Predictive factors for dose escalation with Cyberknife in oligometastatic disease


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Alison Tree1, Nicholas van As1, Karen Thomas1, Vincent Khoo1
1Royal Marsden NHS Foundation Trust, London, UK

Background

Radical treatment of oligometastatic disease (OMD) with stereotactic ablative radiotherapy (SABR) may improve cancer outcomes but there is no consensus on the optimal dose for SABR . This study evaluates the maximal safe SABR dose escalation to OMD involving nodes or bone.

Method

We selected 15 patients with OMD (5 with OMD in the thorax, 5 in abdomen, 5 in pelvis) whom were treated with SABR. The maximal potential dose escalation within dose constraints was assessed [1], to a maximum of 45 Gy in 3 fractions.

Results

Mean PTV volume was 29.8cc (range 6.3-97.2 cc). Nine cases had nodal OMD and 6 had bone OMD. Mean distance to the nearest serial organ at risk (OAR) was 5.9 mm. For 7/15 cases this organ was small bowel and for 3/15 cases it was the spinal cord. The mean projected treatment time was 48 minutes per fraction.

Mean dose achievable to the planning target volume (PTV) was 34.7 Gy (range 23.0 to 45.0 Gy) representing a 16% dose escalation on our protocol. In only 3 cases could a dose of 45 Gy in 3 fractions be given within constraints. The rate of achievable dose escalation was higher in the pelvis (mean 40.2 Gy) than in the abdomen (32.1 Gy) or thorax (31.8 Gy).

Distance to the nearest serial OAR was significantly correlated with dose achievable (Spearmans coefficient 0.85, p=0.01). If this distance was >5mm, an average dose of 43.8 Gy was possible, compared to a dose of 30.6 Gy if <5mm. PTV volume is not predictive in this series.

Conclusion

We can predict ability to dose escalate in OMD by measuring the distance to the nearest relevant serial organ. Further clinical data is needed to establish if maximizing the SABR dose achieves a higher rate of local control for OMD.