[Volumetric and geometric changes of parotids occurring during IMRT for nasopharyngeal carcinoma (NPC) using daily...

To study the volumetric and geometric changes of parotids occurring during IMRT treatment for NPC.

The pre-treatment CBCT images of 10 patients were obtained using kV-CBCT and transmitted back to TPS. The gross tumor volumes (GTVs) and volumes of parotids were manually drawn slice by slice on the CBCT images by the same person using image analysis software system Tomcon 3.0. For position changes, the center-of-mass (COM) were calculated using Pinnacle 8.0 software. Then the distance from the parotid to the atlas was assessed.

The volumetric changes of GTVs and parotids were correlated with the days of treatment. The volumes of GTVs decreased with a mean total loss of 9.85 cm3 (5.10-21.40 cm3) and a median change rate of 0.20 cm3/d (0.10-0.46 cm3/d). The volumes of left parotids decreased with a median total loss of 5.70 cm3 (3.20-12.00 cm3) and a median change rate of 0.13 cm3/d (0.07-0.27 cm3/d). The volumes of right parotids decreased with a median total loss of 8.05 cm3 (3.00-13.40 cm3) and a median change rate of 0.18 cm3/d (0.06-0.29 cm3/d). The distance between the left parotid and the atlas decreased with a median total loss of 3.45 mm (0.70-7.60 mm) and a median change rate of 0.07 mm/d (0.01-0.11 mm/d). The distance between the right parotids and the atlas decreased with a median total loss of 2.60 mm (1.50-4.40 mm) and a median change rate of 0.05 mm/d (0.03-0.12 mm/d).

Significant reductions of GTVs occur after the third or forth week of IMRT treatment for NPC, which indicates that a second plan of CT scanning or treatment replanning might be necessary after 3-4 weeks of treatment to minimize radiations to vital organs.