I have this in my inherited GraphicsScene:
Qt Code: 
  1. void MyGraphicsScene::mousePressEvent ( QGraphicsSceneMouseEvent * event )
  2. {
  3.   event->ignore();
  4.  
  5.   // Reset selectionArea
  6.   setSelectionArea(QPainterPath());
  7.  
  8.   // Always remember to call parents mousePressEvent
  9.   QGraphicsScene::mousePressEvent(event);
  10. }
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and

Qt Code: 
  1. void MyGraphicsScene::mouseReleaseEvent ( QGraphicsSceneMouseEvent * event )
  2. {
  3.   event->ignore();
  4.  
  5.   bool ctrl = (event->modifiers() == Qt::ControlModifier);
  6.  
  7.   QPainterPath tmpPath = selectionArea();
  8.   if(tmpPath.isEmpty())
  9.   {
  10.     // if ctrl pressed, then toggle selection
  11.     emit select(event->scenePos(), ctrl);
  12.   }
  13.   else
  14.   {
  15.     // if ctrl pressed, then add selection
  16.     emit select(tmpPath, ctrl);
  17.   }
  18.  
  19.   // Always remember to call parents mousePressEvent
  20.   QGraphicsScene::mouseReleaseEvent(event);
  21.  
  22. }
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I just inherited QGraphicsScene locally just to re-implement these two functions. This is the complete code I use in these two functions. You may not need all, but what I do, is to emit the signal with the path (selection rectangle) and if the user has pressed ctrl (adding elements)

Hope this helps and I didn't misunderstand your question.