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They may genuinely not know. If she responds to the radiotherapy then the answer could be very different than if she doesn't
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They want to focus on what they can do, rather than making guesses based on population statistics. Projected survival times are notoriously unreliable.
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They don't want to make promises they can't keep, either too optimistic or pessimistic
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They may not think your mum (who is the patient, and as a mentally competent adult is the one who should be receiving the information directly and finding out what she wants to know) is ready to hear potentially bad news yet, or she may not want to discuss it in front of you.