6. Reconstruction of Dose Distribution on CT-Image of a Patient

Manabu Mizota, Tatsuaki Kanai, Takashi Akagi, Haruo Yamashita, Munefumi Shimbo, Yasuyuki Futami, Ken Yusa and Masahiro Endo

Keywords: dose distribution, radiotherapy, RTP quality assurance

From the standpoint of quality assurance in radiotherapy, it is very important to compare the dose distributions realized by an irradiation system with the distribution planned by a treatment planning (RTP) system. For comparison of the two dose distributions, it is necessary to convert the dose distri butions on CT images to those in a water phantom or convert the measured dose distributions to those on CT images. We developed tools for the visualization and comparison of these distributions.

To calculate the distribution on a CT-image from the measured dose, CT-images on the same planes as the measurements are extracted from a set of CT umages in the RTP system. Water equivalent depth to each point on the plane from a beam source is calcufated individually and an expected dose is derived by mapping on the measured distribution. Fitting of the depth-dose curve to the calculated SOBP curve also gives a biological distribution. Once calculated, dose distribution information can be easily handled to make a comparison to other calculations or to display in any form, such as color-coded isodose lines, or overlaid on the gray-scale CT-image.

As one example, Fig.3 illustrates differences between the RTP calculation distribution and one derived by this method using the same beam for treatment. In this figure, irradiation from the left side is separated into two fields by a block for eyeball protection. There are some under-dose or over dose areas near the border of the irradiation field, which is distinguished by colors.

In this way, quantitative comparison of dose distributions are made with anatomical information, which also gives a verification of the irradiation system in a very straightforward way. Together with developing a rapid dosimetry system, we intend to make use of this method to check the therapeutnc beam for each patient.

Fig.3. Colored biological dose difference between planning and reconstructed distribution from measurement. Areas of over 10% difference in the normalized to maximum value are painted.