Aspects on the optimal photon beam energy for radiation therapy

The selection of optimal photon beam energy is investigated both for realis tic clinical bremsstrahlung beams and for monoenergetic photon beams. The p hoton energies covered in this investigation range from Co-60 to bremsstrah lung and monoenergetic beams with maximum energies up to 50 MeV. One head a nd neck tumor and an advanced cervix tumor are investigated and the influen ce of beam direction is considered. It is shown that the use of optimized i ntensity modulated photon beams significantly reduces the need of beam ener gy selection. The most suitable single accelerator potential will generally be in the range 6-15 MV for both superficially located and deep-seated tar gets, provided intensity-modulated dose delivery is employed. It is also sh own that a narrow penumbra region of a photon beam ideally should contain l ow-energy photons (less than or equal to 4 MV), whereas the gross tumor vol ume, particularly when deep-seated targets are concerned, should be irradia ted by high-energy photons. The regions where low photon energies are most beneficial are where organs at risk are laterally close to the target volum e. The situation is completely changed when uniform or wedged beams are use d. The selection of optimal beam energy then becomes a very important task in line with the experience from traditional treatment techniques. However, even with a large number of uniform beam portals, the treatment outcome is substantially lower than with a few optimized intensity-modulated beams.