Purpose/Objective(s)
To calculate planning target volume (PTV) margins for chest wall and regional nodal targets using daily orthogonal kV imaging, and to study residual setup error after kV alignment using volumetric cone-beam computed tomography (CBCT).
Methods and Materials
Twenty-one post-mastectomy patients were treated with IMRT with 7mm PTV margins. Population-based PTV margins were calculated from translational shifts following daily kV positioning and/or weekly CBCT data for each of 8 patients, whose surgical clips were used as surrogates for target volumes. Errors from kV and CBCT data were mathematically combined to generate PTV margins for 3 simulated alignment workflows: 1) skin marks alone, 2) weekly kV imaging, 3) daily kV imaging.
Results
kV data from 613 treatment fractions indicated a 7mm uniform margin would account for 95% of daily shifts if patients were positioned using only skin marks. Total setup errors incorporating both kV and CBCT data were larger than those from kV alone, yielding PTV expansions of 7mm anterior-posterior (AP), 9mm left-right (LR), and 9mm superior-inferior (SI). Required PTV margins following weekly kV imaging were similar in magnitude as alignment to skin marks, but rotational adjustments of patients were required in 32%±17% of treatments. These rotations would have remained uncorrected without the use of daily kV imaging. Despite the use of daily kV imaging, CBCT data taken at the treatment position indicates that an anisotropic PTV margin of 6mm AP, 4mm LR, 8mm SI must be retained to account for residual errors.
Conclusions
CBCT provides additional information on three-dimensional reproducibility of treatment setup for chest wall targets. 3D data indicate that a uniform 7mm PTV margin is insufficient in the absence of daily IGRT. Inter-fraction movement is greater than suggested by two-dimensional imaging, thus a margin of at least 4–8mm must be retained despite the use of daily IGRT.
On average, 'cw' shifts correlated with kV shifts but exhibited significant inter-patient variability and larger rotations than 'all'. Differences between AlignRT and kV were ∼3mm for initial patient positioning. The lack of a one-to-one correspondence between surface and kV shifts in any single session must be further investigated before clinical implementation.
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