When do you add a third field for IMPORT-LOW style partial breast irradiation?
When would you utilize a 3rd field (anterior oblique) to reduce breast V40Gy when treating with IMPORT-LOW style PBI?
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I don’t usually use the 3rd field and treat with min tangent with FIF to improve homogeneity.
If planning to do conformal RT then prefer an APBI dose of 30 in 5 with VMAT technique.
My standard "whole breast" fields are now similar to the approach used in the IMPORT-LOW trial, though more generous - that is, the field borders are placed 4-5 cm beyond the excision cavity, compared to the 2.5-3 cm used in the trial. We commonly add a lightly-weighted anterior oblique to improve homogeneity. Even a few percent of the total monitor units going into this field (or sometimes more than one) can work wonders, especially in patients with large separations. The change in lung and heart dose is trivial. We do forward-planned field-in-field 3D-conformal for almost all these cases.
We also define our breast PTV for this phase differently than the IMPORT-LOW trial does. Our PTV is defined as extending from 5 mm below the skin surface to the posterior surface of the chest wall; to 7 mm from the medial and lateral field edges of the tangential fields; and to 10 mm from the superior and inferior edges of the tangential fields. (Note that including the posterior surface of the chest wall gives adequate margin for setup error and breathing for the PTV to extend all the way to the posterior extent of the breast tissue. Defining the PTV as stopping at the anterior chest wall potentially underdoses this area.) We also have a more stringent coverage goal of 95% of the PTV receiving 100% of the prescribed dose. However, this is based on a prescribed dose of 40 Gy in 16 fractions when a boost is to be used or 45 Gy in 18 fractions without a boost, so the absolute target and hot spots doses are similar to other schemes.
My boost and true PBI plans are essentially the same - that is, an expansion of 1.5-2 cm around the excision site to create the PTV, which is then edited so it comes no closer than 5 mm to the skin surface and no deeper than the anterior chest wall or pectoralis muscle. (I will extend the PTV into the chest wall if the tumor is very posterior or there is a positive posterior margin.) We again ask for at least 95% of the PTV to receive 100% of the prescribed dose.
The "IMPORT" in IMPORT-LOW stands for Intensity Modulated Partial Organ RT.
It's interesting that in some of the responses above, question answerers are using seemingly non-IMRT techniques... or at the least, not pointing out that the Intensity Modulated Partial Organ RT trial was perhaps allowed to be published with an incorrect title. I say this with partial tongue in partial cheek. (I.e., maybe some will say the IMPORT-LOW trial didn't use "true" IMRT... looking at the supplementary materials, I think it did. YMMV.)
Data from MROQC e.g. suggests IMRT is the best way to achieve clinically meaningful outcomes in early breast cancer, perhaps vis-a-vis homogeneity.
Please discuss!
I would say, I almost never add a 3rd field. I have seen excellent outcomes and toxicities with mini tangents alone with very low lung and heart doses.