https://aapm.onlinelibrary.wiley.com/doi/pdf/10.1120/jacmp.v...
FIG. 2. ~Color! Illustration of previous 3D CRT technique used at FCCC. Treatment included initial four-field box to small
pelvis, four-field box cone-down to prostate and seminal vessicals, five-field non-coplanar cone-down to prostate only, and
five-field non-coplanar cone-down to prostate only with reduced margins.


http://library.mephi.ru/pdftunnel.php?Z21FAMILY=ИАТЭ НИЯУ МИ...
. Перспективное изображение букетов, состоящих из 9, 11, 13 и 12 изотропных пучков, падающих на полусферический дозиметрический фантом [4]



https://es.termwiki.com/Company:Varian?orderby=desc&type=ter...
inactive field
A treatment field not scheduled for treatment during the current treatment session. The field may have been treated in a previous session or may be scheduled for treatment in a future treatment session. Contrast with Active Field.

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Note added at 14 hrs (2023-10-21 12:33:53 GMT)
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Но более вероятно, что подразумевается «последовательность с постепенным уменьшением размера поля»
https://myharnetthealth.org/glossary/
cone-down: Usually describes a smaller radiation treatment field. A course of radiation treatments can be 1) Initial course, followed by 2)Cone-down #1, followed by 3) Cone-down #2. There are many reasons for using a coned- down field. The primary reason is to limit the dose to sensitive structures.