Total brain irradiation with a central lense shielding block and asymmetric collimators

In the treatment of multiple brain metastases, it is recommendable to irradiate the whole brain including temporal lobe and lamina cribrosa, but not the eye lenses. The disadvantage of former techniques using simple rightangled opposing lateral fields and further using individual blocks and positioning masks will be demonstrated and a new technique presented. Without simulation, the patient's head will be positioned on the accelerator table so that the horizontal room laser touches the corners of the eyelids. By isocentric adjustment of the gantry angle to 90 or 270 , the eye lenses can be protected by a round shielding block which is placed in the central beam. The 4 collimators are separate adjusted, beginning with the caudal field border, which extends from the lower orbita to the incisura intertragica, and proceeding with the other 3 collimators corresponding to the head outline. Preliminary irradiation portal film reveals the described target volume. The isocentric irradiation of the opposing field is arranged by mirror imaged input of the field parameters without changing the block. This technique allows a daily good reproducible irradiation of the whole brain including temporal lobe and lamina cribrosa by optimum protection of the eye lenses. The elimination of simulation, individual blocks and positioning masks and the practice of using only one block for the isocentric irradiation from both sides and for all patients is time saving and economical. (orig.)