Evaluation and verification of the QF ix Encompass TM couch insert for intracranial stereotactic radiosurgery

• Published in: Journal of applied clinical medical physics Vol. 19; no. 4; pp. 222 - 229
• Main Authors: Snyder, Karen Chin, Xhaferllari, Ilma, Huang, Yimei, Siddiqui, M. Salim, Chetty, Indrin J., Wen, Ning
• Format: Journal Article
• Language: English
• Published: Malden Massachusetts John Wiley & Sons, Inc 01-07-2018
• Subjects: Accuracy; Automation; Brain cancer; Carbon fibers; Dosimetry; Radiation therapy; United States > US
• ISSN: 1526-9914; 1526-9914
• DOI: 10.1002/acm2.12387

Abstract
The
QF
ix Encompass
TM
stereotactic radiosurgery (
SRS
) immobilization system consists of a thermoplastic mask that attaches to the couch insert to immobilize patients treated with intracranial
SRS
. This study evaluates the dosimetric impact and verifies a vendor provided treatment planning system (
TPS
) model in the Eclipse
TPS
. A thermoplastic mask was constructed for a Lucy 3D phantom, and was scanned with and without the Encompass
TM
system. Attenuation measurements were performed in the Lucy phantom with and without the insert using a pinpoint ion chamber for energies of 6x
FFF
, 10x
FFF
and 6X, with three field sizes (2 × 2, 4 × 4, and 6 × 6 cm
2
). The measurements were compared to two sets of calculations. The first set utilized the vendor provided Encompass
TPS
model (Encompass
TPS
), which consists of two structures: the Encompass and Encompass base structure. Three
HU
values for the Encompass (200, 300, 400) and Encompass Base (−600, −500, −400) structures were evaluated. The second set of calculations consists of the Encompass insert included in the external body contour (Encompass
EXT
) for dose calculation. The average measured percent attenuation in the posterior region of the insert ranged from 3.4%–3.8% for the 6x
FFF
beam, 2.9%–3.4% for the 10x
FFF
, and 3.3%–3.6% for the 6X beam. The maximum attenuation occurred at the region where the mask attaches to the insert, where attenuation up to 17% was measured for a 6x
FFF
beam. The difference between measured and calculated attenuation with either the Encompass
EXT
or Encompass
TPS
approach was within 0.5%.
HU
values in the Encompass
TPS
model that provided the best agreement with measurement was 400 for the Encompass structure and −400 for the Encompass base structure. Significant attenuation was observed at the area where the mask attaches to the insert. Larger differences can be observed when using few static beams compared to rotational treatment techniques.