Four-dimensional computed tomography-based treatment planning for intensity-modulated radiation therapy and proton therapy for distal esophageal cancer

Abstract

Purpose: To compare three-dimensional (3D) and four-dimensional (4D) computed tomography (CT)-based treatment plans for proton therapy or intensity-modulated radiation therapy (IMRT) for esophageal cancer in terms of doses to the lung, heart, and spinal cord and variations in target coverage and normal tissue sparing.

Methods and materials: The IMRT and proton plans for 15 patients with distal esophageal cancer were designed from the 3D average CT scans and then recalculated on 10 4D CT data sets. Dosimetric data were compared for tumor coverage and normal tissue sparing.

Results: Compared with IMRT, median lung volumes exposed to 5, 10, and 20 Gy and mean lung dose were reduced by 35.6%, 20.5%, 5.8%, and 5.1 Gy for a two-beam proton plan and by 17.4%, 8.4%, 5%, and 2.9 Gy for a three-beam proton plan. The greater lung sparing in the two-beam proton plan was achieved at the expense of less conformity to the target (conformity index [CI], 1.99) and greater irradiation of the heart (heart-V40, 41.8%) compared with the IMRT plan(CI, 1.55, heart-V40, 35.7%) or the three-beam proton plan (CI, 1.46, heart-V40, 27.7%). Target coverage differed by more than 2% between the 3D and 4D plans for patients with substantial diaphragm motion in the three-beam proton and IMRT plans. The difference in spinal cord maximum dose between 3D and 4D plans could exceed 5 Gy for the proton plans partly owing to variations in stomach gas filling.

Conclusions: Proton therapy provided significantly better sparing of lung than did IMRT. Diaphragm motion and stomach gas-filling must be considered in evaluating target coverage and cord doses.

Fig. 1

Transverse views of the dose distributions in the photon intensity-modulated radiation therapy (IMRT) plan and the two- and three-beam proton plans for patient 13. The thick yellow lines delineate the internal clinical target volume (ICTV).

Fig. 2

Comparisons from dose-volume histograms for the photon intensity-modulated radiation therapy (IMRT) plan (dashed lines) and the proton plan (solid lines) for patient 13.

Fig. 3

The amplitude of tumor motion (panel a) and differences in the endpoints in plans calculated with 4D CT versus single CT for the prescribed dose to the ICTV (b), the lung V5 (c), the lung mean dose (d), the cord maximum dose (e), and the heart V40 (f) for IMRT and for the two-beam and three-beam proton plans. IMRT, intensity-modulated radiation therapy; Proton 2B, two-beam proton therapy; Proton 3B, three-beam proton therapy.

Fig. 4

The effect of beam weighting on heart sparing for the three-beam proton plan for patient 13. Panel at left shows beam arrangement; 0, 110, and 210 refer to the beam angles (in degrees). Panel at right shows dose-volume histograms of three-beam plans using beam weights of 1, 0.2, 1 (circles) or 1, 2, 1 (squares), and IMRT plan (triangles). The green, blue, orange and red colors denote ICTV, total lung, heart and spinal cord.

Fig. 5

The effects of diaphragmatic motion on the dose distribution for patient 14 in the intensity-modulated radiation therapy (IMRT) plan (upper panels) and the three-beam proton plan (bottom panels). The panels at left show the dose distribution on the averaged CT; the panels at right show the dose distribution on the T50 phase of the 4D CT.

Fig. 6

The effects of differences in stomach gas filling on dose distribution on an averaged CT scan (a), a T00 phase scan (b), a T30 phase scan (c), and a T50 phase scan (d) from 4D CT.