# Which statistical test should I use (comparing two cohorts)?

So I am doing some research in the field of Medical Physics, in particular radiation treatment with photons and protons.

Now, I have two cases I am unsure about.

1) Here I have several patients. Each patient is treated (only simulated) with both photons and protons. From those treatment plans one can get several metrics out describing how good/bad each modality is. So for example, the mean dose to the target/tumour should be pretty much the same for both modalities, i.e. it shouldn't change much. However, how much radiation the brainstem gets (since this is radiation to the brain) is different depending on modality - and patient. So what tests could I do here? For me it's a bit unclear. I could compare between modalities, and see what is best, but that is only two data points for each patient. And since the brainstem toxicity can be VERY different for each patient, is there really any good way to use statistics on all patients as a group?

So the hypothesis would be something like: Is one treatment modality better than the other one, or are they the same.

2) Same patient group. And also photon vs. proton related. However, here we look at the movement within the brain instead. So we take weekly images, and see if anything has moved (target etc.), and then use the same dose calculation on each of these images to see if by using the same treatment plan as originally intended, how much difference would we get in brainstem toxicity and target coverage. So here I have for example brainstem toxicity for each scan/week (7 in total) for each patient. What tests would I use here? First a test to see if the weekly scans/movement has anything important to say, and then after than compare photons and protons? (See image for data example)

I guess the hypothesis is pretty much the same as before. Is one better than the other when looking at how they perform during weekly movement in patient.

I'm quite new to this field, and my statistic skills are kind of rusty at the moment. So I'm looking for help until my statistics course will begin later this year :)

• It might be better if you split this into two separate questions. And you could improve them by clarifying what your specific hypothesis is in the first question ('better' is hard for us to judge), and your experimental design in the second one ('...by using the same treatment plan as originally intended' is also unclear).
– mkt
Jul 25, 2017 at 4:49