Computed tomography for four-dimensional dose calculation: Effect of detector array length

Four-dimensional computed tomography (4DCT) is susceptible to a geometrical error when respiration changes upon patient shift. 4DCT using 16-cm detector arrays, not needing the shift, has been shown to improve geometrical accuracy, compared with a conventional 4DCT (4-cm array), albeit with some Hounsfield unit discrepancies. The 4DCTs were validated for 4D planning.

A lung-mimicking phantom containing a spherical target with ten respiratory traces and three tumor-shaped targets with their traces was respectively imaged when, to each of the ten-positions/phases, the lung was moved (1) step-wisely by helical scan at each movement (ground truth; 4DCT_GT), (2) continuously by 4D scan with 16 cm detectors (4DCT₁₆), and (3) similarly with 4 cm detectors (4DCT₄). Respiratory irregularities affected 4DCT₄ only due to shifts. Treatment plans included volumetric modulated arc therapy (VMAT) and intensity-modulated proton therapy (IMPT) on averaged CT images from 4DCT₁₆ and 4DCT₄. IMPT was targeted to internal gross target volumes (iGTVs) with density overrides (1.0 g/cm³), while VMAT to iGTVs+3 mm, with the objective V_{prescribed dose(PD)} ≥ 95%. The plans were recalculated on each phase image of 4DCT_GT and those of 4DCT₁₆ were additionally calculated on its phase images. The calculated doses were registered to the maximum-exhale image, and summed.

(1) 4DCT₁₆: IMPT plans met the objective, despite under-coverages in certain phase images, achieving 98.95 ± 1.09% and 98.27% in averaged V_PD for the spherical and tumor targets, respectively. VMAT plans showed 97.26 ± 1.60% and 94.41% for the respective targets. (2) 4DCT₄: IMPT plans did not meet the objective with 93.83 ± 9.29% and 97.38% for them. VMAT plans yielded 93.20 ± 5.81% and 91.17% for them. (3) 4DCT₁₆ versus 4DCT_GT: 4D doses based on 4DCT₁₆ matched those on 4DCT_GT with 62.76 Gy versus 62.76 ± 0.19 Gy for IMPT plans and 63.79 Gy versus 63.90 ± 0.28 Gy for VMAT plans.

The 4DCT₁₆ demonstrated accurate and reliable imaging of a lung tumor in 4D dose by IMPT plans. However, it could be affected by phase-sorting uncertainty.