Range-Shifting Helmet for Proton Radiation Therapy

摘要

Using the range-shifting helmet with fixed-beam proton therapy eliminates the lateral penumbra and reduces spot size for shallow tumors (ie, < 4 cm). Its development and implementation has enabled therapists at Mayo Clinic proton beam facilities to more precisely and safely treat patients who have shallow tumors. The need for a range-shifting helmet arose because all proton systems have a minimum energy and subsequently a minimum depth of treatment. The minimum energy for the Mayo Clinic proton beam facility's pencil-scanning beam fixed-snout system is 71 MeV, which corresponds to a minimum Bragg peak depth of 4 cm in water. To treat shallower tumors, a range shifter must be added to the nozzle to absorb proton energy and decrease the protons' range. Initially, the Hitachi ProBeat system was equipped with 2 internal range shifters made of acrylonitrile butadiene styrene to decrease the depth at which the facility treated. These range shifters are 2.5 cm and 4.5 cm thick and mount inside the snout of the proton machine, 42.5 cm from isocenter. When protons interact with the range-shifting device, they scatter and diverge. The farther the device is from the patient, the larger the spot size, scatter, and the penumbra when it hits the patient, a situation that worsens the dose distribution and reduces therapists' ability to treat small, shallow tumors precisely.