Magnetic modification of the electron-dose distribution in tissue and lung phantoms

Moderately strong transverse-magnetic fields were used to modify conventional electron-dose distribution in tissue- and lung-equivalent phantoms. Magnetically modified symmetrical-isodose contours and central-axis depth-dose curves were measured for central fields in the range of 9--18 kG, field gradients of approximately 5 kG/cm, and accelerator energies of 10--45 MeV. To the extent that our experimental field strengths and gradients can be reproduced clinically, the measurements showed that magnetic distributions can be generated (in tissue) which are superior to conventional distributions for the treatment of tumors lying at depths less than approximately 7 cm provided that the tumor cross-section dimensions are equal to or greater than tumor depth. The surface dose in tissue is typically reduced by approximately 40% compared to the conventional surface dose for treating the same tumor volume. For the lung phantom data, a significant reduction (less than approximately 50%) in the integrated central-axis dose to healthy tissue was achieved for tumor depths of 10--14 cm. The possibility of reproducing our experimental magnetic fields and gradients inside a patient under realistic clinical conditions is discussed.