Shielding considerations for tomotherapy.

Tomotherapy presents an evolutionary modality that holds forth the promise of better dose conformation to tumor volumes with a concomitant reduction in radiation-induced damage to surrounding normal structures. This delivery technique also presents a new set of radiation protection challenges that impact upon the design of the shielding vault required to house such a unit. A formalism is presented to determine the requisite amounts of shielding for both the primary beam and leakage radiation associated with a generic tomotherapy unit. A comparison is made with the shielding requirements for a conventional linear accelerator operated in a standard manner. Substantial differences in the amount of both primary and secondary shielding are indicated. A tomotherapy primary beam shield is both reduced in width by a factor of almost 10 and increased in thickness by more than a tenth value layer in comparison to a conventional accelerator. Furthermore, the secondary shielding requirements are enhanced by more than two tenth value layers with respect to conventional shielding demands.