Three-dimensional distribution of radiation within the breast: an intercomparison of departments participating in the START trial of breast radiotherapy fractionation.
OBJECTIVE
To examine the ability of computer planning systems to calculate the dose to the breast correctly in three dimensions. Both the absolute dose at the center of the breast and the accuracy of the isodose distributions were investigated.


METHODS
Measurements were performed in a water-filled breast phantom using an ionization chamber. Thirty-six sets of data obtained during the Standardization of Breast Radiotherapy breast fractionation trial quality assurance program were included in the analysis. The planning systems were grouped according to the algorithms used on the basis of the definitions given in International Commission on Radiation Units and Measurements Report No. 24.


RESULTS
Thirty-two of the 36 planning systems overestimated the dose to the center of the breast, with a mean measured/calculated dose ratio of 0.979 (SD 0.013). The relative dose within 2 cm of the lung was also overestimated.


CONCLUSIONS
Only one algorithm (collapsed cone) investigated in this study was able to calculate the dose at the center of the breast correctly in tangential breast radiotherapy. With modern algorithms, it is important to include a correction for the lower density of the lung, because the dose close to the interface between breast and lung tissue will also be lower than anticipated.
Venables
Karen
K
Marie Curie Research Wing, Mount Vernon Hospital, Northwood, Middlesex, UK. karen_venables@hotmail.com
Winfield
Elizabeth A
EA
Aird
Edwin G A
EG
Hoskin
Peter J
PJ
D001940:Breast / Q000528:radiation effects*; D001943:Breast Neoplasms / Q000532:radiotherapy*; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D015203:Reproducibility of Results