Abstract 146P
Background
While treating lung tumours, respiratory motion is the biggest challenge. Four-dimensional (4D) computed tomography (CT) provides maximal intensity projection (MIP) sequence by encompassing respiratory motion to delineate internal target volume (ITV) in lung tumours. This study aimed to analyse the inter-fraction variations of the centre of mass (COM) of the ITV in lung tumours with the help of MIP sequences of 4D Planning and 4D kilo voltage (kV) cone beam CT (CBCT).
Methods
This pilot study enrolled a total of nine patients who had pathologically proven lung cancer. All the patients had tumours in the left lung. Patients were simulated with 4D CT, ITV was contoured and a planning target volume (PTV) margin of 7mm was given. During treatment ITV motions were monitored with 4D kV CBCT. A total of 104 treatment fractions were analysed after excluding the artefacts. The mean COM variation of ITV in the Left-Right (LR), Supero-Inferior (SI) and Antero-Posterior (AP) directions were calculated.
Results
The ITV motion was >5mm, >7 mm and >10 mm was seen in 40, 14 and 7 fractions which contributed to 38%, 13% and 6% of total fractions respectively. The mean COM of ITV varied in LR, SI and AP directions were 0.26 ±0.24 cm (p<0.001), 0.44 ±0.43 cm (p<0.001) and 0.41 ±0.39 cm (p<0.001) respectively. Compared to non-mediastinal location, tumours close to mediastinum expressed high motion in AP direction (0.56 ± 0.42 cm vs 0.31 ± 0.30 cm, p=0.093). The ITV motion as per different lung lobe location is summarized in the table.
Table: 146P
| LR | SI | AP | |
| Upper lobe | 0.29 ± 0.27 cm | 0.41 ± 0.24 cm | 0.28 ± 0.28 cm |
| Middle lobe | 0.20 ± 0.20 cm | 0.36 ± 0.27 cm | 0.30 ± 0.30 cm |
| Lower lobe | 0.29 ± 0.23 cm | 0.56 ± 0.46 cm (p=0.162) | 0.68 ± 0.43 cm (p=0.007) |
Conclusions
This is the first pilot study that compared inter-fraction variation of ITV using MIP sequences of 4D CT and 4D kV CBCT. The mean inter-fraction variation of the ITV falls in the acceptable range encompassed by an adequate PTV margin (7 mm). A PTV margin of 5mm is inadequate (mean ITV motion is about 4mm and 38% of treatment fractions had ITV motion of >5mm) for lung tumours even after 4D CT simulation. High AP motion in tumour close to mediastinum signifies the importance of cardiac motion management in addition to lung motion management.
Legal entity responsible for the study
PGIMER.
Funding
Has not received any funding.
Disclosure
The author has declared no conflicts of interest.