Hello, Bubr
Many of the words that sound "odd" in English come from other languages.
"Radius" comes from Latin, "phenomenon" comes from Greek.
Sometimes, specially in the technical language of science, such words have not been thouroughly "naturalised" and they keep their foreign plural forms. I don't know if people use "radiuses", and I've never seen "phenomenons", so I can't possibly tell you if these plurals are correct.
There are many foreign words, however, that tend to conform to the English plural, and yet others that seem to be still at the half-way stage between both.
When in doubt, I personally will choose the foreign plurals.
You have to be careful because, in some cases, the two plurals still appear to have different meanings, as in:
- index
1. indexes: tables of contents
2. indeces: algebraical signs
- formula
1. formulas: fixed expressions or set forms of words
2. formulae: mathematical term
- medium
1. mediums: people who claim communication with spirits
2. media: means, agencies.
I'm not sure this information is 100% up to date, but it may help if you take it as "general guidelines".
Miriam