The objective of this work was to assess the relative impact of
radiobiological parameters and radiation dose escalation on Tumor Control
Probability for prostate cancer patients treated with radiation.
Radiobiological parameters included α/β ratios, cell surviving
fraction at 2 Gy (SF
$_{2}$
) and clonogenic cell density (CCD). Using the
Niemierko method, TCP was calculated in ten prostate cancer patients as a
function of increasing radiation doses (70–140 Gy), α/β ratios
(1.5–20), SF
$_{2}$
(0.3–0.7) and CCD (10–20 million cells/cm
$^{3}$
). At 70
Gy and CCD of 10 million/cm
$^{3}$
, TCP was above 99% for SF
$_{2}$
of 0.3 or
0.4, 97.4%–98.6% for SF
$_{2}$
of 0.5 and less than 2% for SF
$_{2}$
of
0.6 or 0.7. With dose escalation, TCP values above 99% were demonstrated at
80 Gy for SF
$_{2}$
of 0.5 and 100 Gy for SF
$_{2}$
of 0.6. For SF
$_{2}$
of 0.7,
TCP above 99% was demonstrated with 100 Gy and CCD of 10
$^{4}$
cells/cm
$^{3}$
or 140 Gy and CCD of 10
$^{7}$
cells/cm
$^{3}$
. TCP decreased with
lower α/β of 1.5, but at a much smaller scale compared to
SF
$_{2}$
changes. TCP modeling predicts that SF
$_{2}$
and CCD are dominant
predictors of radioresistance in prostate cancer. Radiation doses of 100 Gy or
greater may be required for tumors with SF
$_{2}$
of 0.6 or above. Relating
clinical tumor prognostic indicators such as Gleason score and PSA to
radiobiological parameters will allow us to identify subsets of patients in
need of higher radiation doses and adjuvant therapy to maximize treatment
outcomes.
