Pelvic organ motion and dosimetric implications during horizontal patient rotation for prostate radiation therapy

PURPOSE

Gantry-free radiation therapy systems utilizing patient rotation would be simpler and more cost effective than the conventional gantry-based systems. Such a system could enable the expansion of radiation therapy to meet global demand and reduce capital costs. Recent advances in adaptive radiation therapy could potentially be applied to correct for gravitational deformation during horizontal patient rotation. This study aims to quantify the pelvic organ motion and the dosimetric implications of horizontal rotation for prostate intensity-modulated radiation therapy (IMRT) treatments.

METHODS

Eight human participants who previously received prostate radiation therapy were imaged in a clinical magnetic resonance imaging (MRI) scanner using a bespoke patient rotation system (PRS). The patients were imaged every 45 degrees during a full roll rotation (0-360 degrees). Whole pelvic bone, prostate, rectum, and bladder motion were compared to the supine position using dice similarity coefficient (DSC) and mean absolute surface distance (MASD). Prostate centroid motion was compared in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) direction prior to and following pelvic bone-guided rigid registration. Seven-field prostate IMRT treatment plans were generated for each patient rotation angles under three adaption scenarios: No plan adaption, rigid planning target volume (PTV)-guided alignment to the prostate, and plan re-optimization. Prostate, rectum, and bladder doses were compared for each adaption scenario.

RESULTS

Pelvic bone motion within the PRS of up to 53 mm relative to the supine position was observed for some participants. Internal organ motion was greatest at the 180-degree PRS couch angle (prone), with prostate centroid motion range < 2 mm LR, 0 mm to 14 mm SI, and -11 mm to 4 mm AP. Rotation with no adaption of the treatment plan resulted in an underdose to the PTV -- in some instances up to 75% (D95%: 78 ± 0.3 Gy at supine to 20 ± 15.0 Gy at the 225-degree PRS couch angle). Bladder dose was reduced during the rotation by up to 98% (V60 Gy: 15.0 ± 9.4% supine to 0.3 ± 0.5% at the 225-degree PRS couch angle). In some instances, the rectum dose increased during rotation (V60Gy: 20.0 ± 4.5% supine to 25.0 ± 15.0% at the 135-degree PRS couch angle). Rigid PTV-guided alignment resulted in PTV coverage which, though statistically lower (P < 0.05 for all D95% values), was within 1 Gy of the supine plans. Plan re-optimization resulted in a statistically equivalent PTV coverage compared to the supine plans (P > 0.05 for all D95% metrics and all within ±0.4 Gy). For both rigid PTV-guided alignment and plan re-optimization, rectum dose volume metrics were reduced compared to the supine position between the 90- and 225-degree PRS couch angles (P < 0.05). Bladder dose volume metrics were not impacted by rotation.

CONCLUSION

Pelvic bone and internal organ motion are present during patient rotation. Rigid PTV-guided alignment to the prostate will be a requirement if prostate IMRT is to be safely delivered using patient rotation. Plan re-optimization for each PRS couch angle to account for anatomical deformations further improves the PTV coverage.

Technical Note: Experimental results from a prototype high-field inline MRI-linac

PURPOSE

The pursuit of real-time image guided radiotherapy using optimal tissue contrast has seen the development of several hybrid magnetic resonance imaging (MRI)-treatment systems, high field and low field, and inline and perpendicular configurations. As part of a new MRI-linac program, an MRI scanner was integrated with a linear accelerator to enable investigations of a coupled inline MRI-linac system. This work describes results from a prototype experimental system to demonstrate the feasibility of a high field inline MR-linac.

METHODS

The magnet is a 1.5 T MRI system (Sonata, Siemens Healthcare) was located in a purpose built radiofrequency (RF) cage enabling shielding from and close proximity to a linear accelerator with inline (and future perpendicular) orientation. A portable linear accelerator (Linatron, Varian) was installed together with a multileaf collimator (Millennium, Varian) to provide dynamic field collimation and the whole assembly built onto a stainless-steel rail system. A series of MRI-linac experiments was performed to investigate (1) image quality with beam on measured using a macropodine (kangaroo) ex vivo phantom; (2) the noise as a function of beam state measured using a 6-channel surface coil array; and (3) electron contamination effects measured using Gafchromic film and an electronic portal imaging device (EPID).

RESULTS

(1) Image quality was unaffected by the radiation beam with the macropodine phantom image with the beam on being almost identical to the image with the beam off. (2) Noise measured with a surface RF coil produced a 25% elevation of background intensity when the radiation beam was on. (3) Film and EPID measurements demonstrated electron focusing occurring along the centerline of the magnet axis.

CONCLUSIONS

A proof-of-concept high-field MRI-linac has been built and experimentally characterized. This system has allowed us to establish the efficacy of a high field inline MRI-linac and study a number of the technical challenges and solutions.

Quantitative evaluation of diffusion-weighted imaging techniques for the purposes of radiotherapy planning in the prostate

OBJECTIVE

Diffusion-weighted imaging (DWI) is an important technique for the localization of prostate cancer, and its response assessment during treatment with radiotherapy (RT). However, it has known limitations in terms of distortions and artefacts using standard acquisition techniques. This study evaluates two alternative methods that offer the promise of improved image quality and the potential for more reliable and consistent diffusion data.

METHODS

Three DWI techniques were investigated; single-shot echoplanar imaging (EPI), EPI combined with reduced volume excitation (ZOOMit; Siemens Healthcare, Erlangen, Germany) and read-out segmentation with navigator-echo correction (RESOLVE; Siemens Healthcare). Daily measurements of apparent diffusion coefficient (ADC) value were made in a quality assurance phantom to assess the repeatability of each sequence. In order to evaluate the geometric integrity of these sequences, ten normal volunteers were scanned, and the prostate was contoured to compare its similarity with T2 weighted images.

RESULTS

Phantom ADC values were significantly higher using the standard EPI sequence than those of the other two sequences. Differences were also observed between sequences in terms of repeatability, with RESOLVE and EPI performing better than ZOOMit. Overall, the RESOLVE sequence provided the best agreement for the in vivo data with smaller differences in volume and higher contour similarity than T2 weighted imaging.

CONCLUSION

Important differences have been observed between each of the three techniques investigated with RESOLVE performing the best overall. We have adopted this sequence for routine RT simulation of prostate patients at Liverpool Cancer Therapy Centre.

ADVANCES IN KNOWLEDGE

This work will be of interest to the increasing number of centres wanting to incorporate quantitative DWI in a clinical setting.

Commissioning of a new wide-bore MRI scanner for radiotherapy planning of head and neck cancer

OBJECTIVE

A combination of CT and MRI is recommended for radiotherapy planning of head and neck cancers, and optimal spatial co-registration is achieved by imaging in the treatment position using the necessary immobilisation devices on both occasions, something which requires wide-bore scanners. Quality assurance experiments were carried out to commission a newly installed 1.5-T wide-bore MRI scanner and a dedicated, flexible six-channel phased array head and neck coil.

METHODS

Signal-to-noise ratio (SNR) and spatial signal uniformity were quantified using a homogeneous aqueous phantom, and geometric distortion was quantified using a phantom with water-filled fiducials in a grid pattern. Volunteer scans were also used to determine the in vivo image quality. Clinically relevant T1 weighted and T2 weighted fat-suppressed sequences were assessed in multiple scan planes (both sequences fast spin echo based). The performance of two online signal uniformity correction schemes, one utilising low-resolution reference scans and the other not utilising low-resolution reference scans, was compared.

RESULTS

Geometric distortions, for a ±35-kHz bandwidth, were <1 mm for locations within 10 cm of the isocentre rising to 1.8 mm at 18 cm away. SNR was above 50, and uniformity in the axial plane was 71% and 95% before and after uniformity correction, respectively.

CONCLUSION

The combined performance of the wide-bore scanner and the dedicated coil was adjudged adequate, although superior-inferior spatial coverage was slightly limited in the lower neck.

ADVANCES IN KNOWLEDGE

These results will be of interest to the increasing number of oncology centres that are seeking to incorporate MRI into planning practice using dedicated equipment.

The potential for an enhanced role for MRI in radiation-therapy treatment planning

The exquisite soft-tissue contrast of magnetic resonance imaging (MRI) has meant that the technique is having an increasing role in contouring the gross tumor volume (GTV) and organs at risk (OAR) in radiation therapy treatment planning systems (TPS). MRI-planning scans from diagnostic MRI scanners are currently incorporated into the planning process by being registered to CT data. The soft-tissue data from the MRI provides target outline guidance and the CT provides a solid geometric and electron density map for accurate dose calculation on the TPS computer. There is increasing interest in MRI machine placement in radiotherapy clinics as an adjunct to CT simulators. Most vendors now offer 70 cm bores with flat couch inserts and specialised RF coil designs. We would refer to these devices as MR-simulators. There is also research into the future application of MR-simulators independent of CT and as in-room image-guidance devices. It is within the background of this increased interest in the utility of MRI in radiotherapy treatment planning that this paper is couched. The paper outlines publications that deal with standard MRI sequences used in current clinical practice. It then discusses the potential for using processed functional diffusion maps (fDM) derived from diffusion weighted image sequences in tracking tumor activity and tumor recurrence. Next, this paper reviews publications that describe the use of MRI in patient-management applications that may, in turn, be relevant to radiotherapy treatment planning. The review briefly discusses the concepts behind functional techniques such as dynamic contrast enhanced (DCE), diffusion-weighted (DW) MRI sequences and magnetic resonance spectroscopic imaging (MRSI). Significant applications of MR are discussed in terms of the following treatment sites: brain, head and neck, breast, lung, prostate and cervix. While not yet routine, the use of apparent diffusion coefficient (ADC) map analysis indicates an exciting future application for functional MRI. Although DW-MRI has not yet been routinely used in boost adaptive techniques, it is being assessed in cohort studies for sub-volume boosting in prostate tumors.

A method to produce and validate a digitally reconstructed radiograph-based computer simulation for optimisation of chest radiographs acquired with a computed radiography imaging system

OBJECTIVES

The purpose of this study was to develop and validate a computer model to produce realistic simulated computed radiography (CR) chest images using CT data sets of real patients.

METHODS

Anatomical noise, which is the limiting factor in determining pathology in chest radiography, is realistically simulated by the CT data, and frequency-dependent noise has been added post-digitally reconstructed radiograph (DRR) generation to simulate exposure reduction. Realistic scatter and scatter fractions were measured in images of a chest phantom acquired on the CR system simulated by the computer model and added post-DRR calculation.

RESULTS

The model has been validated with a phantom and patients and shown to provide predictions of signal-to-noise ratios (SNRs), tissue-to-rib ratios (TRRs: a measure of soft tissue pixel value to that of rib) and pixel value histograms that lie within the range of values measured with patients and the phantom. The maximum difference in measured SNR to that calculated was 10%. TRR values differed by a maximum of 1.3%.

CONCLUSION

Experienced image evaluators have responded positively to the DRR images, are satisfied they contain adequate anatomical features and have deemed them clinically acceptable. Therefore, the computer model can be used by image evaluators to grade chest images presented at different tube potentials and doses in order to optimise image quality and patient dose for clinical CR chest radiographs without the need for repeat patient exposures.

A method to optimize the processing algorithm of a computed radiography system for chest radiography

A test methodology using an anthropomorphic-equivalent chest phantom is described for the optimization of the Agfa computed radiography "MUSICA" processing algorithm for chest radiography. The contrast-to-noise ratio (CNR) in the lung, heart and diaphragm regions of the phantom, and the "system modulation transfer function" (sMTF) in the lung region, were measured using test tools embedded in the phantom. Using these parameters the MUSICA processing algorithm was optimized with respect to low-contrast detectability and spatial resolution. Two optimum "MUSICA parameter sets" were derived respectively for maximizing the CNR and sMTF in each region of the phantom. Further work is required to find the relative importance of low-contrast detectability and spatial resolution in chest images, from which the definitive optimum MUSICA parameter set can then be derived. Prior to this further work, a compromised optimum MUSICA parameter set was applied to a range of clinical images. A group of experienced image evaluators scored these images alongside images produced from the same radiographs using the MUSICA parameter set in clinical use at the time. The compromised optimum MUSICA parameter set was shown to produce measurably better images.

Differentiation of benign and malignant sub-1 cm breast lesions using dynamic contrast enhanced MRI

The purpose of this work is to assess the additional benefit of MRI-based morphology and quantification of contrast enhancement in the differential diagnosis of sub-1cm breast lesions. Forty-three women with suspected breast cancer were examined using X-ray mammography, ultrasound mammography, and MRI. Dynamic contrast imaging was performed and relative enhancement at various time-points was calculated. The dynamic data was also processed using a two-compartment pharmacokinetic model. Radiological interpretation of high-resolution post-contrast images revealed a similar accuracy (69%) compared to X-ray mammography (69%) and ultrasound mammography (67%). The best individual parameter calculated from the dynamic images was found to be the exchange rate constant which revealed a diagnostic accuracy of 0.74 +/- 0.08. When information from the post-contrast images and dynamic data was combined in a logistic regression model a diagnostic accuracy of 0.92 +/- 0.03 was achieved. In conclusion, MR imaging of small breast lesions is feasible and the incorporation of quantitative MR derived parameters is beneficial.

Comparison of quantitative T2 mapping and diffusion-weighted imaging in the normal and pathologic prostate

In this study, diffusion-weighted images of the human prostate were successfully obtained, enabling quantification of apparent diffusion coefficients (ADCs) in normal and pathologic regions. A dual acquisition fast spin-echo sequence was used for accurate T2 calculation. T2 values were significantly higher in the peripheral zone than the central gland (P = 0.015). No significant correlations were found in either normal or pathologic tissue between ADC values and relaxation rates for all three gradient directions and the orientationally averaged water diffusion coefficient. Evidence suggesting that diffusional anisotropy is present in normal prostatic tissue is also detailed, with significant differences noted between the z-component and both the x- and y-components of the ADC for peripheral zone (P < 0.040) and central gland (P < 0.001).

Bilateral open breast coil and compatible intervention device

Dynamic contrast-enhanced breast MRI is an extremely sensitive method for breast lesion detection. For MR-only detected lesions it is essential that needle biopsy or localization prior to surgery is carried out under MR guidance. This work describes a bilateral open breast coil and prototype intervention device, which may be used in these situations. Results demonstrate that the open coil provides images superior to those obtained with a conventional closed breast coil. Initial phantom tests with the intervention device indicate a potential for this system to be used in the MR-guided localization of breast lesions. J. Magn. Reson. Imaging 2000;12:984-990.

Magnetic resonance imaging screening in women at genetic risk of breast cancer: imaging and analysis protocol for the UK multicentre study. UK MRI Breast Screening Study Advisory Group

The imaging and analysis protocol of the UK multicentre study of magnetic resonance imaging (MRI) as a method of screening for breast cancer in women at genetic risk is described. The study will compare the sensitivity and specificity of contrast-enhanced MRI with two-view x-ray mammography. Approximately 500 women below the age of 50 at high genetic risk of breast cancer will be recruited per year for three years, with annual MRI and x-ray mammography continuing for up to 5 years. A symptomatic cohort will be measured in the first year to ensure consistent reporting between centres. The MRI examination comprises a high-sensitivity three-dimensional contrast-enhanced assessment, followed by a high-specificity contrast-enhanced study in equivocal cases. Multiparametric analysis will encompass morphological assessment, the kinetics of contrast agent uptake and determination of quantitative pharmacokinetic parameters. Retrospective analysis will identify the most specific indicators of malignancy. Sensitivity and specificity, together with diagnostic performance, diagnostic impact and therapeutic impact will be assessed with reference to pathology, follow-up and changes in diagnostic certainty and therapeutic decisions. Mammography, lesion localisation, pathology and cytology will be performed in accordance with the UK NHS Breast Screening Programme quality assurance standards. Similar standards of quality assurance will be applied for MR measurements and evaluation.

Dynamic contrast-enhanced MRI in the differentiation of breast tumors: user-defined versus semi-automated region-of-interest analysis

Dynamic contrast-enhanced MR mammography is an increasingly used method of evaluating breast pathology. The purpose of this study was to compare two semi-automated methods of region of interest (ROI) analysis with a user-defined method, in the discrimination of breast tumors using dynamic contrast-enhanced MRI. Results are presented from the retrospective analysis of 81 malignant and 36 benign breast lesions. The study demonstrates the importance of a consistent ROI strategy and also shows that semi-automated approaches offer a standardized method, which may improve the discrimination of primary breast tumors. J. Magn. Reson. Imaging 1999; 10:945-949.

Dynamic contrast-enhanced MRI in the differentiation of breast tumors: user-defined versus semi-automated region-of-interest analysis

Dynamic contrast-enhanced MR mammography is an increasingly used method of evaluating breast pathology. The purpose of this study was to compare two semi-automated methods of region of interest (ROI) analysis with a user-defined method, in the discrimination of breast tumors using dynamic contrast-enhanced MRI. Results are presented from the retrospective analysis of 81 malignant and 36 benign breast lesions. The study demonstrates the importance of a consistent ROI strategy and also shows that semi-automated approaches offer a standardized method, which may improve the discrimination of primary breast tumors. J. Magn. Reson. Imaging 1999; 10:945-949.

A simple and realistic tissue-equivalent breast phantom for MRI

A simple and inexpensive breast phantom is described for use in magnetic resonance imaging. The work demonstrates the similarity of the phantom materials to in vivo breast tissue in terms of T(1) relaxation times. The phantom is also qualitatively compared with images acquired from a patient with a primary breast lesion.J. Magn. Reson. Imaging 10:968-971, 1999.

Differentiation of prostatic carcinoma and benign prostatic hyperplasia: correlation between dynamic Gd-DTPA-enhanced MR imaging and histopathology

One of the major factors limiting the staging accuracy of conventional magnetic resonance imaging (MRI) for prostatic carcinoma, is the similarity in signal intensity between tumor and coexisting benign prostatic hyperplasia (BPH). As neovascularity is an independent indicator of pathological state, dynamic contrast-enhanced MRI may yield additional information. This study correlates the histopathological findings from 12 radical prostatectomy patients on a region-by-region basis, with pharmacokinetic modeling of dynamic contrast-enhanced (0.2 mmol dimeglumine gadopentetate/kg), fast multiplanar spoilt gradient-recalled echo images, using a two-compartment simplex minimization technique. Quantitative analysis demonstrated differences in the amplitude of the initial contrast upslope and contrast exchange rate between tumor and fibromuscular BPH (P<0.03 and P<0.03, respectively) and for the contrast exchange rate between tumor and fibroglandular BPH (P<0.04), providing improved delineation of intraprostatic tumor extent compared with conventional imaging techniques.

In vivo magnetic resonance spectroscopy and dynamic contrast enhanced imaging of the prostate gland

The quantitation of in vivo 1H MR spectroscopy and dynamic contrast enhanced MR imaging is described for patients with histologically confirmed prostate adenocarcinoma and benign prostatic hypertrophy (BPH). Results are presented which suggest that combined use of these techniques may be helpful in improving the characterization of prostate pathologies and ultimately increase the staging accuracy of magnetic resonance.

A simple method for the correction of endorectal surface coil inhomogeneity in prostate imaging

A simple method is described that corrects for the intensity inhomogeneity experienced when using endorectal surface coils in prostate imaging. The method uses a series of proton-density-weighted images, which are acquired to demonstrate the sensitivity profile of the coil and used to correct T2-weighted images acquired in the same plane. The results demonstrate improved image uniformity and visualization of prostatic anatomy.

In vivo quantification of citrate concentration and water T2 relaxation time of the pathologic prostate gland using 1H MRS and MRI

We have previously reported a striking correlation between water T2 relaxation time and citrate concentration in the normal prostate (Liney G.P.; Lowry M.; Turnbull L.W.; Manton D.J.; Knowles A.J.; Blackband S.J.; Horsman A. Proton MR T2 maps correlate with the citrate concentration in the prostate. NMR Biomed. 9:59-64; 1996). In this study we present data from similar studies of the pathologic gland. The findings support the hypothesis that measurement of both citrate concentration and water T2 relaxation time in vivo may aid the differentiation of prostatic carcinoma from benign disease and normal tissue.

Quantification of citrate concentration in the prostate by proton magnetic resonance spectroscopy: zonal and age-related differences

A commercial phased-array multicoil was used to acquire water-suppressed localized proton spectra of the two major anatomical regions of the prostate. The signal-to-noise ratio and spectral resolution allowed identification of peaks from choline and creatine, as well as a major peak from citrate. Quantification of the citrate peak using experimentally determined relaxation parameters with tissue water as an internal concentration reference revealed a marked variability between different volunteers. Nevertheless, in each case, the citrate concentration was up to fourfold greater in the peripheral zone than in the central gland. Furthermore, the difference in citrate concentration between these two regions was positively correlated with the subjects age. The results indicate a consistent difference in cellular function between the major anatomical regions within the prostate and may have important consequences for the application of magnetic resonance spectroscopy to the diagnosis of prostatic pathology.

Comparison of conventional single echo and multi-echo sequences with a fast spin-echo sequence for quantitative T2 mapping: application to the prostate

The accuracy of water T2 maps generated from a fast spin-echo (FSE) sequence was compared with data obtained by conventional single and multi-echo spin-echo pulse sequences using a commercial gel phantom. Spatially localized stimulated echo acquisition mode (STEAM) proton spectroscopy was also used to confirm the reported water T2 values of the gels contained in the phantom. The FSE sequence was shown to be superior in accuracy to both the single and multi-echo spin echo sequences and comparable to STEAM, producing results that were within 10% of known values. The effectiveness of the FSE sequence was further demonstrated by generating T2 maps of the normal and diseased prostate in clinically acceptable imaging times, resulting in comparable T2 values to those obtained using STEAM. Accurate quantitative T2 maps can be produced with the FSE sequence.

Proton MR T2 maps correlate with the citrate concentration in the prostate

A significant correlation between water T2 relaxation time and citrate concentration in the normal prostate is demonstrated using spatially localized spectroscopy and water T2 maps. These data imply that MR images may be used to infer the concentration of citrate at high spatial resolution. Preliminary results in patients demonstrate that this relationship is maintained in benign prostatic hyperplasia (BPH) and prostatic carcinoma. The determination of citrate concentration and water T2 may aid the differentiation of prostate carcinoma from BPH and normal tissue.