Multilayer Gafchromic film detectors for breast skin dose determination in vivo.

Assessment of skin dose delivered to patients from radiotherapy x-ray beams should be performed both inside and outside the prescribed treatment fields. A multilayer Gafchromic film detector which has high sensitivity for detection of radiation can be used to measure skin dose in a two-dimensional map over the skin surface if required. This is an advantage over other detectors, which only provide point dose estimates. A study of 25 patients undergoing breast irradiation was performed to analyse the ability of the multilayer detector to analyse skin dose and to assess both in-field and out-of-field radiation doses delivered during tangent field breast irradiation. Results show that the main contributor to total skin dose within the treatment field was delivered by exit dose. However, outside the field, most dose was delivered by entry beams. Patients with smaller breast separations where found, in general, to receive a higher total skin dose from entry and exiting beams at the central axis. Results also showed that a significant skin dose was delivered outside the treatment field and the main cause of this dose was from electron contamination from entry beams. The multilayer Gafchromic film detector provided adequate skin dose assessment within one fraction of treatment for in vivo results.