"relentless progression" is correct and usable in written English.
You can use it to describe a situation in which a process or action unfolds steadily and without interruption. For example, "The relentless progression of technology has led to rapid advancements in many fields."
In general, at the disease onset an estimated 85 90% of patients have a relapsing-remitting clinical course that is characterized by clearly defined relapses with full or partial recovery, whereas the remaining 10 15% have primary-progressive multiple sclerosis, which represents a relentless progression from the beginning with no remission (Compston and Coles, 2008).
Each part is meant to do its work in relentless progression.
In 2002 she contracted breast cancer, which was held at bay but eventually returned with relentless progression.
And Greece must continue to meet the relentless progression of obligations to bondholders, civil servants and pensioners.
Parkinson's disease is a neurodegenerative illness that affects five million people worldwide and no treatments are available to stop its relentless progression.
There remains, however, a major need to slow down or reverse the relentless progression of the disease to prevent the evolution of disabling motor and nonmotor features that continue to cause disability despite the existing symptomatic approaches.
Other than the use of corticosteroids, little is available to affect the relentless progress of the disease, leading many families to use dietary supplements in hopes of reducing the progression or severity of muscle wasting.
So one way to view this is as a battle between two powerful networking companies with similar digital business models that are both able to keep getting bigger and bigger by linking people to more people in relentless geometric progression.
This brain cancer exhibits a relentless malignant progression with characteristics of widespread invasion, destruction of normal brain tissue, resistance to conventional therapeutic approaches, and certain death.
Glioblastoma multiform (GBM) is a lethal intracranial cancer, which exhibits a relentless malignant progression and is highly resistant to conventional multimodal therapies.
Despite relentless local progression during the 5 years, the tumour never developed distant metastasis.
