PROTON BEAMS IN CLINICAL RADIATION THERAPY

Abstract

Proton beam therapy has high potential for substantial improvements in the efficacy of radiation therapy due to the physical characteristics of the beams. Namely, the range of the protons is finite and is dependent on the particle energy. This permits the design of beams which penetrate just sufficient to irradiate the target tissues but to give no dose to the deeper non-target tissues. As a consequence, there can be important reductions of treatment volumes for some anatomic sites. The benefit from the smaller treatment volumes is that patient tolerance is increased and accordingly, dose to the target is raised with a resultant higher tumor control probability. As lesser volumes of normal tissues will be irradiated, the frequency and severity of radiation injury will be cut. An important truism is that radiation damage never appears in unirradiated tissues.
Clinical applications of proton beam therapy have yielded clinical gains in situations where smaller treatment volumes and higher doses to the targets have been realized. These are discussed. Results of proton therapy for 2800 patients with uveal melanoma treated by 70 CGE/5 fractions or 60 CGE/4 fractions from 3 centers show local control within the globe and survival rates to be 96% and 80% respectively. The local control rate for the high dose area was>99%. For chondrosarcoma and chordoma of the skull base and cervical spine, the 5 year actuarial local control rates were 76% and 67% respectively [176 and 39 patients respectively] in the series at the MGH/HCL. Quite promising results are being obtained for smaller groups of cancer patients with tumors at other sites. Also, the experience in the treatment of patients with AVMs by proton/helium ion beams are good and competitive with those from “Gamma Knife” and “Stereotactic Linear Accelerators”.
The design of clinical evaluations of proton radiation therapy is discussed. The two main points of attention in such studies are: 1] selection of the appropriate tumor targets; and 2] definition of the control treatment. For the latter, the highest feasible technology photon arm is considered a must. The challenge is not merely to achieve results superior to the medical standard of today but rather that of the end of the decade when the photon therapy in major centers will be based upon systems featuring 3 D planning, optimization software, on-line portal imaging, multileaf collimation etc. The choice of target tumors is recommended to concentrate on those with tumor control probabilities in the range of 0.2-0.6 for the high tech photon therapy, viz on the steep portion of the dose response curve for local control. This is so in order that the numbers of patient required to demonstrate the gain from an increment in dose of ≈10-15% be reasonably easy to obtain.