Access (in Controller) an unmapped field nestled in a form collection

In a form I have nestled a delete checkbox in a collection of Answers. (each answer have a delete checkbox so that I can delete any answer easily)

But I do not succeed to access the delete value from the controller

Here the code: First I populated my form with the Answers

 $Answers = $repository->liste_Answers_Child_Of_A_Question($question); // return a list of answers  
 $Answers = array('Answers' => $Answers);
 $form = $this->createform(new AnswersType(),$Answers);

AnswersType (with a s)

$builder->add('Answers','collection', array('type'=> new AnswerType()));

AnswerType

$builder->add('Answer_text','text')
        ->add('delete','checkbox',array(
              'mapped'=> false,))

public function setDefaultOptions(OptionsResolverInterface $resolver)
{
    $resolver->setDefaults(array('data_class' => 'Site\BlogBundle\Entity\Answer'))
}

PS: I do not want to use the 'allow_delete' option combined with jquery.