Comparative treatment planning between proton and X-ray therapy in locally advanced rectal cancer

Conformal treatment planning with megavoltage X-rays and protons for medically inoperable patients with a large rectal cancer has been studied in an attempt to determine if there are advantages of using protons instead of X-rays.

Three dose plans were made for each of the six patients: one proton plan, including three beams covering the primary tumour and adjacent lymph nodes and three boost beams covering the primary tumour: one X-ray plan, eight beams including a boost with four beams and one mixed plan with four X-ray beams and a boost with three proton beams. A three dimensional treatment-planning systems, TMS, was used. The evaluation of the different plans was made by applying the biological models TCP and NTCP on the dose distributions in terms of dose-volume histograms.

The comparison shows advantages of using protons instead of X-rays for all six patients, but in three of them, the advantage is only marginal. The dose-limiting organ at risk is the small bowel, but the proton plan and the mixed plan also spare the bladder and the femoral heads better. At 5% NTCP in any risk organ, the calculated mean TCP value for the six patients is increased by 14%-units with the proton plan and 8%-units with the mixed plan compared to X-rays only.

Proton beam therapy has potential advantages when treating medically inoperable patients with a large rectal cancer over conventional X-ray therapy. Since the benefits are comparatively small, although clinically worthwhile, large randomised studies are needed.