The prediction of radiation-induced liver dysfunction using a local dose and regional venous perfusion model

Evolution of Response-Based Radiotherapy for Hepatocellular Cancer

ABSTRACT

Stereotactic body radiation therapy has emerged as a safe and effective treatment modality for properly selected hepatocellular cancer (HCC) patients with normal liver function. However, many HCC patients have reduced baseline liver function due to underlying cirrhosis or prior liver-directed therapies. Therefore, because of the increased risk of hepatotoxicity, the use of stereotactic body radiation therapy for patients with reduced liver function has been approached with caution. Individualized, response-based radiotherapy incorporates models, imaging tools, and biomarkers that determine the dose-response relationship of the liver before, during, and after treatment and has been useful in reducing the likelihood of liver damage without sacrificing tumor control. This review discusses the evolution of response-based radiotherapy for HCC and highlights areas for further investigation.

Role of Functional MRI in Liver SBRT: Current Use and Future Directions

Stereotactic body radiation therapy (SBRT) is an emerging treatment for liver cancers whereby large doses of radiation can be delivered precisely to target lesions in 3-5 fractions. The target dose is limited by the dose that can be safely delivered to the non-tumour liver, which depends on the baseline liver functional reserve. Current liver SBRT guidelines assume uniform liver function in the non-tumour liver. However, the assumption of uniform liver function is false in liver disease due to the presence of cirrhosis, damage due to previous chemo- or ablative therapies or irradiation, and fatty liver disease. Anatomical information from magnetic resonance imaging (MRI) is increasingly being used for SBRT planning. While its current use is limited to the identification of target location and size, functional MRI techniques also offer the ability to quantify and spatially map liver tissue microstructure and function. This review summarises and discusses the advantages offered by functional MRI methods for SBRT treatment planning and the potential for adaptive SBRT workflows.

Effect of stereotactic body radiotherapy on regional metabolic liver function investigated in patients by dynamic [18F]FDGal PET/CT

Purpose

Stereotactic body radiotherapy (SBRT) is increasingly used for treatment of liver tumors but the effect on metabolic liver function in surrounding tissue is largely unknown. Using 2-deoxy-2-[¹⁸F]fluoro-d-galactose ([¹⁸F]FDGal) positron emission tomography (PET)/computed tomography (CT), we aimed to determine a dose–response relationship between radiation dose and metabolic liver function as well as recovery.

Procedures.

One male subject with intrahepatic cholangiocarcinoma and five subjects (1 female, 4 male) with liver metastases from colorectal cancer (mCRC) underwent [¹⁸F]FDGal PET/CT before SBRT and after 1 and 3 months. The dose response was calculated using the data after 1 month and the relative recovery was evaluated after 3 months. All patients had normal liver function at time of inclusion.

Results

A linear dose–response relationship for the individual liver voxel dose was seen until approximately 30 Gy. By fitting a polynomial curve to data, a mean TD₅₀ of 18 Gy was determined with a 95% CI from 12 to 26 Gy. After 3 months, a substantial recovery was observed except in tissue receiving more than 25 Gy.

Conclusions

[¹⁸F]FDGal PET/CT makes it possible to determine a dose–response relationship between radiation dose and metabolic liver function, here with a TD₅₀ of 18 Gy (95% CI 12–26 Gy). Moreover, the method makes it possible to estimate metabolic recovery in liver tissue.

TNFR1 and the TNFα axis as a targetable mediator of liver injury from stereotactic body radiation therapy

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Elevated soluble TNFR1 levels are predictive of liver toxicity among patients receiving radiation.

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Soluble TNFR1 levels do not independently predict liver toxicity when included in models with ALBI and mean liver dose.

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Data suggest that liver inflammation mediates toxicity after liver irradiation and that the TNFα axis is associated with this inflammation.

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Future studies of should evaluate approaches that target pre-treatment inflammation to reduce the risk of toxicity.

Introduction

Radiation therapy for the management of intrahepatic malignancies can adversely affect liver function. Liver damage has been associated with increased levels of inflammatory cytokines, including tumor necrosis factor alpha (TNFα). We hypothesized that an inflammatory state, characterized by increased soluble TNFα receptor (sTNFR1), mediates sensitivity of the liver to radiation.

Materials/Methods

Plasma samples collected during 3 trials of liver radiation for liver malignancies were assayed for sTNFR1 level via enzyme-linked immunosorbent assay (ELISA). Univariate and multivariate logistic regression and longitudinal models were used to characterize associations between liver toxicity (defined as a ≥2-point increase in Child-Pugh [CP] score within 6 months of radiation treatment) and sTNFR1 levels, ALBI score, biocorrected mean liver dose (MLD), age, and baseline laboratory values.

Results

Samples from 78 patients given liver stereotactic body radiation therapy [SBRT] (92%) or hypofractionated radiation were examined. There was a significant association between liver toxicity and sTNFR1 levels, and higher values were associated with increased toxicity over a range of mean liver doses. When ALBI score and biocorrected dose were included in the model with sTNFR1, baseline ALBI score and change in ALBI (ΔALBI) were significantly associated with toxicity, but sTNFR1 was not. Baseline aminotransferase levels also predicted toxicity but not independently of ALBI score.

Conclusions

Elevated plasma sTNFR1 levels are associated with liver injury after liver radiation, suggesting that elevated inflammatory cytokine activity is a predictor of radiation-induced liver dysfunction. Future studies should determine whether administration of agents that decrease inflammation prior to treatment is warranted.

Serum Levels of Hepatocyte Growth Factor and CD40 Ligand Predict Radiation-Induced Liver Injury

BACKGROUND: Declining liver function is a concerning side effect associated with radiation therapy. Biomarkers of liver toxicity would be useful in personalizing therapy. METHODS: As part of two prospective clinical trials examining adaptive radiation therapy, we collected serum samples from patients receiving liver radiation. We performed a screen of 22 cytokines using a multiplex assay then used ELISA to quantify the cytokines of greatest interest. Subjects were split into screening and validation cohorts. Toxicity was defined as an increase in Child-Pugh score of 2 points or greater within 6 months. Logistic regression models were used to estimate the relationship between our toxicity endpoint and serum cytokine concentrations. RESULTS: Our initial screen (46 subjects, 11 events) identified hepatocyte growth factor (HGF), CD40L (CD154), and eotaxin (CCL11) as potentially predictive of toxicity. We then tested these markers in an expanded patient cohort (104 subjects, 18 events) with a batch correction due to varying age of the samples which confirmed that high HGF and low CD40L were associated with a subsequent decline in liver function following radiation therapy. Multivariate analysis factoring in baseline Child-Pugh score and mean liver radiation dose demonstrated that HGF and CD40L were potentially predictive of toxicity (HGF OR 4.3, P = .009; CD40L OR 0.5 P = .06). Additionally, higher than median baseline HGF levels (1.4 ng/ml) were significantly associated with decreased survival following liver radiation (27.1 vs 14.5 months, P = .03). CONCLUSIONS: Our study identifies high HGF and low CD40L as potential markers of liver toxicity following radiation therapy.

Abdominal DCE-MRI reconstruction with deformable motion correction for liver perfusion quantification

PURPOSE

Abdominal dynamic contrast-enhanced (DCE) MRI suffers from motion-induced artifacts that can blur images and distort contrast-agent uptake curves. For liver perfusion analysis, image reconstruction with rigid-body motion correction (RMC) can restore distorted portal-venous input functions (PVIF) to higher peak amplitudes. However, RMC cannot correct for liver deformation during breathing. We present a reconstruction algorithm with deformable motion correction (DMC) that enables correction of breathing-induced deformation in the whole abdomen.

METHODS

Raw data from a golden-angle stack-of-stars gradient-echo sequence were collected for 54 DCE-MRI examinations of 31 patients. For each examination, a respiratory motion signal was extracted from the data and used to reconstruct 21 breathing states from inhale to exhale. The states were aligned with deformable image registration to the end-exhale state. Resulting deformation fields were used to correct back-projection images before reconstruction with view sharing. Images with DMC were compared to uncorrected images and images with RMC.

RESULTS

DMC significantly increased the PVIF peak amplitude compared to uncorrected images (P << 0.01, mean increase: 8%) but not compared to RMC. The increased PVIF peak amplitude significantly decreased estimated portal-venous perfusion in the liver (P << 0.01, mean decrease: 8 ml/(100 ml·min)). DMC also removed artifacts in perfusion maps at the liver edge and reduced blurring of liver tumors for some patients.

CONCLUSIONS

DCE-MRI reconstruction with DMC can restore motion-distorted uptake curves in the abdomen and remove motion artifacts from reconstructed images and parameter maps but does not significantly improve perfusion quantification in the liver compared to RMC.

Rigid-body motion correction of the liver in image reconstruction for golden-angle stack-of-stars DCE MRI

PURPOSE

Respiratory motion can affect pharmacokinetic perfusion parameters quantified from liver dynamic contrast-enhanced MRI. Image registration can be used to align dynamic images after reconstruction. However, intra-image motion blur remains after alignment and can alter the shape of contrast-agent uptake curves. We introduce a method to correct for inter- and intra-image motion during image reconstruction.

METHODS

Sixteen liver dynamic contrast-enhanced MRI examinations of nine subjects were performed using a golden-angle stack-of-stars sequence. For each examination, an image time series with high temporal resolution but severe streak artifacts was reconstructed. Images were aligned using region-limited rigid image registration within a region of interest covering the liver. The transformations resulting from alignment were used to correct raw data for motion by modulating and rotating acquired lines in k-space. The corrected data were then reconstructed using view sharing.

RESULTS

Portal-venous input functions extracted from motion-corrected images had significantly greater peak signal enhancements (mean increase: 16%, t-test, P <  0.001) than those from images aligned using image registration after reconstruction. In addition, portal-venous perfusion maps estimated from motion-corrected images showed fewer artifacts close to the edge of the liver.

CONCLUSIONS

Motion-corrected image reconstruction restores uptake curves distorted by motion. Motion correction also reduces motion artifacts in estimated perfusion parameter maps. Magn Reson Med 79:1345-1353, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A mouse radiation-induced liver disease model for stereotactic body radiation therapy validated in patients with hepatocellular carcinoma

PURPOSE

Lower radiation tolerance of the whole liver hinders dose escalations of stereotactic body radiation therapy (SBRT) in hepatocellular carcinoma (HCC) treatment. This study was conducted to define the exact doses that result in radiation-induced liver disease (RILD) as well as to determine dose constraints for the critical organs at risk (OARs) in mice; these parameters are still undefined in HCC SBRT.

METHODS

This study consisted of two phases. In the primary phase, mice treated with helical tomotherapy-based SBRT were stratified according to escalating radiation doses to the livers. The pathological differences, signs [such as mouse performance status (MPS)], and serum aspartate aminotransferase (AST)/alanine aminotransferase (ALT)/albumin levels were observed. Radiation-induced disease severities of the OARs were scored using systematic evaluation standards. In the validation phase in humans, 13 patients with HCC who had undergone radiotherapy before hepatectomy were enrolled to validate RILD pathological changes in a mouse study.

RESULTS

The evaluation criteria of the mouse liver radiotherapy-related signs were as follows: MPS ≥ 2.0 ± 0.52, AST/ALT ≥ 589.2 ± 118.5/137.4 ± 15.3 U/L, serum albumin ≤ 16.8 ± 2.29 g/L. The preliminary dose constraints of the OARs were also obtained, such as those for the liver (average dose ≤ 26.36 ± 1.71 Gy) and gastrointestinal tract (maximum dose ≤ 22.63 Gy). Mouse RILD models were able to be developed when the livers were irradiated with average doses of ≥31.76 ± 1.94 Gy (single fraction). RILD pathological changes in mice have also been validated in HCC patients.

CONCLUSIONS

Mouse RILD models could be developed with SBRT based on the dose constraints for the OARs and evaluation criteria of mouse liver radiotherapy-related signs, and the authors' results favor the study of further approaches to treat HCC with SBRT.

An Overdetermined System of Transform Equations in Support of Robust DCE-MRI Registration With Outlier Rejection

Quantitative hepatic perfusion parameters derived by fitting dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) of liver to a pharmacokinetic model are prone to errors if the dynamic images are not corrected for respiratory motion by image registration. The contrast-induced intensity variations in pre- and postcontrast phases pose challenges for the accuracy of image registration. We propose an overdetermined system of transformation equations between the image volumes in the DCE-MRI series to achieve robust alignment. In this method, we register each volume to every other volume. From the transforms produced by all pairwise registrations, we constructed an overdetermined system of transform equations that was solved robustly by minimizing the L_1/2-norm of the residuals. This method was evaluated on a set of 100 liver DCE-MRI examinations from 35 patients by examining the area under spikes appearing in the voxel time–intensity curves. The robust alignment procedure significantly reduced the area under intensity spikes compared with unregistered volumes (P < .001) and volumes registered to a single reference phase (P < .001). Our registration procedure provides a larger number of reliable time–intensity curve samples. The additional reliable samples in the precontrast baseline are important for calculating the postcontrast signal enhancement and thereby for converting intensity to contrast concentration. On the intensity ramp, retained samples help to better describe the uptake dynamics, providing a better foundation for parameter estimation. The presented method also simplifies the analysis of data sets with many patients by eliminating the need for manual intervention during registration.

Local and Global Function Model of the Liver

PURPOSE

To develop a local and global function model in the liver based on regional and organ function measurements to support individualized adaptive radiation therapy (RT).

METHODS AND MATERIALS

A local and global model for liver function was developed to include both functional volume and the effect of functional variation of subunits. Adopting the assumption of parallel architecture in the liver, the global function was composed of a sum of local function probabilities of subunits, varying between 0 and 1. The model was fit to 59 datasets of liver regional and organ function measures from 23 patients obtained before, during, and 1 month after RT. The local function probabilities of subunits were modeled by a sigmoid function in relating to MRI-derived portal venous perfusion values. The global function was fitted to a logarithm of an indocyanine green retention rate at 15 minutes (an overall liver function measure). Cross-validation was performed by leave-m-out tests. The model was further evaluated by fitting to the data divided according to whether the patients had hepatocellular carcinoma (HCC) or not.

RESULTS

The liver function model showed that (1) a perfusion value of 68.6 mL/(100 g · min) yielded a local function probability of 0.5; (2) the probability reached 0.9 at a perfusion value of 98 mL/(100 g · min); and (3) at a probability of 0.03 [corresponding perfusion of 38 mL/(100 g · min)] or lower, the contribution to global function was lost. Cross-validations showed that the model parameters were stable. The model fitted to the data from the patients with HCC indicated that the same amount of portal venous perfusion was translated into less local function probability than in the patients with non-HCC tumors.

CONCLUSIONS

The developed liver function model could provide a means to better assess individual and regional dose-responses of hepatic functions, and provide guidance for individualized treatment planning of RT.

A nonhuman primate model of human radiation-induced venocclusive liver disease and hepatocyte injury

BACKGROUND

Human liver has an unusual sensitivity to radiation that limits its use in cancer therapy or in preconditioning for hepatocyte transplantation. Because the characteristic veno-occlusive lesions of radiation-induced liver disease do not occur in rodents, there has been no experimental model to investigate the limits of safe radiation therapy or explore the pathogenesis of hepatic veno-occlusive disease.

METHODS AND MATERIALS

We performed a dose-escalation study in a primate, the cynomolgus monkey, using hypofractionated stereotactic body radiotherapy in 13 animals.

RESULTS

At doses ≥40 Gy, animals developed a systemic syndrome resembling human radiation-induced liver disease, consisting of decreased albumin, elevated alkaline phosphatase, loss of appetite, ascites, and normal bilirubin. Higher radiation doses were lethal, causing severe disease that required euthanasia approximately 10 weeks after radiation. Even at lower doses in which radiation-induced liver disease was mild or nonexistent, latent and significant injury to hepatocytes was demonstrated by asialoglycoprotein-mediated functional imaging. These monkeys developed hepatic failure with encephalopathy when they received parenteral nutrition containing high concentrations of glucose. Histologically, livers showed central obstruction via an unusual intimal swelling that progressed to central fibrosis.

CONCLUSIONS

The cynomolgus monkey, as the first animal model of human veno-occlusive radiation-induced liver disease, provides a resource for characterizing the early changes and pathogenesis of venocclusion, for establishing nonlethal therapeutic dosages, and for examining experimental therapies to minimize radiation injury.

Hepatic blood perfusion estimated by dynamic contrast-enhanced computed tomography in pigs: limitations of the slope method

OBJECTIVE

The aim of this study was to determine whether dynamic contrast-enhanced computed tomography (DCE-CT) and the slope method can provide absolute measures of hepatic blood perfusion from the hepatic artery (HA) and portal vein (PV) at experimentally varied blood flow rates.

MATERIALS AND METHODS

Ten anesthetized 40-kg pigs underwent DCE-CT of the liver during periods of normocapnia (normal flow), hypocapnia (decreased flow), and hypercapnia (increased flow), which were induced by adjusting the ventilation. Reference blood flows in the HA and PV were measured continuously by surgically placed ultrasound transit-time flowmeters. For each capnic condition, the DCE-CT-estimated absolute hepatic blood perfusion from the HA and PV were calculated using the slope method and compared with flowmeter-based absolute measurements of hepatic perfusions and relative errors were analyzed.

RESULTS

The relative errors (mean ± SEM) of the DCE-CT based perfusion estimates were -21% ± 23% for HA and 81% ± 31% for PV during normocapnia, 9% ± 23% for HA and 92% ± 42% for PV during hypocapnia, and 64% ± 28% for HA and -2% ± 20% for PV during hypercapnia. The mean relative errors for HA were not significantly different from 0 during hypocapnia and normocapnia, and the DCE-CT slope method could detect relative changes in HA perfusion between scans. Infusion of contrast agent led to significantly increased hepatic blood perfusion, which biased the PV perfusion estimates.

CONCLUSIONS

Using the DCE-CT slope method, HA perfusion estimates were accurate at low and normal flow rates, whereas PV perfusion estimates were inaccurate and imprecise. At high flow rate, both HA perfusion estimates were significantly biased.

GPU-accelerated voxelwise hepatic perfusion quantification

Voxelwise quantification of hepatic perfusion parameters from dynamic contrast enhanced (DCE) imaging greatly contributes to assessment of liver function in response to radiation therapy. However, the efficiency of the estimation of hepatic perfusion parameters voxel-by-voxel in the whole liver using a dual-input single-compartment model requires substantial improvement for routine clinical applications. In this paper, we utilize the parallel computation power of a graphics processing unit (GPU) to accelerate the computation, while maintaining the same accuracy as the conventional method. Using compute unified device architecture-GPU, the hepatic perfusion computations over multiple voxels are run across the GPU blocks concurrently but independently. At each voxel, nonlinear least-squares fitting the time series of the liver DCE data to the compartmental model is distributed to multiple threads in a block, and the computations of different time points are performed simultaneously and synchronically. An efficient fast Fourier transform in a block is also developed for the convolution computation in the model. The GPU computations of the voxel-by-voxel hepatic perfusion images are compared with ones by the CPU using the simulated DCE data and the experimental DCE MR images from patients. The computation speed is improved by 30 times using a NVIDIA Tesla C2050 GPU compared to a 2.67 GHz Intel Xeon CPU processor. To obtain liver perfusion maps with 626 400 voxels in a patient's liver, it takes 0.9 min with the GPU-accelerated voxelwise computation, compared to 110 min with the CPU, while both methods result in perfusion parameters differences less than 10(-6). The method will be useful for generating liver perfusion images in clinical settings.

Prediction of liver function by using magnetic resonance-based portal venous perfusion imaging

PURPOSE

To evaluate whether liver function can be assessed globally and spatially by using volumetric dynamic contrast-enhanced magnetic resonance imaging MRI (DCE-MRI) to potentially aid in adaptive treatment planning.

METHODS AND MATERIALS

Seventeen patients with intrahepatic cancer undergoing focal radiation therapy (RT) were enrolled in institution review board-approved prospective studies to obtain DCE-MRI (to measure regional perfusion) and indocyanine green (ICG) clearance rates (to measure overall liver function) prior to, during, and at 1 and 2 months after treatment. The volumetric distribution of portal venous perfusion in the whole liver was estimated for each scan. We assessed the correlation between mean portal venous perfusion in the nontumor volume of the liver and overall liver function measured by ICG before, during, and after RT. The dose response for regional portal venous perfusion to RT was determined using a linear mixed effects model.

RESULTS

There was a significant correlation between the ICG clearance rate and mean portal venous perfusion in the functioning liver parenchyma, suggesting that portal venous perfusion could be used as a surrogate for function. Reduction in regional venous perfusion 1 month after RT was predicted by the locally accumulated biologically corrected dose at the end of RT (P<.0007). Regional portal venous perfusion measured during RT was a significant predictor for regional venous perfusion assessed 1 month after RT (P<.00001). Global hypovenous perfusion pre-RT was observed in 4 patients (3 patients with hepatocellular carcinoma and cirrhosis), 3 of whom had recovered from hypoperfusion, except in the highest dose regions, post-RT. In addition, 3 patients who had normal perfusion pre-RT had marked hypervenous perfusion or reperfusion in low-dose regions post-RT.

CONCLUSIONS

This study suggests that MR-based volumetric hepatic perfusion imaging may be a biomarker for spatial distribution of liver function, which could aid in individualizing therapy, particularly for patients at risk for liver injury after RT.

Correction of arterial input function in dynamic contrast-enhanced MRI of the liver

PURPOSE

To develop a postprocessing method to correct saturation of arterial input function (AIF) in T1-weighted dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) for quantification of hepatic perfusion.

MATERIALS AND METHODS

The saturated AIF is corrected by parameterizing the first pass of the AIF as a smooth function with a single peak and minimizing a least-squares error in fitting the liver DCE-MRI data to a dual-input single-compartment model. Sensitivities of the method to the degree of saturation in the AIF first-pass peak and the image contrast-to-noise ratio were assessed. The method was also evaluated by correlating portal venous perfusion with an independent overall liver function measurement.

RESULTS

The proposed method corrects the distorted AIF with a saturation ratio up to 0.45. The corrected AIF improved hepatic arterial perfusion by -23.4% and portal venous perfusion by 26.9% in a study of 12 patients with liver cancers. The correlation between the mean voxelwise portal venous perfusion and overall liver function measurement was improved by using the corrected AIFs (R(2) = 0.67) compared with the saturated AIFs (R(2) = 0.39).

CONCLUSION

The method is robust for correcting AIF distortion and has the potential to improve quantification of hepatic perfusion for assessment of liver tissue response to treatment in patients with hepatic cancers.

Validation of motion correction techniques for liver CT perfusion studies

OBJECTIVES

Motion in images potentially compromises the evaluation of temporally acquired CT perfusion (CTp) data; image registration should mitigate this, but first requires validation. Our objective was to compare the relative performance of manual, rigid and non-rigid registration techniques to correct anatomical misalignment in acquired liver CTp data sets.

METHODS

17 data sets in patients with liver tumours who had undergone a CTp protocol were evaluated. Each data set consisted of a cine acquisition during a breath-hold (Phase 1), followed by six further sets of cine scans (each containing 11 images) acquired during free breathing (Phase 2). Phase 2 images were registered to a reference image from Phase 1 cine using two semi-automated intensity-based registration techniques (rigid and non-rigid) and a manual technique (the only option available in the relevant vendor CTp software). The performance of each technique to align liver anatomy was assessed by four observers, independently and blindly, on two separate occasions, using a semi-quantitative visual validation study (employing a six-point score). The registration techniques were statistically compared using an ordinal probit regression model.

RESULTS

306 registrations (2448 observer scores) were evaluated. The three registration techniques were significantly different from each other (p=0.03). On pairwise comparison, the semi-automated techniques were significantly superior to the manual technique, with non-rigid significantly superior to rigid (p<0.0001), which in turn was significantly superior to manual registration (p=0.04).

CONCLUSION

Semi-automated registration techniques achieved superior alignment of liver anatomy compared with the manual technique. We hope this will translate into more reliable CTp analyses.

The promise of dynamic contrast-enhanced imaging in radiation therapy

Dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) and computed tomography (CT) scanning are emerging as valuable tools to quantitatively map the spatial distribution of vascular parameters, such as perfusion, vascular permeability, blood volume, and mean transit time in tumors and normal organs. DCE MRI/CT have shown prognostic and predictive value for response of certain cancers to chemotherapy and radiation therapy. DCE MRI/CT offer the promise of early assessment of tumor response to radiation therapy, opening a window for adaptively optimizing radiation therapy based upon functional alterations that occur earlier than morphologic changes. DCE MRI/CT has also shown the potential of mapping dose responses in normal organs and tissue for evaluation of individual sensitivity to radiation, providing additional opportunities to minimize risks of radiation injury. The evidence for potentially applying DCE MRI and CT for selection and delineation of radiation boost targets is growing. The clinical use of DCE MRI and CT scanning as a biomarker or even a surrogate endpoint for radiation therapy assessment of tumor and normal organs must consider technical validation issues, including standardization, reproducibility, accuracy and robustness, and clinical validation of the sensitivity and specificity for each specific problem of interest. Although holding great promise, to date, DCE MRI and CT scanning have not been qualified as a surrogate endpoint for radiation therapy assessment or for treatment modification in any prospective phase III clinical trial for any tumor site.

Semiautomated motion correction of tumors in lung CT-perfusion studies

RATIONALE AND OBJECTIVES

To compare the relative performance of one-dimensional (1D) manual, rigid-translational, and nonrigid registration techniques to correct misalignment of lung tumor anatomy acquired from computed tomography perfusion (CTp) datasets.

MATERIALS AND METHODS

Twenty-five datasets in patients with lung tumors who had undergone a CTp protocol were evaluated. Each dataset consisted of one reference CT image from an initial cine slab and six subsequent breathhold helical volumes (16-row multi-detector CT), acquired during intravenous contrast administration. Each helical volume was registered to the reference image using two semiautomated intensity-based registration methods (rigid-translational and nonrigid), and 1D manual registration (the only registration method available in the relevant application software). The performance of each technique to align tumor regions was assessed quantitatively (percent overlap and distance of center of mass), and by a visual validation study (using a 5-point scale). The registration methods were statistically compared using linear mixed and ordinal probit regression models.

RESULTS

Quantitatively, tumor alignment with the nonrigid method compared to rigid-translation was borderline significant, which in turn was significantly better than the 1D manual method: average (± SD) percent overlap, 91.8 ± 2.3%, 87.7 ± 5.5%, and 77.6 ± 5.9%, respectively; and average (± SD) DCOM, 0.41 ± 0.16 mm, 1.08 ± 1.13 mm, and 2.99 ± 2.93 mm, respectively (all P < .0001). Visual validation confirmed these findings.

CONCLUSION

Semiautomated registration methods achieved superior alignment of lung tumors compared to the 1D manual method. This will hopefully translate into more reliable CTp analyses.

Imaging of normal lung, liver and parotid gland function for radiotherapy

There is growing clinical evidence that functional imaging is useful for target volume definition and early assessment of tumour response to external beam radiotherapy. A subject that has perhaps received less attention, but is no less promising, is the application of functional imaging to the prediction or measurement of radiation adverse effects in normal tissues. In this manuscript, we review the current published literature describing the use of positron emission tomography (PET), four-dimensional computed tomography (4D-CT), single photon emission computed tomography (SPECT) and magnetic resonance imaging (MRI) to study normal tissue function in the context of radiotherapy to the lung, liver and head & neck. Published results to date demonstrate that functional imaging can be used to preferentially avoid normal tissues not easily identifiable on solely anatomical images. It is also a potentially very powerful tool for the early detection of radiotherapy-induced normal tissue adverse effects and could provide valuable data for building predictive models of outcome. However, one of the major challenges to building useful predictive models is that, to date, there are very little data available with combined images of normal function, 3D delivered radiation dose and clinical outcomes. Prospective data collection through well-constructed studies which use established morbidity scores is clearly a priority if significant progress is to be made in this area.

Imaging for assessment of radiation-induced normal tissue effects

Imaging can provide quantitative assessment of radiation-induced normal tissue effects. Identifying an early sign of normal tissue damage with imaging would have the potential to predict organ dysfunction, thereby allowing reoptimization of treatment strategies based on individual patients' risks and benefits. Early detection with noninvasive imaging may enable interventions to mitigate therapy-associated injury before its clinical manifestation. Furthermore, successive imaging may provide an objective assessment of the impact of such mitigation therapies. However, many problems make application of imaging to normal tissue assessment challenging, and further work is required to establish imaging biomarkers as surrogate endpoints of clinical outcome. The performance of clinical trials in which normal tissue injury is a clearly defined endpoint would greatly aid in realization of these goals.

Liver function after irradiation based on computed tomographic portal vein perfusion imaging

PURPOSE

To determine whether individual and regional liver sensitivity to radiation could be assessed by measuring liver perfusion during a course of treatment using dynamic contrast-enhanced computed tomography scanning.

METHODS AND MATERIALS

Patients with intrahepatic cancer undergoing conformal radiotherapy underwent dynamic contrast-enhanced computed tomography (to measure perfusion distribution) and an indocyanine extraction study (to measure liver function) before, during, and 1 month after treatment. We hoped to determine whether the residual functioning liver (i.e., those regions showing portal vein perfusion) could be used to predict overall liver function after irradiation.

RESULTS

Radiation doses from 45 to 84 Gy resulted in undetectable regional portal vein perfusion 1 month after treatment. The volume of each liver with undetectable portal vein perfusion ranged from 0 to 39% and depended both on the patient's sensitivity and on dose distribution. There was a significant correlation between indocyanine green clearance and the mean of the estimated portal vein perfusion in the functional liver parenchyma (p < 0.001).

CONCLUSION

This study reveals substantial individual variability in the sensitivity of the liver to irradiation. In addition, these findings suggest that hepatic perfusion imaging may be a marker for liver function and has the potential to be a tool for individualizing therapy.