Prospective Study of Surface Guided Radiation Therapy (SGRT) for Breath Hold SBRT Treatments of the Lung: Analysis of Reliability of Surface Guidance Alone for Internal Tumor Position During Breath Hold

Abstract

<h3>Purpose/Objective(s)</h3> Respiratory motion management with breath hold (BH) is one technique to limit normal lung and other organ dose during SBRT treatments of the lung. Surface guidance (SGRT) has been utilized for BH in larger targets such as breast cancer, but its reliability in BH treatments of small targets has not been investigated. We report the initial results of the reliability of utilizing SGRT alone as a technique for BH during a prospective study of treatment of primary and metastatic lung tumors. <h3>Materials/Methods</h3> An IRB approved prospective study was conducted in patients (pts) with primary NSCLC or lung metastasis undergoing SBRT. Eligible pts had at least 1 cm of respiratory associated motion on free breathing 4DCT. Pts underwent planning CT with BH either in end inspiration or expiration per investigator choice. The GTV was contoured on the BH planning CT and a 5mm margin added for PTV expansion. During each SBRT treatment, pts performed BH and were aligned within tolerance based on the reference BH surface capture created from the BH planning CT. Pts underwent short-arc CBCT during BH for volumetric match of the target. Before shifts were made for target match, pts performed a BH within tolerance, then shifts were made prior to a new reference surface capture being immediately obtained during the same BH. Pts were then treated in BH based on the new reference surface with a tolerance of 3mm translations and 2 degree rotations. Treatment was stopped if pts fell outside of the predefined tolerances. Midway through treatment, pts underwent a second short arc CBCT during BH to verify the reliability of target position during BH treatment utilizing SGRT. Any subsequent shifts on target volumetric match after the second CBCT were recorded. Linear mixed models for repeated measures were used to analyze rotational and translational shift measurements. <h3>Results</h3> 13 pts and 39 SBRT fractions were treated utilizing SGRT alone for BH. All pts were treated with 54 Gy in 3 fractions. Median free breathing range of motion (4D ROM) was 1.3 cm. Mean displacements after mid-treatment CBCT were minimal in all directions as shown in Table 1. Only one observed fraction had a displacement that exceeded the 5mm PTV expansion (longitudinal shift of 0.59 cm with resulting vector shift of 0.68cm). Factors evaluated for association with shift magnitude included age, BMI, gender, tumor location, 4D ROM, and GTV and PTV size. Higher 4D ROM was associated with increased longitudinal and vector shifts (<i>P</i> = 0.019 and 0.046). <h3>Conclusion</h3> SGRT provides a reliable technique for BH SBRT treatments of the lung. Minimal target displacements on mid treatment BH CBCT were observed. Higher 4D ROM on free breathing scans correlated with larger displacements in tumor position utilizing this technique.