TITLE:
A Statistical Index for Evaluating the CTV Coverage Loss of Brain Stereotactic Radiosurgery When the PTV Margin Is Zero
AUTHORS:
Jenghwa Chang
KEYWORDS:
Stereotactic Radiosurgery, PTV Margin, Coverage Probability
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.11 No.1,
February
28,
2022
ABSTRACT: Purpose: To develop a new statistical index “percent CTV (clinical target vo-lume) coverage probability” (%CCP), defined as the probability that a specific percent (e.g., 95%) of the CTV can be reliably
covered by the prescription dose, for
evaluating the coverage loss of brain (fractionated) stereotactic radiosurgery (SRS/fSRS) when
the PTV (planning target volume) margin is zero. Methods: The random variable Q for CTV percent coverage was derived using a previously developed
model for CTV random motion that follows a three-dimensional (3D) independent normal distribution with a zero mean and a standard deviation of ????(for translation) or ????(for rotation). Assuming both CTV and PTV are spherical with the same diameter dCTV, the
cumulative distribution function of Q could be obtained analytically using the relation of sphere-sphere intersection. The
%CCP was then derived as the reliability function of Q and was used to quantify the coverage loss for selected dCTV. Results: The 95%-95% clinical goal (95% of the times, at least 95% of the CTV is covered) is not achievable
with dCTV mm. For common CTVs (dCTV mm) encountered in SRS/fSRS, only 60%-90% of the CTV
could be reliably covered by the prescription dose 95% of the time. For ????=0.5mm and ???? =0.4?, the 95% CCP was the highest
when the distance between the CTV
and the isocenter ???????=0 and gradually decreased with the increasing ???????. Conclusions: The %CCP was successfully derived for evaluating the
CTV coverage loss for brain SRS/fSRS. When the PTV margin is zero, the 95%-95% clinical goal cannot be achieved for most targets (dCTV mm).