What is the probability to drop the Ghost backpack from the Pale Worm?

Question

I wanna know if the probability to drop a  Ghost Backpack from  The Pale Worm is known; my friends and I kill it every week and still no one dropped one of them.

Answer 1

No statitcs about Ghost Backpack  on internet, but it's certain that he dropped from this boss.

source: https://www.tibiabosses.com/gallery/ (searching the Pale Worm)

and statitcs about loot from boss

Loot (Toggle % view) 

• 3× Crystal Coin (100%)
• 2× Diamond (45.45%)
• Moonstone (45.45%)
• Ultimate Mana Potion (43.18%)
• Supreme Health Potion (29.55%)
• 6× Ultimate Spirit Potion (27.27%)
• Ivory Comb (25%)
• Mastermind Potion (20.45%)
• Bullseye Potion (18.18%)
• Berserk Potion (18.18%)
• White Gem (13.64%)
• Death Toll (11.36%)
• Silver Hand Mirror (11.36%)
• Amber (Item) (6.82%)
• Cursed Bone (6.82%)
• Spectral Scrap of Cloth (4.55%)
• Amber with a Bug (4.55%)
• Ravenous Circlet (2.27%)
• Soulful Legs (2.27%)
• Giant Topaz (2.27%)
• Giant Amethyst (2.27%)
• Angel Figurine (2.27%)
• Pale Worm's Scalp ?
• Ring of Souls ?
• Fabulous Legs ? 
• Ghost Backpack ?
• Ghost Chestplate ?
• Phantasmal Axe ?

source: https://tibia.fandom.com/wiki/The_Pale_Worm

but in Tibiopedia.pl. (https://tibiopedia.pl/monsters/The_Pale_Worm)  is 5.56% for Ring of Souls so I think that for others the drop may be the same or slightly smaller.