  Grade II gliomas are slowly growing primary brain tumors that affect mostly
young patients and become fatal after a few years. Current clinical handling
includes surgery as first line treatment. Cytotoxic therapies (radiotherapy RT
or chemotherapy QT) are used initially only for patients having a bad
prognosis. Therapies are administered following the 'maximum dose in minimum
time' principle, what is the same schedule used for high grade brain tumors.
Using mathematical models describing the growth of these tumors in response to
radiotherapy, we find that a extreme protraction therapeutical strategy, i.e.
enlarging substantially the time interval between RT fractions, may lead to a
better tumor control. Explicit formulas are found providing the optimal spacing
between doses in a very good agreement with the simulations of the full
three-dimensional mathematical model approximating the tumor spatio-temporal
dynamics. This idea, although breaking the well-stablished paradigm, has
biological meaning since in these slowly growing tumors it may be more
favourable to treat the tumor as the different tumor subpopulations move to
more sensitive phases of the cell cycle.
