Intensity-modulated radiotherapy by means of static tomotherapy: A ...

832 M. Oldham and S. Webb: Intensity-modulated radiotherapy 832
FIG. 7. Comparison of predicted dotted lines with measured solid lines
absolute isodose lines cGy for the single intensity modulated field irradiation.
The predicted dose distribution corresponding to the nine
intensity-modulated fields as found by the forward dose calculation
step outlined in Sec. II A is shown in Fig. 8a.
Isodoses, interpolated from dose points on a2mmgrid spacing,
are shown after the distribution has been normalized to
the maximum dose in the plane. As the distribution was only
computed for the region inside the circular outer OAR of
Fig. 2, the very low isodoses follow this OAR at the edges.
In reality the isodoses will not assume this circularity at the
edge. At delivery, Fig. 5 shows that a total of almost 200
monitor units were delivered to the perspex phantom via the
62 component vane configurations. This arrangement led to a
maximum dose in the PTV as predicted by the planning software
of 0.82 Gy Appendix B. On completion of the delivery,
the film was developed and the delivered dose distribution
was scanned Fig. 8b via an optical densitometer
Wellhöffer WP102. The scan performed was a 2D net scan
with each line of the scan containing 16 values per mm along
its length, and with line spacing of 0.5 cm. The 2D netscan
lines were performed in both the x and y directions. Conversion
from optical density values to dose values was achieved
using a calibration curve obtained from irradiations of other
verification films in the same pack.
The predicted and measured dose distributions Figs. 8a
and 8b were compared by photocopying the predicted distribution
onto an OHP transparency so that it could be overlaid
on top of the measured distribution. The general agreement
between the two dose distributions is very good. The
90% isodose line of the delivered distribution clearly shows
two concavities where the OARs are adjacent to the PTV.
FIG. 8. Isodoses of the computer-calculated and film-measured dose distributions.
Isodose lines on both plots are starting from outter lines and moving
inwards 30%, 40%, 50%, 60%, 70%, 75%, 80%, 85%, 90%, 95%,
100% of the maximum dose. A scale-drawing of the PTV and OAR’s has
been superimposed onto the isodose lines. a The predicted dose distribution
as found by the forward dose calculation which takes into account the
penumbral characteristics of the delivery technique. b The delivered dose
distribution as measured by verification film.
Lower isodose lines also clearly show that the dose in the
OARs has been kept relatively very low compared to the
dose in the PTV. All the main features of the predicted dose
distribution are seen in the delivered except for the artificial
circularity of low-dose isodoses at the edges. The 90% isodoses
are consistently in spatial agreement to within 3 mm
except for in the lower right hand region where the predicted
isodose makes a small kink into the body of the PTV. At the
50% isodose level consistent spatial agreement is again
found to within 3 mm, the largest deviation being about 5
mm. Although the general shape of the distributions agree
well, the delivered distribution is asymmetrically hotter on
Medical Physics, Vol. 24, No. 6, June 1997