Sorry to disappoint, but most "small" isotope packages used for medical treatment or industrial use either go in the cargo hold of passenger aircraft in specially designed, tested & certified transportation packages. Larger radioactive sources either go by sea, or on cargo aircraft.
There are a few reasons that they now go in the cargo hold, rather than way out at the wing tip.
Firstly, the rules on transporting radioactive sources have changed to very severly limit the amount of radiation that you can detect on the surface of the package that contains a radioactive source, so each package has to be specifically designed & tested to suit the radioactive source being transported within it.
Secondly, the transport package has to be proof against a hell of a lot of normal & fairly abnormal transport circumstances and has to keep the radioactive source secure [lockfast] between shipper & receiver, so that even if it gets dropped after being left in a bucket of water for a week, and then slightly burnt, there won't be a measurable radiation level at any surface of the package in excess of the specified (low) level.
Thirdly, if you "lost" a radioactive source way back when the Comet was designed, it wasn't really a huge deal, especially if it was somewhere over the Empire. Now it's classed as somewhat worse than child abuse combined with smuggling class "A"drugs inside fluffy kittens that you've killed especially to stuff with the drugs, and there is almost no end to the efforts you are legally required to go to to prove that you've done everything possible to retreive the lost source.
Even with all this it's still one of the easier items for the Pilot in Command to refuse to transport, either for passenger aircraft or for freight aircraft, although most of the freight guys are quite sensible.
Any other small, secret compartments in any aircraft are now between the Cabin Crew & HM Customs.
Donkey
NRPB Trained Radiation Protection Supervisor (Lapsed)