Initial experience delivering stereotactic radiotherapy to a gluteal metastasis on a 1.5T MR Linac

Abstract

Purpose or Objective This work reports on our initial experience delivering stereotactic ablative radiotherapy (SABR) to a gluteal metastasis on a 1.5T Elekta Unity MR Linac (MRL) (Elekta AB. Stockholm, Sweden). Materials and Methods A patient with non-small cell lung cancer (NSCLC) was found to have an 8mm right-sided gluteal oligometastasis on staging FDG PET-CT (stage IVa, oligometastatic disease). Discussion at local SABR multi-disciplinary team (MDT) highlighted challenges in localising the sub-centimetre gluteal metastasis on cone-beam CT (CBCT) imaging, therefore a referral was made for treatment on the MRL within the ethically approved, imaging study PRIMER (Clinicaltrials.gov NCT02973828). A preliminary MR scan was performed to ensure the lesion was identifiable. Vendor approved MR sequences (Table 1) were used to determine lesion conspicuity as, ultimately, only these sequences are sanctioned for clinical use on the Unity MRL. A further spectrally selective attenuated inversion recovery (SPAIR) sequence was performed to suppress fat signal and provide further confidence in disease visibility. The patient then underwent CT and MR planning scans (pCT and pMR respectively), positioned supine with arms on thorax and pelvis immobilised with knee and foot step. pCT was performed for the provision of electron density data to inform treatment planning. pCT and pMR image sets were imported into the Monaco treatment planning system (v 5.40.01, Elekta AB. Stockholm, Sweden) for target volume and organ at risk (OAR) delineation and a prescription of 30Gy in 3 fractions was planned. The T2 SPAIR sequence was co-registered with the 6 minute T2 3D Tra sequence to further aid target delineation. Results Delivery of SABR (30Gy in 3 fractions over 6 days) to a gluteal oligometastasis was successfully completed on the Unity MRL following the Adapt to Position (ATP) workflow. Target visualisation on pMR was superior to pCT, in which the 8mm lesion was barely visible (Figure 1). Consensus review showed that the 6 minute T2 3D Tra sequence provided superior confidence in target visibility and was therefore used for planning and daily plan adaptation/verification. Median treatment time was 52:08 (range 49:19 - 57:16). The patient did not report any adverse effects. Conclusion We have successfully completed SABR delivery to a sub-centimetre oligometastasis within the right gluteus medius muscle on the Elekta Unity MRL following the ATP workflow, a first at our institution. This would not have been possible on a conventional linac due to the poor soft-tissue contrast associated with CT and CBCT imaging. Workflows have been established that will facilitate a future MR guided, soft tissue tumour service.