StoppingTargetBiomarker-class: Stop based on probability of target biomarker
In 0liver0815/onc-crmpack-test: Object-Oriented Implementation of CRM Designs

Stop based on probability of target biomarker

target: the biomarker target range, that needs to be reached. For example, (0.8, 1.0) and scale="relative" means we target a dose with at least 80% of maximum biomarker level.
scale: either relative (default, then the target is interpreted relative to the maximum, so must be a probability range) or absolute (then the target is interpreted as absolute biomarker range)
prob: required target probability for reaching sufficient precision

# As example, here is the rule for: 
#   stopping the study if there is at least 0.5 probability that the biomarker 
#   (efficacy) is within the biomarker target range of [0.9, 1.0] (relative to the
#   maximum for the biomarker).

myStopping <- StoppingTargetBiomarker(target = c(0.9, 1),
                                      prob = 0.5)