Investigation of sliced body volume (SBV) as respiratory surrogate

The purpose of this study was to evaluate the sliced body volume (SBV) as a respiratory surrogate by comparing with the real‐time position management (RPM) in phantom and patient cases. Using the SBV surrogate, breathing signals were extracted from unsorted 4D CT images of a motion phantom and 31 cancer patients (17 lung cancers, 14 abdominal cancers) and were compared to those clinically acquired using the RPM system. Correlation coefficient (R), phase difference (D), and absolute phase difference ([Formula: see text]) between the SBV‐derived breathing signal and the RPM signal were calculated. 4D CT reconstructed based on the SBV surrogate (4D [Formula: see text]) were compared to those clinically generated based on RPM (4D [Formula: see text]). Image quality of the 4D CT were scored ([Formula: see text] and [Formula: see text] , respectively) from 1 to 5 (1 is the best) by experienced evaluators. The comparisons were performed for all patients, and for the lung cancer patients and the abdominal cancer patients separately. RPM box position (P), breathing period (T), amplitude (A), period variability ([Formula: see text]), amplitude variability ([Formula: see text]), and space‐dependent phase shift (F) were determined and correlated to [Formula: see text]. The phantom study showed excellent match between the SBV‐derived breathing signal and the RPM signal ([Formula: see text] , [Formula: see text] , [Formula: see text]). In the patient study, the mean (± standard deviation (SD)) R, D, [Formula: see text] , T, [Formula: see text] , A, [Formula: see text] , and F were [Formula: see text] , [Formula: see text] ([Formula: see text]), 11.4% ([Formula: see text]), 3.6 ([Formula: see text]) s, 0.19 ([Formula: see text]), 6.6 ([Formula: see text]) mm, 0.20 ([Formula: see text]), and 0.40 ([Formula: see text]) s, respectively. Significant differences in R and [Formula: see text] ([Formula: see text] and 0.001, respectively) were found between the lung cancer patients and the abdominal cancer patients. 4D [Formula: see text] slightly outperformed 4D [Formula: see text]: the mean ([Formula: see text]) [Formula: see text] and [Formula: see text] were 2.6 ([Formula: see text]) and 2.9 ([Formula: see text]), respectively, for all patients, 2.5 ([Formula: see text]) and 3.1 ([Formula: see text]), respectively, for the lung cancer patients, and 2.6 ([Formula: see text]) and 2.8 ([Formula: see text]), respectively, for the abdominal cancer patients. The difference between [Formula: see text] and [Formula: see text] was insignificant for the abdominal patients ([Formula: see text]). F correlated moderately with [Formula: see text] ([Formula: see text]). The correlation between SBV‐derived breathing signal and RPM signal varied between patients and was significantly better in the abdomen than in the thorax. Space‐dependent phase shift is a limiting factor of the accuracy of the SBV surrogate. PACS number: 87.59.bd