[Dosimetric assessment of external treatment plans for breast cancer: the significance of dose prescription selection].

PURPOSE

To analyze the treatment plans of breast irradiation performed with two tangential beams, to discuss the importance of dose prescription, and to estimate the differences in delivered dose due to various dose prescription methods.

MATERIAL AND METHODS

A survey was performed between the Hungarian radiotherapy centers in order to compare the dose prescription methods. Then, treatment plans of 125 breast cancer patients treated in our department were evaluated. The irradiations were performed with cobalt unit, and with 6 and 9 MV photon beams of linear accelerators. The dose distributions were normalized to izocenter, then dose values in five points in central plane; local medial, lateral and central maximums (D(med), D(lat) and D(cent)); volumetric maximum and its location were determined. To characterize the dose to lung and to heart at left-sided tumors the central lung distance (CLD) and maximum heart distance (MHD) were used. Based on the results, estimation was made to assess the differences between delivered dose due to various dose prescriptions applied at the institutions.

RESULTS

Four types of dose prescription are currently used in our country, and most frequently the isocenter is selected as a reference point. In the central plane the calculated dose in all but one points differed only a little from the dose to isocenter. The mean D(med), D(lat) and D(cent) were 107%, 107% and 101%, respectively. The volumetric maximum was on average 13% higher than the dose to isocenter. Regarding the beam qualities, this value was 16%, 13% and 11% for cobalt unit, 6 MV and 9 MV photon beams, respectively. The mean CLD and MHD were 1.9 and 0.8 cm, respectively. The difference between delivered doses at the institutions was 6% on average, but in extreme cases it can be as high as 20%.

CONCLUSIONS

Three-dimensional treatment planning and plan evaluation are recommended at breast irradiation, especially for large breasts. Since the various dose prescriptions may result in significant differences in the delivered doses, use of a standard dose prescription protocol is recommended.