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      Planning target volume margins for prostate radiotherapy using daily electronic portal imaging and implanted fiducial markers

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          Abstract

          Background

          Fiducial markers and daily electronic portal imaging (EPI) can reduce the risk of geographic miss in prostate cancer radiotherapy. The purpose of this study was to estimate CTV to PTV margin requirements, without and with the use of this image guidance strategy.

          Methods

          46 patients underwent placement of 3 radio-opaque fiducial markers prior to prostate RT. Daily pre-treatment EPIs were taken, and isocenter placement errors were corrected if they were ≥ 3 mm along the left-right or superior-inferior axes, and/or ≥ 2 mm along the anterior-posterior axis. During-treatment EPIs were then obtained to estimate intra-fraction motion.

          Results

          Without image guidance, margins of 0.57 cm, 0.79 cm and 0.77 cm, along the left-right, superior-inferior and anterior-posterior axes respectively, are required to give 95% probability of complete CTV coverage each day. With the above image guidance strategy, these margins can be reduced to 0.36 cm, 0.37 cm and 0.37 cm respectively. Correction of all isocenter placement errors, regardless of size, would permit minimal additional reduction in margins.

          Conclusions

          Image guidance, using implanted fiducial markers and daily EPI, permits the use of narrower PTV margins without compromising coverage of the target, in the radiotherapy of prostate cancer.

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          Most cited references19

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          Long-term results of the M. D. Anderson randomized dose-escalation trial for prostate cancer.

          Deborah A Kuban, Susan Tucker, Lei Dong … (2008)
          To report the long-term results of a randomized radiotherapy dose escalation trial for prostate cancer. From 1993 to 1998, a total of 301 patients with stage T1b to T3 prostate cancer were accrued to a randomized external beam dose escalation trial using 70 Gy versus 78 Gy. The median follow-up is now 8.7 years. Kaplan-Meier analysis was used to compute rates of prostate-specific antigen (PSA) failure (nadir + 2), clinical failure, distant metastasis, disease-specific, and overall survival as well as complication rates at 8 years post-treatment. For all patients, freedom from biochemical or clinical failure (FFF) was superior for the 78-Gy arm, 78%, as compared with 59% for the 70-Gy arm (p = 0.004, and an even greater benefit was seen in patients with initial PSA >10 ng/ml (78% vs. 39%, p = 0.001). The clinical failure rate was significantly reduced in the 78-Gy arm as well (7% vs. 15%, p = 0.014). Twice as many patients either died of prostate cancer or are currently alive with cancer in the 70-Gy arm. Gastrointestinal toxicity of grade 2 or greater occurred twice as often in the high dose patients (26% vs. 13%), although genitourinary toxicity of grade 2 or greater was less (13% vs. 8%) and not statistically significantly different. Dose-volume histogram analysis showed that the complication rate could be significantly decreased by reducing the amount of treated rectum. Modest escalation in radiotherapy dose improved freedom from biochemical and clinical progression with the largest benefit in prostate cancer patients with PSA >10 ng/ml.
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            The probability of correct target dosage: dose-population histograms for deriving treatment margins in radiotherapy.

            M van Herk, P Remeijer, C. Rasch … (2000)
            To provide an analytical description of the effect of random and systematic geometrical deviations on the target dose in radiotherapy and to derive margin rules. The cumulative dose distribution delivered to the clinical target volume (CTV) is expressed analytically. Geometrical deviations are separated into treatment execution (random) and treatment preparation (systematic) variations. The analysis relates each possible preparation (systematic) error to the dose distribution over the CTV and allows computation of the probability distribution of, for instance, the minimum dose delivered to the CTV. The probability distributions of the cumulative dose over a population of patients are called dose-population histograms in short. Large execution (random) variations lead to CTV underdosage for a large number of patients, while the same level of preparation (systematic) errors leads to a much larger underdosage for some of the patients. A single point on the histogram gives a simple "margin recipe." For example, to ensure a minimum dose to the CTV of 95% for 90% of the patients, a margin between CTV and planning target volume (PTV) is required of 2.5 times the total standard deviation (SD) of preparation (systematic) errors (Sigma) plus 1.64 times the total SD of execution (random) errors (sigma') combined with the penumbra width, minus 1.64 times the SD describing the penumbra width (sigma(p)). For a sigma(p) of 3.2 mm, this recipe can be simplified to 2.5 Sigma + 0.7 sigma'. Because this margin excludes rotational errors and shape deviations, it must be considered as a lower limit for safe radiotherapy. Dose-population histograms provide insight into the effects of geometrical deviations on a population of patients. Using a dose-probability based approach, simple algorithms for choosing margins were derived.
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              Prostate gland motion assessed with cine-magnetic resonance imaging (cine-MRI).

              B Sharpe, Frank A. Vicini, Marcel van Herk … (2005)
              To quantify prostate motion during a radiation therapy treatment using cine-magnetic resonance imaging (cine-MRI) for time frames comparable to that expected in an image-guided radiation therapy treatment session (20-30 min). Six patients undergoing radiation therapy for prostate cancer were imaged on 3 days, over the course of therapy (Weeks 1, 3, and 5). Four hundred images were acquired during the 1-h MRI session in 3 sagittal planes through the prostate at 6-s intervals. Eleven anatomic points of interest (POIs) have been used to characterize prostate/bony pelvis/abdominal wall displacement. Motion traces and standard deviation for each of the 11 POIs have been determined. The probability of displacement over time has also been calculated. Patients were divided into 2 groups according to rectal filling status: full vs. empty rectum. The displacement of POIs (standard deviation) ranged from 0.98 to 1.72 mm for the full-rectum group and from 0.68 to 1.04 mm for the empty-rectum group. The low standard deviations in position (2 mm or less) would suggest that these excursions have a low frequency of occurrence. The most sensitive prostate POI to rectal wall motion was the mid-posterior with a standard deviation of 1.72 mm in the full-rectum group vs. 0.79 mm in the empty-rectum group (p = 0.0001). This POI has a 10% probability of moving more than 3 mm in a time frame of approximately 1 min if the rectum is full vs. approximately 20 min if the rectum is empty. Motion of the prostate and seminal vesicles during a time frame similar to a standard treatment session is reduced compared to that reported in interfraction studies. The most significant predictor for intrafraction prostate motion is the status of rectal filling. A prostate displacement of <3 mm (90%) can be expected for the 20 min after the moment of initial imaging for patients with an empty rectum. This is not the case for patients presenting with full rectum. The determination of appropriate intrafraction margins in radiation therapy to accommodate the time-dependent uncertainty in positional targeting is a topic of ongoing investigations for the on-line image guidance model.
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                Author and article information

                Journal
                Journal ID (nlm-ta): Radiat Oncol
                Title: Radiation Oncology (London, England)
                Publisher: BioMed Central
                ISSN (Electronic): 1748-717X
                Publication date Collection: 2010
                Publication date (Electronic): 10 June 2010
                Volume: 5
                Page: 52
                Affiliations
                [1 ]Department of Radiation Oncology, Tom Baker Cancer Center and University of Calgary; 1331 29 St NW, Calgary AB, T2N 4N2, Canada
                [2 ]Department of Medical Physics, Saskatoon Cancer Center; 20 Campus Drive, Saskatoon SK, S7N 4H4, Canada
                [3 ]Department of Radiation Oncology, Saskatoon Cancer Center; 20 Campus Drive, Saskatoon SK, S7N 4H4, Canada
                [4 ]Department of Radiation Oncology, Cross Cancer Institute; 11560 University Ave, Edmonton AB, T6G 1Z2, Canada
                [5 ]Department of Radiation Oncology, Allan Blair Cancer Center; 4101 Dewdney Avenue, Regina SK, S4T 7T1, Canada
                Article
                Publisher ID: 1748-717X-5-52
                DOI: 10.1186/1748-717X-5-52
                PMC ID: 2896366
                PubMed ID: 20537161
                SO-VID: 3ea5dcad-c62f-4886-adea-dc159f6aa283
                Copyright © Copyright ©2010 Skarsgard et al; licensee BioMed Central Ltd.
                License:

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                Date received : 3 March 2010
                Date accepted : 10 June 2010
                Categories
                Subject: Research

                ScienceOpen disciplines: Oncology & Radiotherapy
                Data availability:
                ScienceOpen disciplines: Oncology & Radiotherapy

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