KAPIL MISHRACSU/CLEVELAND CLINIC

ANDREW GODLEY

Image Guided Radiation Therapy (IGRT) shifts patients to best align the target with the treatment beams

However, IGRT is unable to correct for change in target shape, or position of organs at risk relative to the target

• Need to adapt plan per daily image• The most common form of IGRT is kV cone-beam CT

(CBCT), which lacks accurate electron density informationCT Deformed CT

CBCT

We propose to deform the planning CT to match the anatomy of the CBCT and hence provide the electron density.

We will test our proposal by using CT-on-rails (CTOR) images as these have correct electron density

Determine the dosimetric difference between plans calculated on CT-on-rails images and a deformed planning CT

- Five prostate only IMRT patients treated with CTOR image guidance were selected. These patients were re-planned on Panther (Prowess Inc.) with a uniform 5 mm PTV expansion, prescribed 78 Gy.

Deform planning CT to CT-on-Rails Five CT-on-Rails were chosen for each patient

• Load both images into Prowess• Contours were drawn on CTOR and copied to deformed CT The original treatment plan was copied to both the CTOR

and DPCT, keeping the center of the prostate as the isocenter

The plans were then calculated using the collapsed cone heterogeneous dose engine of Prowess

DVH parameters were compared

PROSTATE(100%)

PROSTATE(98%)

PROSTATE(95%)

PTV(98%)

PTV(95%)

PTV(Mean Dose)

CTOR 7918.3 8058 8106 7672 7809 8193.4

Deformed 7909.5 8057 8118 7682 7816 8201.3

1Day 0.12% 0.01% 0.15% 0.13% 0.09% 0.1%

5Day Ave 0.11% 0.09% 0.15% 0.1% 0.19% 0.12%

Total Ave 0.5% 0.2% 0.2% 0.3% 0.3% 0.3%

RECTUM(V70 Gy)

RECTUM(V 60 Gy)

RECTUM(Mean Dose)

BLADDER(V70 Gy)

BLADDER(V60 Gy)

BLADDER(Mean Dose)

CTOR 8.5 15.4 3848.4 7.4 10.2 1680

Deformed 8.3 15.8 3832.4 7.4 10.2 1678

1 Day Ave 2.35% 2.6% 0.42% 0.0% 0.0% 0.12%

5 Day Ave 1.41% 2.59% 0.58% 0.39% 0.2% 0.11%

Total Ave 1.5% 1.4% 0.3% 1.5% 0.7% 0.3%

More patients Contrast errors in deformation with errors in dose

Do bad images have high dosimetric error? Deformation errors away from beams? Will the deformation accuracy when deforming to lower

contrast CBCTs be sufficient to get an accurate electron density?

Further comparisons such as gamma index between the doses could be performed

Initial results show negligible difference between dose calculated on deformed CT and CTOR

Thus the deformed planning CTs are a suitable substitute for electron density

Next question is whether the deformation accuracy when deforming to lower contrast CBCTs will be sufficient to get an accurate electron density

THANK YOU