Two lesions between buttocks that are burning when they get warm, 68M

[Yoddleman]

Short bio first:
*     Age: 68
*     Sex: male
*     Height: 175cm
*     Duration of complaint: around 3 months
*     Location: between buttocks
*     Any existing relevant medical issues: glioblastoma (after resection and RT, before chemo); type II diabetes
*     Current medications: dexamethazone, potassium, magnesium, omeprazole, metformin
*     Include a photo if relevant: NSFW obviously: https://imgur.com/a/pA6KtJe

Writing on behalf of my grandpa. He's currently undergoing treatments for his brain tumor (after surgical resection and RT, before chemo currently) and this red lesion appeared on his right buttock around three months ago, after his first hospital stay. It stayed more or less the same - dry, redish and stinging when it warmed up so he had to make sure to change sitting position often. We've tried plethora of things like hydrocortisonum ointment, sudocrem, clotrimazolum, antiseptic sprays, baby powder, all-in-one OTC wound creams with allantoinum and dexpanthenolum - but to no avail.

After his last hospital stay (6 weeks, undergoing RT) he also developed a second lesion of the same type on the left buttock. As can be seen on the photo, this one is bigger and more crater-like and grandpa says that now not only is it getting stingy when it warms up but also when he sits for a moment and tries to stand up, it feels like it dried up a bit and is tearing open which causes significant discomfort and some pain.

What may this be? Sure, grandpa spent a lot of time laying down in the hospitals but it's not like he's immobile and he wasn't moving the whole day or anything. He did the rehabilition excercises every day, tried his best to move as much as possible and applied the sprays and creams himself every day while at the hospital too, but it did not help much.

What else can we try? The hospital staff always just recommended spraying this with antiseptic sprays and using soothing creams like sudocrem but as it shows, it did not really help at all :(

[CHIN_C]

Bedsores, also called pressure ulcers and decubitus ulcers, are injuries to skin and underlying tissue resulting from prolonged pressure on the skin. Bedridden or immobilized patients in hospitals and nursing homes are particularly at risk.

Staging:

Stage 1: The area looks red and feels warm to the touch.
Stage 2: The area looks more damaged and may have an open sore, scrape, or blister.
Stage 3: The area has a crater-like appearance due to damage below the skin's surface.
Stage 4: The area is severely damaged and a large wound is present. Muscles, tendons, bones, and joints can be involved.

Prevention:

Reposition at least every 2 hours.
Protect at-risk areas with extra cushioning.
Perform regular (at least daily) skin checks of at-risk areas.
Keep skin lubricated and free of excess moisture.
Provide nutritious meals that include protein, vitamins, and minerals.

Treatment:

Clinical Practice Guideline from American College of Physicians:
https://www.acpjournals.org/doi/10.7326/M14-1568