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Radiotherapy for Prostate Cancer Using RapidArc: Does Radiation Energy Make a Difference?

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H Sidhique

H Sidhique1*, S Kaliyappa2, T Kron3, J Cramb4, M Durai5,(1)Continental Hospitals, Hyderabad, India (2) Action Cancer Centre, New Delhi, India (3) Peter MacCallum Cancer Centre, Melbourne (4) Peter MacCallum Cancer Centre, Melbourne,(5) Shidhivinayaka Cancer Centre, MiraJ

SU-E-T-663 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The aim of our investigation was to determine the difference between treatment plans using 6 and 15MV X-rays when employing RapidArc for radiotherapy of locally advanced prostate cancer

Methods:In a retrospective study of five patients 4 plans were developed each (Varian Eclipse V8.9.08). PTV1 consisted of prostate plus seminal vesicles while PTV2 contained bilateral pelvic lymph nodes. RapidArc plans were individually optimized for 6 and 15MV X-rays to develop the best possible plan in each case. To avoid bias in the comparison two additional plans were created in which the optimization constraints were exchanged to ensure differences between the objectives did not affect the results. Homogeneity Index (HI), Conformity Indices (CI) and several dose/volume parameters were evaluated for PTV and organs at risk. Number of monitor units (mus) was used a surrogate for integral peripheral dose.

Results:Plans developed with higher X-ray energy reduced the dose to rectum and bladder without compromising target coverage. The largest differences between plans was found to be in the medium dose range with a dose reduction of 6, 5.5 and 2.5% of the prescription dose for 50% of the volume of rectum, bladder and intestines, respectively. The exchange of optimization constraints confirmed these findings and resulted as expected in reduced HI in all cases and a small reduction of PTV dose in the case of 6MV. The number of mus was generally smaller for plans employing 15MV.

Conclusion:Our study showed that it is possible to demonstrate an advantage for the use of 15MV over 6MV when treating pelvic lesions with RapidArc if optimisation parameters are chosen to maximise the benefit for each energy.

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