Session Item

Oligo-progression of irradiated metastatic site or further recurrence are becoming an open issue in the multi-integrated clinical management of oligometastatic disease (OMD). In this clinical scenario, re-irradiation with stereotactic technique, could represent the finest and repeatable treatment option for enhancing the course of OMD in term of local control and progression free survival.
Moreover, the introduction of hybrid magnetic resonance imaging Stereotactic Body Radiotherapy (SBRT) allows real-time tumor tracking (cine-MRI) and to perform online adaptive radiotherapy (oART). Aim of this study is to retrospectively evaluate the efficacy and toxicity profile of Magnetic Resonance image-guided repeated SBRT (MRIg-reSBRT) in a OMD settings.

We retrospectively analyzed pts with in-field liver or perihepatic abdominal carcinosis recurrence candidate to MRIg-reSBRT.
Second SBRT doses were determined based on the tumor and distance to Organs at Risks (OaRs).
Toxicities were assessed using the Common Terminology Criteria for Adverse Events (CTCAE) scale version 5. Overall response rate (ORR= partial and complete response) and clinical benefit (CB= ORR and stable disease) were assessed at 12 months from the end of reSBRT.  
Progression-free survival (PFS) and overall-survival (OS) at 12 months were calculated using the Kaplan Meier analysis.

From July 2019 to January 2020 18 pts completed prescribed course of MRIg-reSBRT for 24 metastatic liver lesions, with a mean interval of 8,61 months (range 2-28) from the first SBRT.
First stereotactic treatment was delivered on MR-Linac except for two patients. Table 1 summarize pts characteristic. At least an overlap of the 5% isodoses of prescription dose between the first and second SBRT were reported. For a few cases (22%) there were a minimal overlap between highest isodoses, at least 50% of prescription dose. Fig.1
The mean prescribed dose for the first treatment was 43 Gy (range 24-50Gy, mean BEDα/β10=93).
Instead for reSBRT the mean prescribed dose was 41 Gy (range 16-50Gy, mean BEDα/β10=92).
For 15 pts (79%) the total doses were prescribed to the 80% isodose rather for 4 pts (21%) were prescribed to the dmean covering the 95% of the PTV.
The mean tumor volume (the GTV) was 6,8 cm3 (range 1,5-19,9), with median PTV of 13,4 cm3 (range 3,6-34,9). The mean volume encompassed by the prescription isodose was 11,19 cm3.
Mean PTV D2 and Mean PTV D98 were 51(range 16,5- 67,5) and 41(range 15-59,5), respectively.
The average mean liver dose was 3,9 Gy (range 1-10 Gy; median EQD2 2,9 Gy) and 3,7 Gy (range 1,6-8 Gy; median EQD2 3Gy) for the first SBRT and reSBRT, respectively.
We did not report acute and late toxicities >1 at median follow-up of 10,7 months (range 2-34). 
At 1 year OS and PFS was 73,08% and 50% respectively, with an ORR and CB were both 54%.

MRIg-reSBRT is an effective, safe and valid integrated option in a multi-treatment strategy for patients with an oligometastatic metastatic disease.