Mixing intensity modulated electron and photon beams: combining a steep dose fall-off at depth with sharp and depth-independent penumbras and fiat beam profiles

For application in radiotherapy, intensity modulated high-energy electron a nd photon beams were mixed to create dose distributions that feature: (a) a steep dose fall-off at larger depths, similar to pure electron beams, (b) hat beam profiles and sharp and depth-independent beam penumbras, as in pho ton beams, and (c) a selectable skin dose that is lower than for pure elect ron beams. To determine the required electron and photon beam fluence profiles, an inv erse treatment planning algorithm was used. Mixed beams were realized at a MM50 racetrack microtron (Scanditronix Medical AB, Sweden), and evaluated b y the dose distributions measured in a water phantom. The multileaf collima tor of the MM50 was used in a static mode to shape overlapping electron bea m segments, and the dynamic multileaf collimation mode was used to realize the intensity modulated photon beam profiles. Examples of mixed beams were generated at electron energies of up to 40 MeV . The intensity modulated electron beam component consists of two overlappi ng concentric fields with optimized field sizes, yielding broad, fairly dep th-independent overall beam penumbras. The matched intensity modulated phot on beam component has high fluence peaks at the field edges to sharpen this penumbra. The combination of the electron and the photon beams yields dose distributions with the characteristics (a)-(c) mentioned above.