[Intraprostatic calcifications as natural fiducial markers in image-guided radiotherapy for prostate cancer]

Automatic matching using intraprostatic calcifications as a volume of interest in CBCT images during prostate radiotherapy: a comparative study

Aim:

The study aimed to assess the clinical feasibility of employing an automatic match during cone beam computed tomography (CBCT) imaging using prostatic calcifications within the 95% isodose set as the region of interest.

Materials and methods:

CBCT images were analysed on the 5th fraction in 34 patients evaluating the difference between standard manual soft tissue anatomy matching versus auto calcification matching. An assessment of the clinical feasibility of using prostatic calcifications during matching alongside considering the effect a more automated matching process has been conducted on interobserver variability.

Results:

The standard deviation values of the difference between the soft tissue match (baseline) versus automatic calcification matches fluctuated around 1 mm in all three axes for all of the matches carried out. The interobserver variability observed between the two radiographers was 0·055, 0·065 and 0·045 cm in the vertical, longitudinal and lateral axes, respectively.

Findings:

The clarity of the calcifications on the CBCT images might explain the low interobserver variability displayed by the two matching radiographers. A calcification provides a clear starting point for image matching before commencing a check of volumetric coverage, if the matching process begins in the same place, it can allow for a standardisation of matching technique between radiographers.

Observed high incidence of prostatic calculi with the potential to act as natural fiducials for prostate image guided radiotherapy

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Prostatic calculi are a frequent radiological finding and may aid prostate IGRT.

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Incidence of prostatic calculi in a population of radiotherapy patients is reported.

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Significant proportion of patients have calculi detectable on radiotherapy images.

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Prostatic calculi may reduce the need for surgically implanted markers.

Purpose

This study aims to quantify the incidence and distribution of prostatic calculi in a population of prostate radiotherapy patients and assess their potential role in prostate image guided radiotherapy (IGRT).

Methods & materials

A retrospective analysis of trans-rectal ultrasound (TRUS), computed tomography (CT) planning and treatment verification cone beam CT (CBCT) scans from radical prostate radiotherapy patients (external beam and brachytherapy) between 2012 and 2014 was undertaken by a single experienced observer. An internationally validated schema from the Prostate Imaging Reporting and Data system (PIRADS) was used to map the location of calculi. The association of calculi with patient and disease characteristics was explored. Data was analysed using SPSS (IBM version 22.0) using descriptive statistical methods and logistic binary regression analysis.

Results

389 scan sets from 254 patients were included in the analysis. The overall incidence of calculi was 85% (n = 218) of which 79% (n = 201) were intra-prostatic calculi. The mean number of intra-prostatic calculi was 2 (range 1–10) and the mean size of calculi was 3.7 mm (range 0.5–15 mm). Calculi were most frequently observed in the posterior of the mid-gland (PI-RADs 3p, 9p) and posterior of the apex (PI-RADs 5p, 11p). 99% (n = 135) of CT planning scans with a corresponding CBCT had calculi in the same PIRADs location and all calculi were visible at the last fraction. There was no statistically significant association of calculi and N stage, M stage or Gleason score.

Conclusions

A significant proportion of prostate radiotherapy patients have prostatic calculi detectable on pre radiotherapy imaging. Calculi observed on CT were also detectable on CBCT in 99% of cases and remain visible at the end of treatment. These findings add to the growing evidence base supporting the potential of calculi as an alternative to fiducial markers to aid prostate IGRT.

The potential failure risk of the cone-beam computed tomography-based planning target volume margin definition for prostate image-guided radiotherapy based on a prospective single-institutional hybrid analysis

Background

The purpose of this study was to evaluate the impact of markerless on-board kilovoltage (kV) cone-beam computed tomography (CBCT)-based positioning uncertainty on determination of the planning target volume (PTV) margin by comparison with kV on-board imaging (OBI) with gold fiducial markers (FMs), and to validate a methodology for the evaluation of PTV margins for markerless kV-CBCT in prostate image-guided radiotherapy (IGRT).

Methods

A total of 1177 pre- and 1177 post-treatment kV-OBI and 1177 pre- and 206 post-treatment kV-CBCT images were analyzed in 25 patients who received prostate IGRT with daily localization by implanted FMs. Intrafractional motion of the prostate was evaluated between each pre- and post-treatment image with these two different techniques. The differences in prostate deviations and intrafractional motions between matching by FM in kV-OBI (OBI-FM) and matching by soft tissues in kV-CBCT (CBCT-ST) were compared by Bland-Altman limits of agreement. Compensated PTV margins were determined and compensated by references.

Results

Mean differences between OBI-FM and CBCT-ST in the anterior to posterior (AP), superior to inferior (SI), and left to right (LR) directions were − 0.43 ± 1.45, − 0.09 ± 1.65, and − 0.12 ± 0.80 mm, respectively, with R² = 0.85, 0.88, and 0.83, respectively. Intrafractional motions obtained from CBCT-ST were 0.00 ± 1.46, 0.02 ± 1.49, and 0.15 ± 0.64 mm, respectively, which were smaller than the results from OBI-FM, with 0.43 ± 1.90, 0.12 ± 1.98, and 0.26 ± 0.80 mm, respectively, with R² = 0.42, 0.33, and 0.16, respectively. Bland-Altman analysis showed a significant proportional bias. PTV margins of 1.5 mm, 1.4 mm, and 0.9 mm for CBCT-ST were calculated from the values of CBCT-ST, which were also smaller than the values of 3.15 mm, 3.66 mm, and 1.60 mm from OBI-FM. The practical PTV margin for CBCT-ST was compensated with the values from OBI-FM as 4.1 mm, 4.8 mm, and 2.2 mm.

Conclusions

PTV margins calculated from CBCT-ST might be underestimated compared to the true PTV margins. To determine a reliable CBCT-ST-based PTV margin, at least the systemic error Σ and the random error σ for on-line matching errors need to be investigated by supportive preliminary FM evaluation at least once.

[Intraprostatic fiducials in stereotactic radiotherapy for prostate cancer]

PURPOSE

Image-guided radiotherapy for prostate cancer is widely used in radiotherapy departments. Intraprostatic gold fiducial markers are used to visualize prostate position and motion before and during treatment. The aim of this report is to describe our experience of implanting intraprostatic fiducial markers under local anesthesia before hypofractionated stereotactic radiotherapy for prostate cancer and to assess its tolerance and reproducibility.

PATIENTS AND METHODS

Over a 6 and a half year period, 226 patients with prostate cancer received a stereotactic radiotherapy using the CyberKnife^® system (Accuray) in our institution. Eighteen patients were treated for recurrence after prostatectomy; these patients were excluded from the study. Among the 208 remaining patients, 94 patients (45.2%) received stereotactic radiotherapy as a boost after external beam radiotherapy (three fractions of 6Gy); 36 patients (17.3%) were had a re-irradiation (six fractions of 6Gy) and 78 patients (37.5%) had a exclusive stereotactic radiotherapy (68 patients received five fractions of 7.25Gy and 11 patients five fractions of 6.25Gy). Four markers were implanted in all patients using transrectal ultrasound; the procedure was performed under local anesthesia, using transperineal access. The four fiducial markers were implanted in two strands with two fiducial each one, 1cm apart. In order to follow the recommendations of the image-guided radiotherapy system, the two strands of the two markers were located on the same plane in the middle of the prostate, at least 2cm apart from the midline. After insertion, correct positioning of fiducials markers was verified by X-ray. Dosimetry scanning was performed after the implantation procedure; prostate position tracking was possible before and during treatment through the kilovoltage incorporated system of the robotic accelerator. Clinical data, X-ray verification and dosimetry scanner have been retrospectively reviewed for all patients.

RESULTS

The tolerance to procedure was excellent; only four patients (1.8%) described pain related to implant. No urinary side effects were reported. Median time from fiducial implantation to dosimetry scanner was 16 days (4-113 days). Four fiducials were found within the prostate at dosimetry scanner in 181 patients and three in 27 remaining patients. All intraprostatic fiducials were used to track the prostate gland before and during treatment.

CONCLUSIONS

Intraprostatic fiducial markers implantation is a safe and reproducible procedure that allows us to have reliable prostate information before and during stereotactic radiotherapy.