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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:
- 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.
- Create a field-in-field from the primary field by using the function “new field in field”.
- On the BEV of the field-in-field, bring in the MLC leaves to cover the 3D isodose line.
- Calculate the plan after finalizing your changes. With any change in the MLC leaf position, the plan must re-calculated for.
- 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.
- 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:
- Field-in-field gives more than 5 MU.
- Field-in-field gives no more than 10-12 MU.
- Maximum of 3 field-in-fields per primary field.