Ok, everyone knows about drinking plenty of water in hot weather, but it’s even more important during treatment. Any fluid, really, except for alcohol and anything that upsets your stomach. If treatment is making you hot anyhow, you might need external measures for cooling–there are various cool packs, cooling gel pillows and those fans you can drape around your neck, but in the interests of economy I favour a damp-to-wet tea towel around the back of my neck, or a damp hanky if stuck on a hot train. Wetting your wrists and ankles also helps cool the rest of you. Wet tea towel on swollen ankles while you put them up. And if all else fails, a tepid–not cold–shower or bath and just air-drying wrapped in a towel if you can. Sit in a child’s paddling pool if you have one or can get one. I also have a little USB desk fan which can dispense a fine mist, but any fan will help.
(Based on my experience of chemo some years ago, and having lived in a HOT climate in the past,)