Role of On-Table Plan Adaptation in MR-Guided Ablative Radiation Therapy for Central Lung Tumors

As patients with centrally located lung tumors are at increased risk of toxicity with stereotactic ablative radiation therapy (SABR), we performed stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) for such patients. We retrospectively analyzed the benefits of daily on-table plan adaptation.

Twenty-five patients with central lung tumors underwent a total of 182 fractions of video-assisted, respiration-gated SMART on the MRIdian (ViewRay, Inc). Risk-adapted fractionation was used to deliver 60 Gy in 8 fractions (n = 20) or 55 Gy in 5 fractions (n = 5). For each fraction, daily MR-guided setup and on-table plan reoptimization, based on planning target volume (PTV) coverage and organ-at-risk (OAR) constraints, was performed. Gated breath-hold delivery was performed under continuous MR guidance. Benefits of daily plan reoptimization were studied by comparing 168 "predicted" plans, which are the calculated baseline plans on the anatomy of the day, with the reoptimized treatment plans.

The reoptimized plan was chosen for treatment in 92% of fractions. On-table plan adaptation improved PTV coverage in 61% of fractions by achieving superior coverage by the prescription dose (V_100%) and a higher median dose (D_50%). Mean increase in PTV V_100% was 4.6% (P < .01) with a median of 91.2% and 95.0% in predicted and reoptimized plans, respectively. The benefits of on-table adaptation persisted in an analysis restricted to fractions in which the PTV change was ≤1 cm³ compared with baseline. On-table plan adaptation reduced the number of OAR planning constraint violations (P < .05). Maximum OAR doses remained mostly stable, with on-table reoptimization avoiding excessive OAR doses in selected cases.

On-table plan reoptimization during breath-hold MR-guided SABR for central lung tumors improves target coverage while avoiding excessive OAR doses. The SMART approach may widen the therapeutic window of SABR in high-risk patients with central lung tumors.