Necrosis of cancer cells releases their content into the interstitial fluid and hence the blood.

Blood plasma is a very complex fluid with a huge dynamic range of protein concentration. See an old but still very valid review by one of the "pope" of plasma protein, Leigh Anderson:

Article The Human Plasma Proteome: History, Character, and Diagnosti...

As you will see many proteins that are normally secreted like hormone and cytokines are only found in trace amount while cytosolic proteins arising from cellular breakdown can be quite abundant.

If you want the full list of proteins that have been identified over the years in the plasma proteome by HUPO groups you can plug the following SPARQL query in the neXtProt advanced search option:

https://www.nextprot.org/proteins/search?mode=advanced

#Proteins which have been detected in the HUPO plasma proteome set

select distinct ?entry where {

?entry a :Entry.

?entry :isoform / :peptideMapping / :evidence / :assignedBy source:PeptideAtlas_human_Blood_Plasma .

}

Best

Amos Bairoch