Pelvic lymph node displacement in high-risk prostate cancer patients treated with image guided intensity modulated radiation therapy with 2 independent target volumes.

Purpose: To evaluate the displacement of the pelvic lymph node (PLN) target when using cone beam computed tomography (CBCT) for localization of the prostate in patients treated with simultaneous integrated boost.
Methods and Materials: High-risk prostate cancer patients treated with image guided intensity modulated radiation therapy with simultaneous integrated boost receiving 60 Gy in 20 fractions to the prostate and proximal seminal vesicles (PTV60) and 44 Gy in the same 20 fractions to the PLN (PTV44) were studied. Two hundred weekly CBCTs of 50 patients were retrospectively reviewed to assess the displacement of the iliac vessels compared with the simulation computed tomography. For each CBCT, possible displacements were analyzed at 3 levels of PTV44: a superior, middle, and inferior slice, making a total of 600 slices reviewed. Geographical miss (GM) was defined when any part of the iliac vessels on the CBCT was outside of the PTV44 contour.
Results: GM was found in 7 of the 600 CBCT slices, all in different patients. All GMs were of ≤5 mm. Four GMs occurred on the middle slice and 3 on the superior slice. In 3 cases, the GM was related to shifts ≥7 mm applied to the prostate.
Conclusions: Our review suggests that for high-risk prostate cancer, the chance of not appropriately covering the PLN target after adjusting the prostate is low. GM was uncommon and in the order of only a few millimeters when it occurred.
(Copyright © 2015 American Society for Radiation Oncology. Published by Elsevier Inc. All rights reserved.)