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Volumetric Modulated Arc Therapy for the Treatment of Cervical Cancer in Lieu of HDR Brachytherapy

C Merrow

C Merrow*, S De Boer, M Podgorsak, Roswell Park Cancer Institute, Buffalo, NY

SU-E-T-635 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To investigate the use of VMAT to deliver the following conceptual GYN brachytherapy (BT) dose distributions: 1. traditional pear-shaped dose distribution with substantial dose gradients, 2. homogeneous dose distribution throughout PTV (BT prescription volume), and 3. increased dose to PTV without OAR overdose.

Methods: A tandem and ovoid BT treatment plan, with the prescription dose of 6 Gy to point A was exported into the VMAT treatment planning system (TPS) and became the baseline for further comparative analysis. The 200%, 150%, 130% 100%, 75% and 50% dose volumes were converted into structures for optimization and evaluation purposes. The 100% dose volume was chosen to be the PTV. Five VMAT plans were created to duplicate the Ir-192 tandem and ovoid inhomogeneous dose distribution. Another five VMAT plans were generated to deliver a homogeneous dose of 6 Gy to the PTV. An additional five VMAT plans were created to increase the dose to the PTV with a homogeneous dose distribution. All plans used BT dose constraints.

Results: On average in the first set of plans, the dose given to 99% of the 200-100% dose volumes agreed within 2% of the BT plan, the 75% dose volumes were within 5% and the 50% dose volumes were within 6.4%. In the second set, the 100% dose volume was within 1% of the original plan. In the last set the maximum increase of dose to the PTV was 8 Gy with similar doses to OARs as the other VMAT plans. The maximum increase of dose was 2.50 Gy to the rectum. The maximum decrease of dose was 0.70 Gy to the bladder.

Conclusion: VMAT was successful at reproducing brachytherapy dose distributions. Alternative dose distributions have the potential to be used in lieu of brachytherapy. However, differences in radiobiology need to be further investigated.

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