Planning Target Volume Margin Assessment of Retroperitoneal Tumors Using Robotic SBRT With Spine Tracking

Abstract

Abstract Objective: Stereotactic body radiation therapy (SBRT) treatment is an emerging salvage modality for treating oligometastatic malignant lesions within the retroperitoneum. Appropriate planning target volume (PTV) margins are essential when delivering SBRT to effectively cover the target volume. Spine tracking uses bony spinal anatomy for localization during treatment delivery on robotic linear accelerator platforms. The aim of this study is to quantify the PTV margin needed when spine tracking is used for intrafraction motion tracking when treating retroperitoneal metastatic lesions with robotic SBRT. Materials and Methods: A single-institution chart review identified 16 patients with retroperitoneal tumors treated with SBRT over 19 courses in 103 fractions. Daily cone-beam CT images registered based on tumor positioning at the time of treatment were analyzed. Van Herk’s margin recipe was used to calculate the additional PTV margin required if spine tracking was used instead of daily tumor imaging. Patients’ tumors were stratified based on PTV proximity to the vertebral column (≤ 1 cm vs > 1 cm) and location within the retroperitoneum (superior vs inferior to renal artery), with descriptive statistics used to compare the differences of shifts based on location. Results: The additional margins calculated by Van Herk’s margin recipe to adequately cover the PTV within the 95% isodose surface for 90% of the entire patient cohort in the vertical, longitudinal, and lateral directions were 2.7, 2.8, and 2.8 mm, respectively. When tumors were stratified by proximity to the vertebral column, average longitudinal (P < .001) and total shifts (P < .001) were statistically significant. Conclusion: When treating retroperitoneal tumors with robotic SBRT, a minimum isometric margin expansion of 5 mm when creating the PTV is recommended if spine tracking is used for intrafraction motion assessment. Target volumes adjacent to the vertebral column may have PTV margins decreased to 4 mm without compromise in target coverage.