A treatment is a short piece of prose, usually produced as a precursor to the writing of a full screenplay. There seem to be several recognised formats for treatments. If you get to send your treatment out to a producer, these formats would doubtless be important to research. But I am presently concerned with using the treatment as a tool to help the writing process.
How is a treatment different from the synopsis of a novel? It is different in degree. A novel synopsis tends to be one or two pages long. Thus the story of a novel of 100,000 words is reduced to between 500 and 1000 words. There is far less story in a screenplay than in a novel. A screenplay of 90 pages (approximately an hour and a half of screen time) is reduced to a treatment of perhaps 9 pages.
The formats are so different that we can't rely on these figures as a mathematical comparison. But the ratios are revealing none-the-less. Moving from novel to synopsis represents a reduction of 1000 to 1. Moving from screenplay to treatment is more like 10 to 1.
Thus, synopsis writing is a much hated occupation. Novelists - myself included - tend to dislike the brutality of the process. Hacking out all the elegant sub-plots and layering of a story. Whereas, I have found treatment writing to be tremendously creative and enjoyable.
Why not write a treatment of a novel as an aid to writing the novel itself? Looking at those ratios, a treatment of a novel would need to be 10,000 words long. A substantial investment. And would it not take away the joy of discovery that I feel whilst writing the novel? I am not sure. Perhaps I should give it a go.