How to add radio element inside another radio element in Zend Framework?

I am trying to write a form consist of multiple radio selections like below.

• Radio 1 Option 1

  • Radio 2 Option 1

  • Radio 2 Option 2

• Radio 1 Option 2

  • Radio 3 Option 1

  • Radio 3 Option 2

I am thinking of creating a custom Radio class to redefine the whole structure. But not sure how to write the decorators.

I made it by taking Radio 1 as the StandaloneRadio Element and put Radio 2 and 3 under each option of Radio 1.