Push HTML to grid field

Dear all.
I'm trying to push HTML code into a grid field editing site.php file. It works but the code is escaped when rendered.
This is the method I used:

$newColumn = $e->getGrid()->addField(new Am_Grid_Field("Usage", "Usage", false))
->setGetFunction(function($record, $grid, $fieldname, $field){
$columnCode = "Some code...";
return $columnCode;
});

I also tried to replace some arbitrary placeholders, like this...

Am_Di::getInstance()->hook->add(Am_Event::AFTER_RENDER, function (Am_Event_AfterRender $event) {
if (preg_match('#/URL.*#i', $_SERVER['REQUEST_URI']))
{
$event->replace('#PLACEHOLDER_CODE#i', "<span class=\"myClass\">Activities</span>");
}
}

...with no success.
It works only for the first page (I'm working on Browse Users page). When I click on "next" or manually search for something, the AFTER_RENDER event is not called anymore.
How can I push HTML inside a field, as it is, without code replacements?
Thanks in advance

I got it!
I had to use the function setRenderFunction.
Here is the code:

$newColumn->setRenderFunction(function($record, $fieldname, $grid, $field){
$columnCode = "";
return $grid->renderTd($columnCode, false); // false turns off the escape function.
});

Watch out the order of the parameters because it is different compared to setGetFunction.
Best regards!