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Biologically Based Prostate Cancer Treatment Planning with Combined Brachytherapy and External Beam Radiotherapy

X Qiu

X Qiu*, L Voros , G Cohen , G Mageras , J Yang , M Zaider , Memorial Sloan-Kettering Cancer Center, New York, NY


TH-AB-BRA-1 (Thursday, July 16, 2015) 7:30 AM - 9:30 AM Room: Ballroom A

Purpose: Currently, when external beam radiotherapy (EBRT) is combined with brachytherapy, they are planned independently of one another. Standard EBRT strives to deliver homogeneous physical dose to target, ignoring the non-homogeneous nature of brachytherapy. Here, we take advantage of the difference in dose distribution and the resulting biologic effects from both modalities, and examine an alternative approach to deliver homogeneous biological effect. Specifically, we implement the new methodology to the combination of permanent seed brachytherapy and IMRT for the treatment of prostate cancer.

Methods: Prostate cancer treatments consisting of 125I (or 103Pd) implants (110 Gy or 100 Gy, respectively) followed by 45 Gy in 25 IMRT fractions are considered. A biological iso-effective dose (IED) is calculated from clinically-derived biologic parameters to determine the desired dose for each voxel within the target and dose constraints for normal tissue. Steep dose gradients are avoided by replacing the voxel doses required by IED with iso-dosimetric optimization structures for IMRT planning. For sub-volumes receiving large brachytherapy doses less than 45 Gy physical dose constraints are imposed; similarly, for cool sub-volumes, more than 45 Gy physical dose constraints are imposed.

Results: Retrospective evaluation of patient cases shows that improvement in therapeutic ratio is obtained with the new IED-based treatment plan optimization. Dose distribution, IED-volume histograms, and IED-equivalent DVH for a 45-fraction IMRT treatment are presented.

Conclusion: We describe a new biological optimization-based treatment planning implementation. IED is calculated (rather than physical dose) while respecting practical aspects of the EBRT plan. The resulting plans predict reduced toxicity to urethra, bladder and rectum. The results require further clinical verification in a larger number of cases. This new methodology can be generalized to any treatment combination.

Funding Support, Disclosures, and Conflict of Interest: Funding Support: Research grant from Varian Medical Systems. Conflict of Interest: Xinjie Qiu, Gig Mageras, Marco Zaider: Varian Medical Systems; Laszlo Voros, Gilad Cohen, Jie Yang: None.

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