Robust Planning for a Patient Treated in Decubitus Position with Proton Pencil Beam Scanning Radiotherapy

Abstract

A challenging case was reported for a patient treated in decubitus position with proton pencil beam scanning. A regular robust plan with the consideration of the uncertainties of translational alignment and range accuracy cannot ensure the target coverage as revealed in two verification computed tomography (CT) scans during the first week of the treatment. The irreproducibility of daily alignment and anatomical variations in such a position is mainly due to patient's roll. To mitigate the interfractional effect on the target coverage, a novel robust optimization against the patient's angular setup uncertainties was implemented to improve the plan quality by introducing two artificial CT image sets by rolling the planning CT three degrees in both clockwise and counter-clockwise directions and adding them into robust optimization scenarios, which was shown to be an effective and simple way to mitigate target dose degradation with respect to interfractional variations. This method can be easily generalized and applied to other situations where angular variations in patient's setup can introduce large dosimetric effects. It is recommended that angularly robust optimization method should be integrated into the treatment planning system as an option particularly for patient's treatment subject to large angular variations, such as the one in the decubitus position reported here.

Keywords: lung tumors; pencil beam scanning; proton raiotherapy; robust planning.

Figure 1. A patient was simulated in a right lateral decubitus position. A whole body Vac-Lok was used for immobilization.

Figure 2. The DVH of the nominal plan in the planning CT (solid line), +3 degree (CW) rotated CT (dashed line), and -3 degree (CCW) rotated CT (dotted line). The red lines denote the iGTV and the dark brown lines denote the CTV.

CCW: Counter-clockwise; CT: Computed tomography; CTV: Clinical target volume; CW: Clockwise; DVH: Dose volume histogram; iGTV: internal gross target volume.

Figure 3. The DVH of the RR plan in the planning CT (solid line), +3 degree (CW) rotated CT (dashed line), and -3 degree (CCW) rotated CT (dotted line). The red lines denote the iGTV and the dark brown lines denote the CTV.

CCW: Counter-clockwise; CT: Computed tomography; CTV: Clinical target volume; CW: Clockwise; DVH: Dose volume histogram; iGTV: internal gross target volume; RR: Roll robust.

Figure 4. The DVH of the nominal plan in the planning CT (solid line), VFCT1 (dotted line), and VFCT2 (dashed line). The red lines denote the iGTV and the dark brown lines denote the CTV.

CT: Computed tomography; CTV: Clinical target volume; DVH: Dose volume histogram; iGTV: internal gross target volume; VFCT: Verification CT.

Figure 5. The DVH of the RR plan in the planning CT (solid line), VFCT1 (dotted line), and VFCT2 (dashed line). The red lines denote the iGTV and the dark brown lines denote the CTV.

CT: Computed tomography; CTV: Clinical target volume; DVH: Dose volume histogram; iGTV: internal gross target volume; RR: Roll robust; VFCT: Verification CT.

Figure 6. The DVH of the left lung of the nominal plan (solid line) and the RR plan (dotted line).

DVH: Dose volume histogram; RR: Roll robust.

Figure 7. An individual beam dose distribution of the nominal plan on the planning CT (left) and three-degree counter clock-wisely rotated planning CT (right). The thick red line denotes the iGTV.

CT: Computed tomography; iGTV: internal gross target volume.