Keywords:
Radiotherapy techniques, Biological effects, Radiobiology, Radiation therapy / Oncology, Cone beam CT, Radiation physics, Head and neck, Computer applications
Authors:
W. Harriss-Phillips, E. Bezak; ADELAIDE/AU
DOI:
10.1594/ranzcr2014/R-0162
Results
Oxic Tumours
Oxic virtual tumour results,
shown in FIGURE 4,
illustrate the total doses required to kill all clonogens or proliferative cells and the effects of the LQC “DL”.
The model predicts tumour control when delivering 73 Gy (all proliferative) or 67 Gy (all clonogens) ±5 Gy for conventional fractionation,
validating the model against typical clinical EBRT conventional prescriptions.
Applying a DL value of 10 and/or 18 increased the total dose required for tumour control (for the largest two d/#) by 11 to 12 (±8) Gy.
In FIGURE 5,
total dose results were analytically converted to
BED and EQD2,
to show that the schedules were roughly equivalent as d/# increased.
Hypoxic Tumours
In order to control hypoxic tumours using 2 Gy dose fractions,
up to 106±11 Gy was required,
without considering reoxygenation,
or else up to 88±4 Gy if reoxygenation was applied. For SBRT schedules,
reoxygenation had significant dose reduction effects for 6 and 9 Gy/#,
while total doses reduced down to approx.
60 Gy for the 15 Gy/# schedule [FIGURE 6].
Applying a LQC DL value was significant for 9 and 12 Gy/#,
when no reoxygenation was applied.
In FIGURE 7,
total dose results were analytically converted to EQD2,
illustrating that the schedules outcomes for SBRT were not uniform and hence not easily predictable analytically.
FIGURES 8 a) and b) illustrate the dynamics of hypoxic tumour cell kill for the two extremes of low and high delivered doses per fraction. It may be noted that many cell remain after tumour control is achieved,
however these are cells without clonogenic potential.
FIGURE 8 c) shows 9 Gy/# cell kill,
with and with reoxygenation,
with a 1 week difference in total treatment time.
These figures are examples from specific simulations using a single random seed.