The accuracy of field shape definition using standard shielding blocks and the consequences for field shape verification.

The accuracy of field shaping using standard shielding blocks has been investigated in 157 fields in the thoracic region, the majority for treatment of lung cancer. Accuracy was defined as the width of the tolerance margin around the prescribed field outline that would be needed to accept the full megavoltage field boundary. The use of several positioning aids was evaluated, including a cardboard template that was created with the data already specified for field shape verification of megavoltage fields. Block positioning via tattoos on the skin gives large random field shape errors: assuming a tolerance margin of +/- 5 mm, approximately 50% of these fields would have to be rejected. When a transparent sheet with a drawing of the required field outline was used as positioning aid on the tray without an explicit quality control of the sheet itself approximately 24% of the fields would not be accepted because of an error in the sheet (systematic error) and approximately 20% because of a discrepancy in block position relative to the actual prescription of the sheet (random error). If block positioning is performed via a cardboard template of which the prescription was checked to be better than 3 mm measured in the plane of the isocentre, approximately 96% of the fields will be accepted assuming a tolerance margin of +/- 5 mm: the risk of misplacement of blocks appeared to be very low. Integration of prescription, preparation of the cardboard template, quality control of the template and verification of the megavoltage field shape results in an easy, accurate and reliable way of field shape definition using standard blocks.