I tried a couple of things Paul, creating reminders on a product (set to interactive) then set the reminder to completed or cancelled neither of which resulted in the cancellation of the previous reminder type in the same group.

The issue with these types of reminders is that they are going to always (atleast in my practice) be highly interactive, another example I can give you is the "Followup Blood Test" that follows a abnormal result as part of a yearly geriatric profile etc.   The time between the original test and followup is always varying, as would followup x-rays.  The trouble with creating a new reminder is what product do you attach it too to cause the reminder to be removed.

Here was something I came up with

In your case:

Create a Reminder Group: Called say X-rays.

Create your Reminder Type Lets Says its called 'Follow UP X-rays" set the params...ie due in 3mths cancel if not followed after 6mths and set a sensitivity. make sure grouped is ticked

I assume this reminder will be manual added when  a vet believes a patient need followup. So make sure its interactive..but you could add it to a product and this will still work.  You need to decide here if the reminder is going to be applied more often than not when the product is sold.

Now Create a reminder called "Cancel X-ray Followups" and add it to the same group.  Set the Interval to 0, Cancel interval to 0 and sensitivity to 0. Dont add any templates. make sure grouped is ticked

Add this reminder to the product sale you want to cancel other x-ray reminders.

So  what happens.

1. It will cancel any other x-ray reminder in the reminder group X-rays.

2. It will generate a reminder due immediately.(will come up in reports) on the next reminder run it will automatically be cancelled without sending the client any communication.

3. Downside.  The pet will be flagged with a reminder bell until the next run.  BUT the name of the reminder would deter staff from actioning anything I imagine.

Summary: Dirty Fix IMO, as it generates unneeded database entries. but it does give you a solution.  The problem as I see it is actually appling your cancellation type to the right product.  IE I have a chronic Stifle lameness I am monitoring for maybe a OCD lesion etc so I am monitoring for joint mice etc.  Unrelatedly on the weekend the pet presents for an x-ray for its front left toe after the owner stood on it.  My weekend vet x-rays and charges for the x-rays, maybe he just charges a single plate ...which you had set up with your reset reminder...suddenly oops you OCD followup is gone. or maybe he wanted to set another recheck reminder for x-rays up when the interactive reminder pops up for 3mths for the front leg...again your ocd reminder is gone (but actually still needs followup)

My 2 cents is these shouldnt be grouped and should be manually managed by staff.  Each situation is potentially unique though and may be ameniable to grouping.

Good luck hope it helps.

Ben