Introduction and Objectives
Modern Image Guided radiotherapy (IGRT) techniques allow physician to relocate the target volume in order to ensure a better CTV coverage by the delivered dose. CTV margin enlargement to obtain the PTV is a critical step during planning procedures and the aim of this study is the evaluation of movements of CTV (due to internal movements and setup errors) obtained by IGRT methods in order to individualize the PTV margin.
Material and Methods
Patients affected by prostate cancer were planned twice during CTV delineation procedure: the 1st time by using a standard CTV to PTV enlargement procedure with margins common to all patients. Afterwards the patients started the treatment and during the 1st week of therapy each one underwent to daily Cone Beam CT (CBCT) scanning before dose delivery (5 CBCT scans for each patient). Subsequently CBCTs were registered with the simulation CT by overlapping the images in order to obtain the superimposition of all CT isocenters, hence allowing the overall movements evaluation both for organ motion and setup movements at the same time. A new CTV-IGRT contour was defined by merging all the positions of the prostate and seminal vesicles in the 6 CTs (1 simulation + 5 CBCT) and finally a further 3 mm margin in all directions was added to obtain the a individualized IGRT based PTV. Both the 1st part of the treatment (before the IGRT based optimization) and the 2nd one were planned using IMRT optimization procedures. The size of PTVs in both phases of the treatment were compared. DVH were collected to compare the dose distribution on bladder and rectum before and after the IGRT optimization process of PTVs in order to evaluate differences in dose distribution due to the PTV optimization procedure
Thirteen prostate cancer patients were enrolled in this study. The shrinking of volumes is always significant comparing the pre-IGRT PTV volume with the post-IGRT optimized PTV using the paired sample T-test. A dosimetric analysis was performed by comparing the volume of the rectum and the volume of the bladder receiving 50 Gy (V50) in all the patients calculating these values on the delineated critical structures on the simulation CT. The reduction of the V50 was significant only for the rectum and not for bladder.
Using CBCT for the optimization of PTV delineation seems to be helpful reducing the amount of the rectal volumes receiving highest dose levels. This fact is due to the PTV shrinkage obtained after a re-delineation and re-plan realized thanks to the CBCT imaging data registered with the simulation CT and this nevertheless a further PTV enlargement over the overlapped PTV delineated on the simulation and cone beam ct scans. In conclusion the CBCT can be considered not only a way to relocate the CTV just during the therapy of the patient but also a useful tool to further optimize the planning procedures on individualized imaging data obtained directly during the treatment phase.
© 2009 European Association of Urology. Published by Elsevier Inc. All rights reserved.