Get Structure field entries if a field is not empty

I’m displaying a calendar (a structure field) grouped by year, which works fine with the code underneath.
But I only need to get the entries that contain a value in the field called ‘retreat’.

  $items = page('reserve')->calendar()->toStructure();
  $callback = function($p) { return $p->date_end()->toDate('Y'); };
  $grouped = $items->groupBy($callback)->flip();
  // Group items by year
  foreach ($grouped->flip() as $year => $itemsPerYear): 
  <h2><?= $year ?></h2>
  <?php // Items date
    foreach ($itemsPerYear as $item): 
      $start = $item->date_start()->toDate('d MMMM');
      $end = $item->date_end()->toDate('d MMMM');
      $date = $start . ' t/m ' . $end; 
      <li><?= $date ?> <?= $item->retreat() ?></li>
  <?php endforeach ?>
<?php endforeach ?>

Forgot about the filter operators:

page('reserve')->calendar()->toStructure()->filterBy('retreat', '!=', '');