  Nowadays there is a growing interest in Particle Therapy treatments
exploiting light ion beams against tumors due to their enhanced Relative
Biological Effectiveness and high space selectivity. In particular promising
results are obtained by the use of $^4$He projectiles. Unlike the treatments
performed using protons, the beam ions can undergo a fragmentation process when
interacting with the atomic nuclei in the patient body. In this paper the
results of measurements performed at the Heidelberg Ion-Beam Therapy center are
reported. For the first time the absolute fluxes and the energy spectra of the
fragments - protons, deuterons, and tritons - produced by $^4$He ion beams of
102, 125 and 145 MeV/u energies on a poly-methyl methacrylate target were
evaluated at different angles. The obtained results are particularly relevant
in view of the necessary optimization and review of the Treatment Planning
Software being developed for clinical use of $^4$He beams in clinical routine
and the relative benchmarking of Monte Carlo algorithm predictions.
