Session Item

To evaluate the intrafraction (IF) motion of pulmonary tumors during stereotactic body radiation therapy (SBRT) using online Cone Beam Computed Tomography (CBCT) data and to determine possible clinical consequences, such as margin adjustment.

The Planning Target Volume (PTV) is defined as the Internal Tumor Volume with a 5 mm expansion.

For each fraction, CBCTs were acquired for set-up correction (scan A), to confirm correct positioning of the patient (scan B) and to monitor the IF motion (scan C, after completion of the fraction). Both a bone match (translations + rotations) and a soft tissue (tumor)match (translations only, rotations used from bone match) were performed for all CBCTs.

In total 6930 CBCTs were reviewed (340 patients, 404 tumors) of SBRT treatments performed in the period 2018-2020. The IF motion of both tumor and patient were assessed (difference between match results of scan B and C for respectively soft tissue (tumor)match and bone match). The IF motion was investigated for dependence on: 
1) tumor localization (lobe)
2) tumor size (GTV) (group 1: volume 0-5 cm³, 2: volume 5-10 cm³, 3: volume 10-15 cm³, 4: ≥ 15 cm³)
3) single fraction dose (which serves as surrogate for the time between scan B and C). 

IF motion patient
The average IF motion amounted to 0.0 ± 1.1 mm, 0.0 ± 1.3 mm and -0.2 ± 1.3 mm respectively for left-right (LR), craniocaudal (CC) and anteroposterior (AP) direction (vector 1.6 ± 1.5 mm). For short fractions (5 Gy/fr), the IF motion was slightly less than for longer fractions (18 Gy/fr) (1.4 ± 1.6 mm (vector) and 1.8 ± 1.5 mm for 3 fractions. The translations and rotations of the IF motion did not show systematic effects.

IF motion tumor
The average IF motion amounted to 0.0 ± 1.3 mm, 0.2 ± 1.6 mm and -0.5 ± 1.7 mm respectively for LR, CC and AP direction (vector 2.1 ± 1.7 mm). Note the systematic 0.5 mm motion in the posterior direction. The average IF motion vector amounted to 2.0 ± 1.4, 1.8 ± 1.1 mm and 2.3 ± 2.1 mm respectively for upper, middle and lower lobe. For 5Gy/fr, the IF motion was slightly less than for 18Gy/fr, 1.4 ± 1.6 mm vs 1.8 ± 1.5 mm (vector). The IF motion did not depend on tumor size.

Tumormatch post CBCT
The soft-tissue (tumor)match of scan C revealed that in only 7% of the fractions the tumor had partially moved outside of the PTV margin. A systematic average motion of the tumor in cranial (0.51 mm) and dorsal (1.3 mm) direction was observed.

The applied PTV margin of 5 mm is appropriate. The IF tumor motion shows systematic components in cranial and dorsal directions. The IF motion slightly depended on the lobe the tumor is located in, as well as on the applied dose-scheme/treatment duration. The differences are too small and the variation between patients is too large for clinical consequences.