Analysis of field and custom block sizes used in radiation therapy--implications for multileaf collimator design

The sizes of radiotherapy treatment fields using customized low melting point alloy blocks have been analyzed for a broad range of treatment fields in a busy radiotherapy department (i.e., 2,000 new cases per year), from small head and neck fields to large mantle fields. The results of this study show that 90% of the blocked fields have an equivalent field size defined by the collimator less than 26 cm and 80%, less than 21 cm. Measurements of the average size of the customized blocks indicate that along one axis 90% have lengths less than 15.5 cm and 80% have lengths less than 13 cm, while for the other axis, 90% have lengths less than 7 cm and 80%, less than 6 cm.