sfWidgetFormSelectRadioSingleable Symfony widget lets you render just one option at a time. Extends sfWidgetFormSelectRadio.

sfWidgetFormSelectRadioSingleable.class.php

<?php

class sfWidgetFormSelectRadioSingleable extends sfWidgetFormSelectRadio
{
    public function formatChoices($name, $value, $choices, $attributes)
    {
        $onlyChoice = !empty($attributes['only_choice']) ? $attributes['only_choice'] : false;
        unset($attributes['only_choice']);

        if ($onlyChoice)
        {
            if (!isset($choices[$onlyChoice]))
            {
                throw new Exception("Option '$onlyChoice' doesn't exist.");
            }

            $key = $onlyChoice;
            $option = $choices[$key];
            $baseAttributes = array(
                'name' => substr($name, 0, -2),
                'type' => 'radio',
                'value' => self::escapeOnce($key),
                'id' => $id = $this->generateId($name, self::escapeOnce($key)),
            );

            if (strval($key) == strval($value === false ? 0 : $value))
            {
                $baseAttributes['checked'] = 'checked';
            }

            return $this->renderTag('input', array_merge($baseAttributes, $attributes));
        } 
        else
        {
            return parent::formatChoices($name, $value, $choices, $attributes);
        }
    }
}

Example usage in a template:

<?php echo $form['item']->render(array('only_choice' => 'item_choice_key')) ?>

Simpler way:
https://gist.github.com/jupeter/5443225