From the Wikipedia article: Fans have occasionally seized upon the two Time Lords' rivalry to speculate that the Master and the Doctor are, in fact, brothers. Such speculation was fueled by the Master's last line in Planet of Fire, which was originally scripted by Peter Grimwade (with permission from script editor Eric Saward) to allude to this as "Doctor, could you do this to your own..."
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Fans have occasionally seized upon the two Time Lords' rivalry to speculate that the Master and the Doctor are, in fact, brothers. Such speculation was fueled by the Master's last line in Planet of Fire, which was originally scripted by Peter Grimwade (with permission from script editor Eric Saward) to allude to this as "Doctor, could you do this to your own..."