Exhibit Hall | Forum 6
Purpose: To perform a dosimetric evaluation of prostate stereotactic body radiotherapy (SBRT) reirradiation using Cyberknife to deliver a comparable dose distribution as HDR brachytherapy for prostate reirradiation
Methods: Two patients with locally recurrent prostate cancer after prior radiation therapy in 2008 and 2015 using linear accelerators (LINACs) were selected for re-treatment using Cyberknife. The SBRT treatment plan was developed using Cyberknife Multiplan with the prescription dose (Dtx) of 34 Gy in 5 fractions to cover >95% of the planning target volume (PTV). The clinical target volume (CTV) was defined to include the magnetic resonance imaging defined gross target volume (GTV) plus 5mm proximal seminal vesicles, and the PTV was defined to include 3-5mm expansion margin around the CTV. The planning goal was to mimic HDR brachytherapy isodose distribution reported by Fuller et al 2019. PTV dose constraints were set to V100>95%, V125=50%-60%, V150=15%-25%, V200=0-2%, and Dmax>150% of Dtx. For organ-at-risk (OAR), Dmax<120% & D50<105% of Dtx for urethra, and Dmax <100% of the Dtx for both the bladder and rectum. The equivalent uniform dose (EUD) was calculated to evaluate the dosimetric equivalency compared to Fuller’s study.
Results: PTV V100=95% & 99%, V125=57.0% & 59.5%, V150=16.8 & 17.1% of Dtx, and the prescription isodose line was 56% & 58%, respectively. The EUD was 43.89 Gy, which was comparable to 42 Gy reported by Fuller. For OARs, the urethra Dmax=37.8Gy & 38.6Gy, and D50=34.3Gy & 35.7Gy; bladder Dmax=33.9Gy & 34.6Gy, and rectum Dmax=33.7Gy & 34.5Gy, respectively.
Conclusion: The PTV dose coverage and OAR doses were within the criteria used for reirradiation prostate HDR brachytherapy, and were comparable to previous reports. SBRT with Cyberknife can be offered as an alternative treatment modality for reirradiation of prostate local recurrences, especially for patients who are not candidates for salvage HDR brachytherapy.