Lights roll over my nose. I want to sneeze but I can’t.

‘OK, hold lovely and still for me now, Casey.’

You’re not meant to know when the protons are firing out of the nozzle and penetrating your skull, but I think I do. It’s like a hum or a vibration, deep, deep down, where Bob is hiding.

I try to picture the beams doing their work. Zap! Bam! Take that, tumour!

‘All done, my love.’ She reappears and helps me to sit up. ‘Be careful stepping down.’

On the way out she gives me a new glass bead for today’s PBT: six done, fourteen to go. I told Mum not to wait earlier, as she’s in a weird mood, so I go straight up to hospital school, not meeting my waffle face reflection in the lift mirror. This weekend Molly was telling me about the end-of-year tests she’s had in chemistry and Spanish and drama, and I’m worried I won’t ever be able to catch up. That’s if Mum even lets me go back to normal school when this is all over.

Will it ever be all over?

When Mr Porter loads up an algebra lesson onto my iPad I’m so focused that it’s only when the little kids start getting fidgety and excited that I realize a whole hour’s gone by.

‘Casey, if you want, you can go and work in the art room while Bear is here,’ Mr Porter says.

‘Bear?’

‘The therapy dog. Remember, from last week? He comes every Monday.’

Last week seems like years ago. I cringe, remembering my pathetic reaction to that little dog.

‘I freaked out because I didn’t expect to see an animal here. Usually I quite like dogs, so I don’t have to hide or anything.’

The teacher nods. ‘Well, if you’re sure. He’s a very good-natured animal. Hey, tell you what: your English project this week is to write an article, so you could write about therapy dogs? His owner, Gemma, has done lots of research.’

I shrug. Mum would still hate the whole idea of a dog here, but if the doctors are cool with it, I can try to be, too.

Because I’m ready this time, I don’t panic when the dog comes in. My classmates go so crazy over Bear that I’m a bit sorry for him. Mr Porter explains about the article.

‘Sure,’ Gemma says. She’s the same sort of age as my mum, nowhere near as pretty, and quite a lot plumper. Though these days everyone is plumper than Mum. But Gemma does have amazingly glossy, dark hair. I touch my own head, hating the falseness of somebody else’s hair. ‘Do you want to chat to me here? Or we can take Bear down to the garden and you can interview him too?’

‘He’s a dog!’ I say.

‘Sorry, that was patronizing of me, wasn’t it?’ she says.

‘It’s OK. Let’s go.’ In the garden it’ll be easier to keep our distance so I don’t get exposed to too many germs.