the treatment and setup isocenter, which should be taken into consideration when making clinical judgment of whether the rotational error could be ignored.

Extending the concept of weighted CT dose index to elliptical phantoms of various aspect ratios.

The purpose of this study was to extend the concept of weighted CT dose index ([Formula: see text]) to the elliptical phantoms. Based on the published body dimension data, eight body aspect ratios were chosen between 1 (perfectly circular) and 1.72 (extremely elliptical). For each aspect ratio, two elliptical cylinders were created digitally to represent adult and pediatric bodies. Their cross-sectional areas were identical to the standard 32- and 16-cm CTDI phantoms. For each phantom, [Formula: see text] at center and periphery were simulated for tube voltages between 70 and 140 kVp using a validated Monte Carlo program. The simulations also provided the average dose over the cross-sectional area, [Formula: see text]. Values of [Formula: see text] and [Formula: see text] allowed linear systems of equations to be established, from which central and peripheral weighting coefficients were solved. Regardless of phantom shape, only two weighting coefficients were needed: [Formula: see text] for the central [Formula: see text] and [Formula: see text] for the average of the four peripheral [Formula: see text]'s. Over the full range of aspect ratios, [Formula: see text] increased linearly from 0.37 to 0.46, whereas [Formula: see text] decreased linearly from 0.63 to 0.54, allowing the concept of [Formula: see text] to be readily extended to the elliptical phantoms. When cross-sectional area (hence volume) was kept constant, all phantoms had the same [Formula: see text] regardless of shape.