MO-FG-CAMPUS-JeP2-05: MRI-Guided Single-Fraction Boost Delivery On Individual Axillary Lymph Nodes.

PURPOSE: The Utrecht MRI-linac (MRL) design enables new MR-guided radiotherapy (RT) approaches. This is a feasibility study for a single-fraction high dose (boost) to individual lymph nodes (LNs) in breast-cancer patients, after breast-conserving surgery (BCS) and hypofractionated whole-breast irradiation (WBI) with conventional axillary RT (AxRT).

METHODS: After written informed consent, 5 breast-cancer patients (cT1-3N0) were enrolled (NL500460.041.14 trial) and underwent 1.5T MRI in supine RT position, after BCS. Axillary levels, based on ESTRO guidelines, and organs-at-risk (OARs) - including lungs, chest wall, plexus and neurovascular bundle (NVB) - were delineated. Pseudo-CT scans (pCTs) were generated by HU bulk-assignment of water, lung, and air. With Monaco treatment-planning software (TPS Elekta), VMAT plans were generated for simultaneous WBI and AxRT, prescribing 16×2.66=42.56Gy (V95%>99% V107%<2cc). Two scenarios were considered: AxRT of levels I-II; AxRT of levels I-IV, depending on boost location. Per patient, 4 LNs with varying axillary locations were selected, delineated, and expanded to PTV with 2-mm margin. Using dedicated MRL TPS, accounting for magnetic-field effects, an IMRT 1×8.5Gy boost was simulated for each LN, to achieve a total target dose of 66Gy EQD2 (α/β=3.5Gy). WBI/ART doses and boost doses were added, and evaluated in EQD2.

RESULTS: For all scenarios, 1×8.5Gy boosts could be simulated within clinical constraints for a 66Gy total dose, in addition to WBI/AxRT. LN target coverage was excellent (V95%>95%, mean >8.5Gy). Additional dose to OARs was limited.

CONCLUSION: Our study explored the concept of LN boosting using on-line MRI guidance. It is feasible to boost individual axillary LNs - with 2-mm margin - with an additional 1×8.5Gy, in all axillary levels, within clinical constraints. This may lead to more personalized RT approaches for patients with involved LNs and may reduce RT-induced toxicity, or the need for axillary surgery. Other LN boost strategies, including dose escalation, are under investigation.