I think you'll struggle to get a definitive answer from anyone who actually knows (and that includes clinicians). The comments on overdose at https://www.medicines.org.uk/emc/product/4651/smpc are:
4.9 Overdose
No case of overdose has been reported.
In animal studies, doses of up to 500 times the recommended human dose resulted in dyspnoea, decreased activity and local irritation at the injection site. In cases of overdose, the patients should be monitored closely and management should be symptomatic and supportive.
It's probably a case of if they're willing to give it to you early or not. Nurses might want permission from oncologist.