Rainbow IMRT and Volumetric Imaging for Anterior Mesenteric Targets

Abstract

Mesenteric masses can considerably shift positions between fractions of radiation therapy (RT). With image-guided RT, we can adapt our treatment delivery daily to target the mass while avoiding surrounding structures. We report the case of a highly-mobile mesenteric mass which required adaptive planning from a traditional “butterfly” intensity modulated radiation therapy (IMRT) plan to an anterior “rainbow” beam arrangement to permit shifting of the isocenter without compromising target coverage or increasing dose to organs-at-risk.

Figure 1:. Diagnostic Pre- and Post-Chemotherapy Imaging Studies.

Baseline PET scan at diagnosis (A), CT at diagnosis (B), post-chemotherapy PET scan (C) and CT scan (D) showing residual mass (arrow)

Figure 2.. Original IMRT Radiation Plan with “Butterfly” Beam Arrangement

A. Axial image from initial butterfly IMRT beam arrangement (Day 0) with three anterior (0°, 25° and 335°) and two posterior (165° and 195°) beams targeting the CTV (day 0, red) and PTV (day 0, teal). Dose distribution in the axial (B), coronal (C), and sagittal (D) dimensions. The 5 Gy isodose line spares the kidneys. The dose volume histogram (DVH) illustrating excellent target coverage with a favorable OAR dose profile (E).

Figure 3.. IMRT Butterfly Plan adaptation after target movement on day one.

A. Axial image from CT-on-rails scan from the first treatment day with isodose lines placed according to the original plan isocenter. The mesenteric mass moved over 4 cm on the first day (CTV_day1 depicted in red and PTV_dayi depicted in blue). B. Axial image of original mass location (CTV_day0 in yellow and PTV_day0 in green) and new location on the first treatment day (CTV_day1 in red and PTV_day1 in teal), C. DVH of the original butterfly plan based on original isocenter location illustrating poor coverage of the CTV and PTV based on the actual location of the mass (CTV_day1 and PTV_day1). D. IMRT butterfly plan applied to shifted Day1 target (CTV_day1 and PTV_day1) based on daily CT-on-rails imaging. The Day0 location of the target is also visualized (CTV_day0 in yellow, PTV_day0 in green). E. The DVH based on CT-on-rails shift to the new target location (CTV_day1 and PTV_day1) results in increased left kidney dose and suboptimal PTV coverage.

Figure 4.. New Rainbow IMRT planning performed after large target shifts.

A. Axial image from initial rainbow IMRT plan with anterior beam arrangement (25°, 65°, 100°, 295°, 320°, 335°) targeting the CTV_day1 (red) and PTV_day1 (teal). A 1.2 cm kidney margin was used as an avoidance structure (arrow). Dose distribution in the axial (B), coronal (C) and sagittal (D) dimensions. The IMRT rainbow plan DVH (E) demonstrated excellent target coverage and low OAR dose.

Figure 5.. Rainbow IMRT utilized successfully throughout treatment with continued mass motion

A. Axial image from CT-on-rails plan with isodose lines in the actual location of the mass on day 2 of treatment. CTV_day2 (red), PTV_day2 (teal), CTV_day1 (yellow), PTV_day1 (green) with DVH based on the fitted position after shift showing excellent coverage of day2 volumes (B). Images depicting location of targets from day 0 (green), day 1 (yellow) and day 2 (red) in axial (C) coronal (D) and sagittal (E) planes.