Movement of a small tumour in contact with the diaphragm: characterisation with four-dimensional CT

Radiotherapy to the thoracic and abdominal regions can require tailoring of the planning target volume (PTV) to compensate for respiratory motion. We evaluated dose variations that might occur to a small target close to the diaphragm. We compared tumour and diaphragm displacement in gated and non-gated four-dimensional computed tomography (CT) of a patient with a peridiaphragmatic lesion, using peak exhalation as a baseline. Diaphragmatic motion was 12.7 mm in the inferior direction. The tumour was noted to move 1.0 mm to the right, 1.1 mm anteriorly, and 1.4 mm superiorly. The tumour moved in the opposite direction from the diaphragm in the vertical axis. This paradoxical motion did not affect the dose distribution because the beam did not irradiate the liver on non-gated treatment plans, and remained within the PTV. We observed minimal movement of a small tumour on 4D CT, in spite of it being in contact with, and moving opposite to, the diaphragm. In this patient, respiratory gating during irradiation was not needed, making it possible to reduce the treatment time.