  Anatomical motion and deformation pose challenges to the understanding of the
delivered dose distribution during radiotherapy treatments. Hence, deformable
image registration (DIR) algorithms are increasingly used to map contours and
dose distributions from one image set to another. However, the lack of
validation tools slows their clinical adoption, despite their commercial
availability. This work presents a novel water-equivalent deformable dosimeter
that simultaneously measures the dose distribution and tracks deformation
vector fields (DVF). The dosimeter in made of an array of 19 scintillating
fiber detectors embedded in a cylindrical elastomer matrix. It is imaged by two
pairs of stereoscopic cameras tracking the position and angulation of the
scintillators, while measuring the dose. The resulting system provides a
precision of 0.3 mm on DVF measurements. The dosimeter was irradiated with
5$\times$3, 4$\times$3 and 3$\times$3 cm$^2$ 6 MV photon beams in both fixed
and deformed conditions. The measured DVF was compared to the one computed with
a DIR algorithm (Plastimatch). The deviations between the computed and measured
DVFs was below 1.5 mm. As for dose measurements, the dosimeter acquired the
dose distribution in fixed and deformed conditions within 1\% of the treatment
planning system calculation and complementary dose validation using the
Hyperscint dosimetry system. Using the demonstrated qualities of scintillating
detectors, we developed a real-time, water-equivalent deformable dosimeter.
Given it's sensor tracking position precision and dose measurements accuracy,
the developed detector is a promising tools for the validation of DIR
algorithms as well as dose distribution measurements under fixed and deformed
conditions.
