How to get the passed options to a Symfony form - php

Question

I'm trying to extend a Symfony 2.8 Form Type, following this http://symfony.com/doc/2.8/form/create_form_type_extension.html
In my ObjectType form I have something like this, where I create my form. My form includes a field of Type MyExtendedType
public function buildForm(FormBuilderInterface $builder, array $options)
{
$passed_options = array('aKey'=>'aValue', ...);
$builder
->add('someotherfield', 'SomeOtherStandardType', ..)
->add('fieldname', 'AppBundle\Form\Type\MyExtendedType', $passed_options)
->add('someotherfield', 'SomeOtherStandardType', ..)
//...
;
}
In my MyExtendedType I have this in which I want to substitute fieldname in FormEvents::PRE_SET_DATA
public function getParent()
{
return 'Symfony\Bridge\Doctrine\Form\Type\EntityType';
}
public function buildForm(FormBuilderInterface $builder, array $options) {
parent::buildForm($builder, $options);
$builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) use ($builder, $options) {
$form = $event->getForm()->getParent();
$modified_options = $options; //this is not correct because there are all the resolved options of the field
$modified_options['aKey'] = 'anotherValue';
$form->add($builder->getName(), 'Symfony\Bridge\Doctrine\Form\Type\EntityType', $modified_options);
});
}
The problem is that in MyExtendedType I need to access only the $passed_options. In $options I have all the resolved options that are initialized from the Form component, and doing as the previous code the field is not properly built, because of the presence of options in contrast with each other (e.g. query_builder, choices, choice_list..).
Here is the question:
how can I access just the $passed_options using the Form component? I don't want to set the properties in the ObjectType in order to have a generic approach.