Treating locally advanced lung cancer with a 1.5T MR-Linac - Effects of the magnetic field and irradiation geometry on conventionally fractionated and isotoxic dose-escalated radiotherapy

Abstract

Purpose: This study investigates the feasibility and potential benefits of radiotherapy with a 1.5T MR-Linac for locally advanced non-small cell lung cancer (LA NSCLC) patients.

Material and methods: Ten patients with LA NSCLC were retrospectively re-planned six times: three treatment plans were created according to a protocol for conventionally fractionated radiotherapy and three treatment plans following guidelines for isotoxic target dose escalation. In each case, two plans were designed for the MR-Linac, either with standard (∼7mm) or reduced (∼3mm) planning target volume (PTV) margins, while one conventional linac plan was created with standard margins. Treatment plan quality was evaluated using dose-volume metrics or by quantifying dose escalation potential.

Results: All generated treatment plans fulfilled their respective planning constraints. For conventionally fractionated treatments, MR-Linac plans with standard margins had slightly increased skin dose when compared to conventional linac plans. Using reduced margins alleviated this issue and decreased exposure of several other organs-at-risk (OAR). Reduced margins also enabled increased isotoxic target dose escalation.

Conclusion: It is feasible to generate treatment plans for LA NSCLC patients on a 1.5T MR-Linac. Margin reduction, facilitated by an envisioned MRI-guided workflow, enables increased OAR sparing and isotoxic target dose escalation for the respective treatment approaches.

Keywords: Dose-escalation; Lung cancer; MR-Linac; MRI-guided radiotherapy.

Fig. 1

Differences in the investigated dose–volume metrics between the plans designed for the MR-Linac with either standard or reduced margins and the conventional linac. Numerically positive differences mark an increase in the respective metric for the MR-Linac plans. Displayed are the first and third quartiles (boxes), medians (bands inside), average values (crosses), standard deviations (whiskers) and outliers (circles).

Fig. 2

Dose distribution of the conventionally fractionated treatment plans for patient 1 displayed over an axial (left), coronal (central) and sagittal (right) slice of the average phase of the 4DCT scan. Plans were created either for a conventional linac with standard margins (top row) or the MR-Linac with reduced margins (bottom row). Marked are the ITV (red) as well as the 95% (blue), 50% (yellow) isodose contours.