Lung tumor motion change during stereotactic body radiotherapy (SBRT): an evaluation using MRI

Abstract

The purpose of this study is to investigate changes in lung tumor internal target volume during stereotactic body radiotherapy treatment (SBRT) using magnetic resonance imaging (MRI). Ten lung cancer patients (13 tumors) undergoing SBRT (48 Gy over four consecutive days) were evaluated. Each patient underwent three lung MRI evaluations: before SBRT (MRI-1), after fraction 3 of SBRT (MRI-3), and three months after completion of SBRT (MRI-3m). Each MRI consisted of T1-weighted images in axial plane through the entire lung. A cone-beam CT (CBCT) was taken before each fraction. On MRI and CBCT taken before fractions 1 and 3, gross tumor volume (GTV) was contoured and differences between the two volumes were compared. Median tumor size on CBCT before fractions1 (CBCT-1) and 3 (CBCT-3) was 8.68 and 11.10 cm3, respectively. In 12 tumors, the GTV was larger on CBCT-3 compared to CBCT-1 (median enlargement, 1.56 cm3). Median tumor size on MRI-1, MRI-3, and MRI-3m was 7.91, 11.60, and 3.33 cm3, respectively. In all patients, the GTV was larger on MRI-3 compared to MRI-1 (median enlargement, 1.54 cm3). In all patients, GTV was smaller on MRI-3m compared to MRI-1 (median shrinkage, 5.44 cm3). On CBCT and MRI, all patients showed enlargement of the GTV during the treatment week of SBRT, except for one patient who showed minimal shrinkage (0.86 cm3). Changes in tumor volume are unpredictable; therefore, motion and breathing must be taken into account during treatment planning, and image-guided methods should be used, when treating with large fraction sizes.

Figure 1

An example of GTV (red line) volume difference between the two CBCT scans for Patient 3: an image taken before SBRT (left) and an image taken after completing the 3rd fraction of SBRT (right).

Figure 2

The CBCT image of the only lesion (Patient 2, tumor #2) for which the GTV (red line) volume difference was smaller on the image after the 3rd fraction compared to the image before the 1st fraction: image taken before SBRT (left) and image taken after treatment on Day 3 of SBRT (right). A prominent artifact, due to a metal clip, is detectable.

Figure 3

An example of GTV (red line) volume difference between the two scans in T1WI for Patient 2, tumor #2: image taken before SBRT (left) and image taken after treatment on Day Three of SBRT (right).