A series of patients treated with complex fields to the thorax and neck utilising individually tailored shielding blocks made from low-melting-point alloy (LMPA), were investigated using treatment verification film to identify field placement errors. A total of 19 patients yielded 171 machine verification films, which were analysed. It was found that on 21% of occasions the intended treatment volume was partially shielded, by more than 1 cm, by the lead blocks prescribed to protect the lungs. On half of these occasions (11% of the total), the areas erroneously shielded contained known disease. Suggestions are made for minimising such errors in the future.