Three TBI
treatments were performed with TomoHD™ tomotherapy equipment. The
immobilization consists of two indexed moldcare couches and a
thermoplastic mask. Due to limitations
in the movement of the table the whole body image is acquired in two series, a
head gantry CT and a feet to gantry CT. In addition, a 4D CT is performed to
contour the lungs. All with a slice thickness of 5mm. In both series, the
target volume is the external contour of the patient, expanded by 5 mm to
ensure that the skin receives the prescribed dose (Dp). It is contoured to the
overlapping area of both plans, where 5 gradient volumes of 2 cm thickness are
generated. Between successive gradient volumes the dose is reduced by 2 Gy, so
that positioning errors do not lead to large dose variations. In the region
prior to the gradient, an overdosage is generated by adding up the plans. To
avoid this, 2 to 4 cm of the patient's contour are prescribed at 95% Dp. The
lungs are delimited on the MIP image and reduced by 1 cm in all directions
allowing a gradient zone from the costal wall, which is also contoured to
verify that it is not underdosed.
The treatment schedule
is 12 Gy in 6 fractions, twice a day, separated by more than 6 hours. The
treatment goals are at least 95% volume coverage at 95% Dp , dose as
homogeneous as possible and maximum lung dose of 10 Gy and minimum of 7 Gy. The
length of both upper and lower plans should be less than 120-126cm due to
limitations of table movement and immobilization indexations. The parameters of the upper plan are: field width=5 cm, pitch=0.287, modulation
factor=2 till 3. The parameters of the lower plan are: field width=5cm, pitch=4,
modulation factor=2.
Both plans
are verified with ArcCheck, the gamma criterion (3 %, 3 mm) is applied. The
overlap region is verified with both ArcCheck and radiochromic, both plans are
irradiated consecutively by rotating the equipment. During treatment,
radiochromic film strips are placed in the overlap region and in regions where
repositioning is less reproducible.