Clinical Assessment of Prostate Displacement and Planning Target Volume Margins for Stereotactic Body Radiotherapy of Prostate Cancer

Abstract

Purpose: To assess the optimal planning target volume (PTV) margins for stereotactic body radiotherapy (SBRT) of prostate cancer based on inter-and intra-fractional prostate motion determined from daily image guidance. Methods and Materials: Two hundred and five patients who were enrolled on two prospective studies of SBRT (8 Gy × 5 fractions) for localized prostate cancer treated at a single institution between 2012 and 2017 had complete inter-and intra-fractional shift data available. All patients had scheduled kilovoltage planar imaging during SBRT with rigid registration to intraprostatic fiducials prior to each of four half-arcs delivered per fraction, as well as cone beam CT verification of anatomy prior to each fraction. Inter-and intra-fractional shift data were obtained to estimate the required PTV margins based on the classic van Herk formula. Inter-and intra-fractional motion were compared between patients with and without severe toxicities using the independent two-sample Wilcoxon test. Results: The margins required to account for inter-fractional motion were estimated to be 0.99, 1.52, and 1.45 cm in lateral (LR), longitudinal (SI), and vertical (AP) directions, respectively. The margins required to account for intra-fractional motion were estimated to be 0.19, 0.27, and 0.31 cm in LR, SI and AP directions, respectively. Large intra-fractional shifts were mostly observed in the SI and AP directions, with 2.0 and 5.4% of patients experiencing average intra-fractional motion >3 mm in the SI and AP directions, respectively, compared with none experiencing mean shifts >3 mm in the LR direction. Six patients experienced grade 3 gastrointestinal or genitourinary toxicity. There were no significant differences in mean inter-or intra-fractional motion in any of the cardinal directions compared to patients without severe toxicity (inter-fractional p = 0.46-0.99, intra-fractional p = 0.10-0.84). Levin-Epstein et al. Prostate Displacement and SBRT Margins Conclusion: The inter-and intra-fractional margins estimated from this study are in line with prior reported values. Intra-fractional prostate motion was generally small with larger margins required for the SI and AP directions, notably just slightly exceeding the commonly used 3 mm posterior PTV margin even with realignment between half-arcs. Development of severe toxicity was not significantly associated with the degree of inter-or intra-fractional motion.

Preliminary Clinical Evaluation of Intrafraction Prostate Displacements for Two Immobilization Systems

“…In order to obtain statistically significant conclusions, further investigation with more patients is necessary. Despite this limitation of having few patients, it is encouraging that the intrafraction displacements and calculated margins found here are similar, and the standard deviations of 0.3-0.5 mm for prostate displacements are smaller than those found in a large study of SBRT patients [10] using the same van Herk formulation [11]. Furthermore, the observed mean displacement in the present study is the largest in the longitudinal and vertical directions, which is also consistent with previous results [10].…”

“…The van Herk formula ( \begin{document}\Delta = 2.5 \Sigma + 0.7 \sigma\end{document} ) was used to calculate the CTV-PTV margins (lateral, longitudinal, vertical) based on intrafraction displacements, where S is the standard deviation of the systematic error and s is the standard deviation of the random error [ 11 ]. \begin{document}\Sigma\end{document} was estimated by the standard deviation of all the patient means, \begin{document}\sigma\end{document} is the standard deviation of random error, which equals the root mean square (RMS) of the standard deviation of each patient’s mean displacements.…”

Magnetic resonance imaging-guided stereotactic body radiotherapy for prostate cancer (mirage): a phase iii randomized trial

“…For patients with cT3b disease, the CTV P will be expanded to cover the entire seminal vesicle. The CTV P will be expanded by 2–5 mm isotropically (typically 4 mm isotropically for CT-guided arm, per recent data from our institution [ 28 ] and 2 mm isotropically for the MRI-guided arm). Note the posterior expansion can be smaller than the expansion in other directions, i.e.…”

MR-Guided Radiotherapy for Prostate Cancer

“…Any corrections to the field may be rendered inaccurate during beam on due to the aforementioned target position diversity ( 31 – 33 ), or otherwise clinicians must extend the planning margin to cover the expected excursion of prostate motion ( 34 ). A small number of non-MRgRT systems have intra-fraction motion solutions such as Cyberknife, which uses KV imaging tracking of fiducial seeds.…”