Collimator angle influence on dose distribution optimization for vertebral metastases using volumetric modulated arc therapy.
OBJECTIVE
The cylindrical symmetry of vertebrae favors the use of volumetric modulated are therapy in generating a dose "hole" on the center of the vertebrae limiting the dose to the spinal cord. The authors have evaluated if collimator angle is a significant parameter for dose distribution optimization in vertebral metastases.


METHODS
Three patients with one-three vertebrae involved were considered. Twenty-one differently optimized plans (nine single-arc and 12 double-arc plans) were performed, testing various collimator angle positions. Clinical target volume was defined as the whole vertebrae, excluding the spinal cord canal. The planning target volume (PTV) was defined as CTV+5 mm. Dose prescription was 5 x 4 Gy(2) with normalization to PTV mean dose. The dose at 1 cm(3) of spinal cord was limited to 11.5Gy.


RESULTS
The best plans in terms of target coverage and spinal cord sparing were achieved by two arcs and Arcl-80 degrees and Arc2-280 degrees collimator angles for all the cases considered (i.e., leaf travel parallel to the spinal cord primary orientation). If one arc is used, only 80 degrees reached the objectives.


CONCLUSIONS
This study demonstrated the role of collimation rotation for the vertebrae metastasis irradiation, with the leaf travel parallel to the spinal cord primary orientation to be better than other solutions. Thus, optimal choice of collimator angle increases the optimization freedom to shape a desired dose distribution.
Mancosu
Pietro
P
Department of Radiation Oncology, IRCCS Istituto Clinico Humanitas, Milano, Italy. pietro.mancosu@humanitas.it
Cozzi
Luca
L
Fogliata
Antonella
A
Lattuada
Paola
P
Reggiori
Giacomo
G
Cantone
Marie Claire
MC
Navarria
Pierina
P
Scorsetti
Marta
M
D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009928:Organ Specificity; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D013125:Spinal Neoplasms / Q000532:radiotherapy* / Q000556:secondary*