Basic Field-in-Field Guide

The field-in-field technique is a method improving dose homogeneity by decreasing regions of high dose by utilizing field-in-fields. A field-in-field or a subfield is a copy of the primary field (e.g. 01 RT MED -> 01 RT MED.0) which can be created by right clicking on a field and selecting “new field in field”. The MLC leaves of the field-in-field can be moved to block regions of high dose, and by transferring monitor units (MU) from the primary field onto the field-in-field via field weighting, dose deposition in these blocked regions will reduce. As long as all the field sizes remain identical and the MLC leaves of the field-in-field do not extend beyond their physical limitation, both can eventually merge to form a sliding window. Doing so will allow a radiation therapist to treat the primary field and its respective field-in-fields in one go allowing for a reduction in treatment time. There are different variations as to how planners may utilize the field-in-field technique, but the following instructions are only a basic overview:

  1. Open the isodose levels tab, and check 3D for the highest isodose line. Change the highest isodose line to a value that is 3%-5% lesser than the current maximum hotspot and on the Beam’s Eye View (BEV) of any field, the select isodose line should appear as a 3D cloud of dose. For example, if the maximum hotspot is 121%, change the highest isodose line to 116%-118%. Changing the maximum hotspot by 3%-5% at a time is a recommendation because blocking a larger surface area can result in creating dips and holes in your dose distribution.
  2. Create a field-in-field from the primary field by using the function “new field in field”.
  3. On the BEV of the field-in-field, bring in the MLC leaves to cover the 3D isodose line.
  4. Calculate the plan after finalizing your changes. With any change in the MLC leaf position, the plan must re-calculated for.
  5. Open the field weighting tab, and transfer weighting from the primary field onto the field-in-field. If there are other field-in-fields present, only perform weighting between the involved fields.
  6. Repeat this procedure for each subfield until the maximum hotspot is satisfactory.

It is recommended that every field-in-field comes close to meeting the following criteria:

  1. Field-in-field gives more than 5 MU.
  2. Field-in-field gives no more than 10-12 MU.
  3. Maximum of 3 field-in-fields per primary field.

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