Robustness of the voluntary breath-hold approach for the treatment of early stage lung cancer with spot scanned proton therapy

Abstract: Background: Intra- and interfractional variations in anatomy can cause unplanned substantial alterations of the dose distribution of the target in proton therapy. Motion management methods might be used in order to minimize anatomical variations. In the proton therapy of lung cancer, a potential motion management technique is the use of the voluntary breath-hold approach. The aim of this study was to investigate the robustness of proton therapy treatment plans of lung cancer towards interfractional variations using voluntary breath-hold. Materials and methods: Fifteen patients previously treated for Non-Small Cell Lung Cancer (NSCLC) or lung metastases with Stereotactic Body Radiation Therapy (SBRT) were included in this study. A voluntary breath-hold CT scan was collected as a part of the planning procedure and following each treatment fraction. The purpose of this treatment planning study was to obtain treatment plans for all the patients and then recalculate these on the repeated breath-hold CT scans, in order to investigate the robustness of the voluntary breath-hold approach. We used the definition of robust being as if the volume of the target receiving 95% of the prescribed dose (V95%) deviated less than 5% during the treatment. Two different plans were made; 2F with two fields and 3F with three fields. The Dose Volume Histogram (DVH) was evaluated for the Gross Target Volume (GTV), the Planning Target Volume (PTV), and the Organs at Risk (OAR) after both a Rigid Image Registration (RIR) recalculation and a Deformable Image Registration (DIR) recalculation of the treatment plans. Results: The results of the study shows that after a RIR recalculation 6/15 (2F) and 9/15 (3F) treatments passed our robustness criterion. For two different DIR recalculations, 9/15 (3F) treatments for the first one and 3/4 (2F) and 2/4 (3F) treatments for the second one also passed our robustness criterion. The study further shows that the robustness was associated with the size of the target, with the slope of the regression curve being significant. No difference in robustness with respect to the number of fields per plan (two or three) was found. Discussion: The results of this study show that for a majority of the cases the voluntary breath-hold approach seems robust. It may be necessary to have a cut off for small targets, as the robustness correlates with the size. Small tumors should be studied in more detail in future investigations. Remaining intrafractional motion during the treatment and the breath-holds, which was not considered in this study, may further compromise the robustness of the approach.