German to English Medical Medical (general) radiation therapy

Still in reference to radiation therapy for a breast cancer patient. The justification for a higher than normal Faktor is:

Mehr als den Durchschnitt uebersteigender Zeitaufwand bei der technisch schwierigen dreidimensionalen Bestrahlung am Linearbeschleuniger mit individueller Feldausblockung mittels Multileafkollimator.

field blocking? Again, this seems to make sense as far as the words are concerned, but I have not been able to find it as a term. Thanks for your help!