  The safety and clinical effectiveness of medical devices are closely
associated with their specific use in clinical treatments. Assuring safety and
the desired clinical effectiveness is challenging. Different people may react
differently to the same treatment due to variability in their physiology and
genetics. Thus, we need to consider the outputs and behaviour of the device
itself as well as the effect of using the device to treat a wide variety of
patients. High-intensity focused ultrasound systems and radiation therapy
machines are examples of systems in which this is a primary concern.
Conventional monolithic assurance cases are complex, and this complexity
affects our ability to address these concerns adequately. Based on the
principle of separation of concerns, we propose separating the assurance of the
use of these types of systems in clinical treatments into two linked assurance
cases. The first assurance case demonstrates the safety of the manufacturer's
device independent of the clinical treatment. The second demonstrates the
safety and clinical effectiveness of the device when it is used in a specific
clinical treatment. We introduce the idea of these separate assurance cases,
and describe briefly how they are separated and linked.
