Session Item

In our clinical practice, breast cancer patients requiring a boost dose to positive axillary lymph nodes, are treated with a simultaneously integrated boost (SIB) technique. Position verification with CBCT is performed for the full axillary lymph node region, the PTV margins cover anatomical variation between boost volume and lymph node regions. With a sequential boost given on a magnetic resonance (MR)-Linac, the plan can be re-optimized to optimally target the boost volume thus allowing smaller treatment margins. The SIB technique is less suited for the MR-Linac, mainly due to the large extend of the treated volume compared to the limited field size on the MR-Linac.

Our aim was to investigate the potential benefit of an MR-Linac (Unity, Elekta) for irradiation of axillary lymph nodes with a sequential boost, relative to our regular SIB technique. The hypothesis is that we achieve a lower organ at risk (OAR) dose using the sequential technique, while the same target dose is achieved.

We performed a plan comparison (Monaco, Elekta) for 7 breast cancer patients who received SIB treatment to axillary lymph node(s), with or without local breast/chest wall radiotherapy. We compared 2 techniques per patient: a SIB technique for the CBCT-Linac (20x2.67Gy using VMAT), and a sequential technique consisting of a primary CBCT-Linac plan (15x2.67Gy using VMAT) and an MR-Linac boost plan (5x2.67Gy with IMRT). To investigate the maximal achievable benefit of the sequential technique, a PTV boost margin of 0mm for the MR-Linac plan was used instead of the 5mm margin used for the SIB technique. Both techniques were converted to EQD2 for DVH analysis with α/β=2 for the brachial plexus (D0.1cc) and α/β=3 for the total lung (V20Gy and Dmean) and humeral head (D2% and Dmean).

All plans were clinically acceptable according to the institutional guidelines. The average DVH parameters were (SIB technique vs. sequential technique) V95% PTV-boost: 98.6% vs. 97.9%; V20Gy total lung: 7% vs. 6.8%; Dmean total lung was similar for both techniques: 4.3Gy; D2% humeral head: 36.5Gy vs. 37.2Gy; Dmean humeral head: 16.5Gy vs 17Gy; D0.1cc brachial plexus: 56.3Gy vs 53.3Gy.

For 4 of the 7 patients there was a larger difference in the D0.1cc of the brachial plexus (figure 1), because the boost location was close to the brachial plexus (figure 2).

Even with a 0mm PTV margin, the overall difference between both techniques is limited. For specific patients with an OAR close to the lymph node boost, the MR-Linac based sequential boost was beneficial over the SIB technique.