Asymmetric field arc rotations

Optimal treatment planning of target volume that surrounds a vital critical structure is often very difficult. Treatment techniques using moving beam therapy with fields asymmetric with respect to rotational axis of the collimator head allow treatment of such target volumes with minimal dose to critical structures. The availability of independent motion of the collimator jaws on new medical accelerators allows easy setting up of asymmetric treatment portals. Therefore, treatment techniques utilizing asymmetric field arc rotations with acceptable dose distributions have been possible. It is sometimes necessary to treat a tumour volume that surrounds a structure with low radiation tolerance. One method that has been used in the past is to use moving beam therapy while a secondary block is placed in the field to shield the sensitive region (Proimos, 1960, 1961; Rawlinson & Cunningham, 1972; Takahashi, 1965). While it is possible to achieve acceptable dose distributions with this technique, there is the undesirable feature of significant transmission through field-shaping blocks. The safety concerns in rotating the machine gantry with heavy blocks and the weight of the blocks have been additional deterring factors. A useful recent development in medical linear accelerators is the availability of field offset facilities (Varian, Siemens, USA; Philips, UK). This enables a photon field to be set with field centre offset from the radiation head rotation axis, thus allowing set-ups of asymmetric portals. This is achieved by independent motion of each collimator jaw or pair of jaws. Treatment techniques using moving beam therapy with fields asymmetric with respect to rotational axis are therefore very easy to implement. These are useful in the treatment of vertebral body and thyroid tumours. In both cases, the critical structure is the spinal cord. A doughnut-shaped isodose distribution is ideally suited for such target volumes. Such distributions are possible with asymmetric field arc rotations. This report describes the concepts, computer simulation and experimental verification of asymmetric field arc rotations with a 6 MV X-ray beam from Philips SL75 linear accelerator. A clinical example is also described where it was possible to generate an optimized treatment plan with acceptable dose distributions for the management of a patient with a vertebral chordoma.