  Liquid scintillating phantoms have been proposed as a means to perform
real-time 3D dosimetry for proton therapy treatment plan verification. We have
studied what effect radiation damage to the scintillator will have upon this
application. We have performed measurements of the degradation of the light
yield and optical attenuation length of liquid scintillator and water-based
liquid scintillator after irradiation by 201 MeV proton beams that deposited
doses of approximately 52 Gy, 300 Gy, and 800 Gy in the scintillator. Liquid
scintillator and water-based liquid scintillator (composed of $5\%$
scintillating phase) exhibit light yield reductions of $1.74 \pm 0.55 \%$ and
$1.31 \pm 0.59 \%$ after $\approx$ 800 Gy of proton dose, respectively. Whilst
some increased optical attenuation was observed in the irradiated samples, the
measured reduction to the light yield is also due to damage to the
scintillation light production. Based on our results and conservative estimates
of the expected dose in a clinical context, a scintillating phantom used for
proton therapy treatment plan verification would exhibit a systematic light
yield reduction of approximately $0.1\%$ after a year of operation.
