Reproducibility of the lung anatomy under active breathing coordinator control: Dosimetric consequences for scanned proton treatments

Abstract

Purpose: The treatment of moving targets with scanned proton beams is challenging. For motion mitigation, an Active Breathing Coordinator (ABC) can be used to assist breath-holding. The delivery of pencil beam scanning fields often exceeds feasible breath-hold durations, requiring high breath-hold reproducibility. We evaluated the robustness of scanned proton therapy against anatomical uncertainties when treating nonsmall-cell lung cancer (NSCLC) patients during ABC controlled breath-hold.

Methods: Four subsequent MRIs of five healthy volunteers (3 male, 2 female, age: 25-58, BMI: 19-29) were acquired under ABC controlled breath-hold during two simulated treatment fractions, providing both intrafractional and interfractional information about breath-hold reproducibility. Deformation vector fields between these MRIs were used to deform CTs of five NSCLC patients. Per patient, four or five cases with different tumor locations were modeled, simulating a total of 23 NSCLC patients. Robustly optimized (3 and 5 mm setup uncertainty respectively and 3% density perturbation) intensity-modulated proton plans (IMPT) were created and split into subplans of 20 s duration (assumed breath-hold duration). A fully fractionated treatment was recalculated on the deformed CTs. For each treatment fraction the deformed CTs representing multiple breath-hold geometries were alternated to simulate repeated ABC breath-holding during irradiation. Also a worst-case scenario was simulated by recalculating the complete treatment plan on the deformed CT scan showing the largest deviation with the first deformed CT scan, introducing a systematic error. Both the fractionated breath-hold scenario and worst-case scenario were dosimetrically evaluated.

Results: Looking at the deformation vector fields between the MRIs of the volunteers, up to 8 mm median intra- and interfraction displacements (without outliers) were found for all lung segments. The dosimetric evaluation showed a median difference in D_98% between the planned and breath-hold scenarios of -0.1 Gy (range: -4.1 Gy to 2.0 Gy). D_98% target coverage was more than 57.0 Gy for 22/23 cases. The D_{1 cc} of the CTV increased for 21/23 simulations, with a median difference of 0.9 Gy (range: -0.3 to 4.6 Gy). For 14/23 simulations the increment was beyond the allowed maximum dose of 63.0 Gy, though remained under 66.0 Gy (110% of the prescribed dose of 60.0 Gy). Organs at risk doses differed little compared to the planned doses (difference in mean doses <0.9 Gy for the heart and lungs, <1.4% difference in V₃₅ [%] and V₂₀ [%] to the esophagus and lung).

Conclusions: When treating under ABC controlled breath-hold, robustly optimized IMPT plans show limited dosimetric consequences due to anatomical variations between repeated ABC breath-holds for most cases. Thus, the combination of robustly optimized IMPT plans and the delivery under ABC controlled breath-hold presents a safe approach for PBS lung treatments.

Keywords: active breathing coordinator control; interfraction reproducibility; intrafraction reproducibility; nonsmall-cell lung cancer; pencil beam scanning.

Figure 1

(a) Coronal view of an MR scan overlaid with the deformation vector fields of a deformable image registration between two MRIs for one volunteer. The lung contour was used to guide the deformable registration algorithm. The seven segments that overlap with the lung contour represent the local regions for which the magnitudes of displacement were analyzed. (b) Range of intrafractional (upper boxplots) and interfractional displacements (lower boxplots) for the five volunteers. The middle line in the boxes indicates the median and the bottom and top edges of the boxes the 25^th and 75^th percentiles. Up to the whiskers the most extreme data points not considered outliers are shown. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

(a)–(e) Fused coronal views of the five patient/volunteer matches. (f)–(i) 3D views of simulated tumor locations for patient number one, including the original tumor location in the left upper lobe (f), left lower lobe (g), right middle lobe (h), and right lower lobe (i). [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

The dose at 98% of the CTV for each simulation and all patients. The tolerance threshold (dotted line) was set to 57.0 Gy of minimum target coverage.. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

The dose to 1 cc of the CTV for each simulation and all patients. The tolerance threshold of the maximum dose (dotted line) was set to 63.0 Gy. [Color figure can be viewed at wileyonlinelibrary.com]