PTV margin definition in hypofractionated IGRT of localized prostate cancer using cone beam CT and orthogonal image pairs with fiducial markers

Abstract

PurposeTo evaluate PTV margins for hypofractionated IGRT of prostate comparing kV/kV imaging or CBCT.Patients and methodsBetween 2009 and 2012, 20 patients with low- (LR), intermediate- (IR) and high-risk (HR) prostate cancer were treated with VMAT in supine position with fiducial markers (FM), endorectal balloon (ERB) and full bladder. CBCT’s and kV/kV imaging were performed before and additional CBCT’s after treatment assessing intra-fraction motion. CTVP for 5 patients with LR and CTVPSV for 5 patients with IR/HR prostate cancer were contoured independently by 3 radiation oncologists using MRI. The van Hark formula (PTV margin =2.5Σ +0.7σ) was applied to calculate PTV margins of prostate/seminal vesicles (P/PSV) using CBCT or FM.Results172 and 52 CBCTs before and after RT and 507 kV/kV images before RT were analysed. Differences between FM in CBCT or in planar kV image pairs were below 1 mm. Accounting for both random and systematic uncertainties anisotropic PTV margins were 5-8 mm for P (LR) and 6-11 mm for PSV (IR/HR). Random uncertainties like intra-fraction and inter-fraction (setup) uncertainties were of similar magnitude (0.9-1.4 mm). Largest uncertainty was introduced by CTV delineation (LR: 1-2 mm, IR/HR: 1.6-3.5 mm). Patient positioning using bone matching or ERB-matching resulted in larger PTV margins.ConclusionsFor IGRT CBCT or kV/kV-image pairs with FM are interchangeable in respect of accuracy. Especially for hypofractionated RT, PTV margins can be kept in the range of 5 mm or below if stringent daily IGRT, ideally including prostate tracking, is applied. MR-based CTV delineation optimization is recommended.

The impact of the three degrees‐of‐freedom fiducial marker‐based setup compared to soft tissue‐based setup in hypofractionated intensity‐modulated radiotherapy for prostate cancer

“…Regarding the percentage of shift agreements within 3 mm between each setup, the same tendency was observed between Barney's study (41.3% for AP, 49.3% for SI, and 87.4% for LR) and ours (97% for AP, 99% for SI, and 100% for LR). Oehler et al compared the setup accuracy between an FM‐based setup with orthogonal kV portal imaging and an ST‐based setup with CBCT for 20 prostate cancer patients with volumetric modulated arc therapy . They found that the shift differences were usually <2 mm in all axes.…”

“…For the correction of a daily patient setup, there are various IGRT techniques such as kilovoltage (kV) and megavoltage (MV) portal imaging with implanted fiducial markers (FMs), ultrasound, electronic portal imaging devices with FMs, and kV and MV cone‐beam computed tomography (CBCT) . In particular, the three degrees‐of‐freedom (3DOF) CBCT‐based setup has become widely used for prostate cancer, and these are the most frequently used systems in the United States .…”

“…Shi et al reported that a rotational shift <2° may not need to be adjusted in a 6DOF‐FM‐based setup used with the ExacTrac x‐ray system . Barney and Oehler et al investigated the accuracy of a 3DOF‐FM‐based setup using the orthogonal kilovoltage portal imaging . However, little has been reported on the efficacy of a 3DOF‐FM‐based setup without rotational shift verification using the ExacTrac x‐ray system.…”

Sensitivity of array detector measurements in determining shifts of MLC leaf positions

“…With the application of stereotactic body radiotherapy (SBRT) to prostate cancer, further reduction of inter‐ and intrafraction prostate motion becomes important 1, 2, 3, 4. Even with daily diagnostic image quality‐guided soft tissue alignment, our previous study found that 6/4 mm (4 mm posterior) planning margin is necessary to account for residual interfraction errors, such as prostate rotation and deformation 5.…”

Evaluating deviations in prostatectomy patients treated with IMRT

“…9 Geometric uncertainties that arise during the treatment include deviations due to systematic errors and randomized errors. 10 Systematic errors occur during the preparation of the treatment, from the data transference to the treatment administration. 11,12 On the other hand, randomized errors appear during setup and they are due to organ motion.…”