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Dosimetric Dependence On Patient Size Reduction Between Prostate IMRT and VMAT

J Chow

J Chow1*, R Jiang2, (1) Princess Margaret Hospital, Toronto, ON, (2) Grand River Regional Cancer Center, Kitchener, ON

SU-E-T-625 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

This study compared the dosimetric impact between prostate intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) when patient weight loss occurs, which resulted in a reduction of external contour in the course of treatment.

Six prostate patients were planned by the 7-beam IMRT and VMAT technique using the same set of target and critical organ dose-volume criteria and prescription dose per fraction (79 Gy per 39 fractions). Based on the original plan, doses in IMRT and VMAT plans were recalculated with the external contour of the patient reduced by 0.5-2 cm anteriorly and laterally to mimic the patient size reduction. Dose coverage and dose-volume points of the targets and critical organs (rectum, bladder and femoral heads) were compared between IMRT and VMAT.

It is found in IMRT plans that increases of the D99% (target volume having 99% of the prescription dose) in PTV and CTV were 4.0 +/- 0.1% per cm of the reduced depth, which were higher than those in VMAT plans (2.7 +/- 0.24% per cm). For increases of the D30% in rectum and bladder, 4.0 +/- 0.2% per cm and 3.5 +/- 0.5% per cm were found in IMRT plans, which were higher than those of VMAT (2.2 +/- 0.2% per cm and 2.0 +/- 0.6% per cm), respectively. The increase of the D5% in right femoral head for IMRT (3.7 +/- 0.1% per cm) was also found higher than VMAT (3.3 +/- 0.1% per cm).

For increases of dose-volume criteria of critical organs in IMRT plans, it is concluded that VMAT would be preferred to IMRT in prostate radiotherapy, when a patient has potentials to have weight loss in the treatment.

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