TITLE:
Multicomponent Assessment of the Geometrical Uncertainty and Consequent Margins in Prostate Cancer Radiotherapy Treatment Using Fiducial Markers
AUTHORS:
Pablo Castro, María Roch, Almudena Zapatero, David Büchser, Julia Garayoa, Cristina Ansón, David Hernández, Carlos Huerga, Margarita Chevalier, Saturnino González, Leopoldo Pérez
KEYWORDS:
Treatment Margin, Fiducial Marker, Prostate, Geometrical Uncertainty
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.7 No.4,
November
20,
2018
ABSTRACT: The aim
is to
compute all sources of geometrical uncertainty in prostate radiotherapy using
fiducial markers and determine the safety treatment margins. Based on the
markers position, correlations between prostate rotation/deformation and rectal
and bladder fillings as well as changes in prostate volume during the treatment
course are analyzed. The
study includes 375 pre-treatment CBCT images from 15 prostate cancer patients
treated with hypofractionated radiotherapy. The position coordinates of the
markers were obtained from each image acquisition. In addition, rectum and
bladder were outlined on CBCTs. The intrafractional error was estimated by an
additional post-treatment CBCT acquired on alternate days. Tau-Kendall analysis
was performed to correlate organ fillings with prostate rotation/deformation.
Delineation uncertainty was assessed from contours of 10 patients performed by
two radiation oncologists and repeated twice. The CT contouring was assisted by
a multiparametric MR approach combining a T2-weighted with diffusion-weighted
imaging, and a gradient recalled echo for fiducial marker identification.
Uncertainty associated to treatment unit was estimated from phantom
measurements. The
obtained clinical margins were 4.4, 7.3, 5.1 mm in the Left-Right,
Superior-Inferior, and Anterior-Posterior directions, respectively, being the
contouring the most important contribution. The mechanical limitations of the
beam delivery system and the associated imaging device entailed errors of the
same order as prostate motion, rotation or deformation. Weak correlations
between variation of the rectal volume and the presence of rotations/deformations
were found (correlation
coefficient 0.182, p = 0.001 for rotations around lateral axis; correlation coefficients
0.1, p - 1.8 mm at the end of the
treatment. In summary we have determined the optimal treatment margins based on
geometrical uncertainty assessment using van Herk formalism. An appropriate preparation
of rectum and bladder involves minimizing the effect of prostate
rotations/deformations. The prostate tends to decrease in size during the
treatment which could influence treatment re-planning strategies.