• cerenkov correction
• chromatic removal
• dosimetry
• output factors
• plastic scintillating fibers
• radiotherapy
• small photon fields

The presented work is about the development of Plastic Scintillating Detectors
(PSDs) for small photon field dosimetry in external beam radiotherapy. PSDs
were chosen for their water-equivalence, energy-independent response and high
spatial resolution, in addition to their real-time response. Several prototypes
have been built, using plastic scintillating fibers optically coupled to either a
plastic or silica clear optical fiber that transmitted the scintillation light to the
read-out system, a Charge-Coupled Device (CCD) camera. The scintillators
sensitive volumes were between 0.196 and 0.785 mm3 , to minimize the effects of
volume averaging also in small field sizes. These prototypes have been tested at
a clinical TrueBeam LINAC by irradiating them with photons. Linearity was
tested and field output factors were measured for various square fields, down to
1 × 1 cm2 , both in solid water and in water. The Čerenkov light contribution,
originating mainly in the clear fiber, was corrected with a chromatic removal
technique by using two optical bandpass filters in distinct wavelength regions.
The results show that, after the Čerenkov correction, the prototypes signal
is linear with dose and the measured output factors are in good agreement
(within 1÷2 %) with those measured with different dosimeters under the same
experimental conditions (reference conditions). Such results outline the already
assessed PSDs effectiveness in small photon fields dosimetry and justify the
effort in developing new prototypes optimized in terms of: detector size, further
reducing volume averaging effects; read-out system, photodiodes could replace
the CCD camera and allow real-time measurements; overall assembling and
optical coupling techniques, to minimize light loss and therefore maximize signal.