Three-dimensional dose reconstruction of breast cancer treatment ...

2381 Louwe et al.: 3-D dose reconstruction 2381
FIG. 2. Dose distributions obtained with film bottom
and calculated with the TPS top at the sagittal film
positions A1–A3 in phantom A, and the transversal film
positions B1–B4 in phantom B. The color scale indicates
deviations from the dose at the isocenter.
direction, a more or less homogeneous dose distribution was
obtained by the application of a wedge to compensate for the
variation of the breast contour. At the cranial and caudal
parts of the breast and at the proximity of the lungs, however,
regions with dose differences up to 10% relative to the prescribed
dose were observed.
A. Validation of TPS calculations with ionization
chamber and film dosimetry
The calculated dose and the dose measured at the isocenter
of phantom B using an ionization chamber were equal
to 150.0 cGy and 150.60.3 cGy 1 SD, respectively. The
difference between these values was less than the tolerated
output fluctuation of the treatment machine. The difference
in isocenter position of the two phantoms resulted in a lower
calculated dose for phantom A of 140.1 cGy at the isocenter.
The dose distributions calculated with the TPS corresponded
very well with the normalized total film dose distributions
for both phantoms Fig. 2. The difference between the calculated
and measured film dose, averaged over all pixels,
was 0.5%1.3% 1 SD and 0.2%2.1% 1 SD for phantoms
A and B, respectively. In the Appendix it is shown that
these deviations are consistent with the observed reproducibility
of the film measurements.
B. Accuracy of 2-D dose reconstruction
If the attenuation correction was applied, the deviation
between the reconstructed and planned total dose distributions
resulting from all treatment beams is 0.3%1.6% 1
SD and 0.3%1.8% 1 SD for phantoms A and B, re-
TABLE I. Average deviation between the dose reconstructed at the isocenter
plane from EPID measurements either with or without attenuation
correction, and the dose calculated with the TPS. Results for the individual
beams MO: medial open, MW: medial wedged, LO: lateral open, LW:
lateral wedged as well as for multiple beams are shown. All values have
been obtained by averaging the observed deviations over all pixels within
the region of interest. The values in brackets represent the corresponding
variations 1 SD.
Phantom A
Phantom B
MO 3.6%2.0% 5.6%8.8% 0.2%1.5% 2.3%5.8%
MW 2.3%1.7% 0.7%7.5% 3.6%1.6% 1.3%5.3%
LO 0.9%2.0% 3.8%7.6% 1.2%3.0% 0.4%4.6%
LW 2.8%2.1% 1.7%7.0% 2.3%2.5% 1.7%3.9%
Sum open 2.2%1.7% 4.3%2.7% 0.3%1.8% 1.3%1.8%
Sum wedge 2.5%1.5% 0.9%1.9% 3.0%1.7% 1.6%1.7%
Total dose 0.3%1.6% 2.9%2.3% 0.3%1.8% 1.6%1.8%
Medical Physics, Vol. 30, No. 9, September 2003