An Image-Guided Study of Setup Reproducibility of Postmastectomy Breast Cancer Patients Treated With Inverse-Planned Intensity Modulated Radiation Therapy

Abstract

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.

Design of a new breast vacuum bag to reduce the global and local setup errors and to reduce PTV margin in post-mastectomy radiation therapy

“…For example, Feng et al . [ 12 ] used kilovolt (KV) image to register the chest wall and correct it. If it was registered with a titanium clip, the PTV margins 5.9 mm (AP), 3.9 mm (LR) and 7.9 mm (SI) were still needed.…”

Intrafractional baseline drift during free breathing breast cancer radiation therapy

“…Using this formalism, the necessary margins to ensure that 90% of the patients would have 95% dose coverage to the CTV were estimated. In 3D, the PTV margins are determined using the formula M ¼ 2.5 P þ 0.7r which has been widely used for margin calculation in BCRT [6,14,15]. The position of the patient's sternum measured with the laser system at the time of portal imaging was compared to the calculated offsets from matching the digital reconstructed radiographs with the portal images.…”

Single‐institution report of setup margins of voluntary deep‐inspiration breath‐hold (DIBH) whole breast radiotherapy implemented with real‐time surface imaging

“…Since setup margins cannot be statistically compared, the data used to calculate them (i.e., mean errors and standard deviations) were compared among subgroups (right‐sided vs left‐sided and 2‐field vs 3‐field) using a nonparametric one‐way analysis of variance (i.e., Kruskal–Wallis H‐test). To account for increases in the type I error rate ( α = 0.05) due to multiple testing in each of the three dimensions, the P ‐value of 0.05 was divided by the number of comparisons made such that results were deemed significant only if P < 0.0167 (i.e., P < 0.05/3).…”