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Introduction

Ovarian cancer is the leading cause of mortality from gynecological cancer globally (1). Taxanes-platinum combination have become the first-line chemotherapeutic regimen used against advanced epithelial ovarian cancer (EOC), which accounts for 85–90% of all ovarian malignancies (2). Taxanes, docetaxel (DTX) included, are widely recognized as a viable treatment method, and have been indicated to inhibit tumor angiogenesis and cell proliferation, induce tumor cell apoptosis and suppress tumor growth (3–5). Despite the encouraging survival benefits, not every patient with EOC responds to therapy in a satisfactory manner due to resistance or insensitivity (6,7). Detecting treatment responses early will benefit these non-responders. At present, cancer antigen 125 detection and the Response Evaluation Criterion in Solid Tumors (RECIST) are widely used to clinically assess treatment responses (8,9). However, size changes due to therapy tend to appear later compared with changes in the underlying tumor functions, including vascularization and vascular permeability (10,11). This emphasizes the need to develop reliable methods for predicting early responses to therapy in order to replace unsuccessful drugs with potentially more effective therapeutic methods.

As a non-invasive imaging method, dynamic contrast enhanced-magnetic resonance imaging (DCE-MRI) contributes to the evaluation of tumor vasculature function and has increasingly been applied in animal experiments and clinical trials to predict early tumor responses to therapy (12–16).

However, it has not yet been determined whether quantitative DCE-MRI is able to predict the early response to DTX in EOC or whether there are associations between DCE-MRI parameters and the tumor size or histopathological changes. Therefore, the present study aimed to investigate whether quantitative DCE-MRI parameters may be used to determine an early treatment response to DTX in induced rat EOC by assessing the association of these parameters with tumor size, vascular endothelial growth factor (VEGF) levels and microvessel density (MVD) of the tumor.

Materials and methods

Tumor model and treatment

All animal experimental procedures were ethically approved by the Institutional Review Board of Jinshan Hospital of Fudan University (Shanghai, China) and were performed according to the Guide for the Care and Use of Laboratory Animals of the National Science and Technology Committee of China. A total of 160 female Sprague-Dawley rats (8 weeks old, 150–200 g; Shanghai Laboratory Animal Research Center, Shanghai, China) underwent surgery to establish orthotopic rat EOC. They were fed sterile water and food and housed under controlled temperature (at 25±1°C) and relative humidity (40–60%) conditions, with a 12–12 h light-dark cycle. The surgical procedures and protocol for the induction of EOC were performed as previously described (17).

MRI scanning

After anesthetization with pentobarbital sodium (40 mg/kg, i.p.) via the caudal vein, rats underwent MRI scanning using a 3.0 T scanner (Verio; Siemens Healthineers, Erlangen, Germany) with a rat coil. The following sequences were obtained: Axial spin echo (SE) T1 weighted image (WI) = [repetition time (TR)/echo time (TE)] = 7.29/2.28 msec; axial, sagittal and coronal turbo SE T2WI with fat saturation = TR/TE = 2,500/93 msec; and turbo SE T2WI = TR/TE = 8,000/98 msec.

For DCE-MRI, pre-contrasted fast low angle shot-two dimensional T1WI with fat saturation (TR/TE = 7.92/2.28 msec) was performed at two different flip angles (3° and 15°) for T1 mapping. Subsequent to the acquisition of four baseline scans, a dose of 0.2 mmol/kg gadopentetate dimeglumine (Magnevist; Bayer AG, Leverkusen, Germany) was administered to the rats via the caudal vein at a rate of 0.3 ml/sec, followed by a bolus injection of 0.4 ml saline at the same rate. A total of 30 phases of images were sequentially acquired with intervals of 6 sec. The scan parameters for the DCE-MRI were as follows: Slice thickness, 1 mm; no gap; spatial in-plane resolution, 224×370; TR/TE = 5.27/2.14 msec; flip angle, 15°; and field of view, 80×62.5 mm. The total acquisition time was 4 min. DCE-MRI was performed in an axial plane covering the entire tumor volume.

DCE-MRI processing and analysis

Using tissue four-dimensional software (Siemens Healthineers) and two-compartment modeling (18), DCE-MRI analysis was performed by two radiologists, each with 10 years of experience in pelvic MRI, who were blinded to the original information By avoiding hemorrhage, necrosis and major vascular structures, regions of interest of 20–50 mm2 in size were manually drawn on the slice to determine the longest diameter of the ovarian tumor types. Quantitative parameters, including the volume transfer constant (Ktrans), rate constant (kep), extravascular extracellular space volume ratio (ve) and initial area under the curve (IAUC), were automatically generated. MRI morphological features, including the tumor size, shape, boundary and mass configuration, were also assessed on T2WI.

Histopathological and immunohistochemical (IHC) analysis

Subsequent to validation (Fig. 1B) following the completion of the MRI scans at every time point, one rat ovary was removed and fixed for hematoxylin and eosin staining to evaluate the histopathology and tumor necrosis. IHC staining of VEGF and cluster of differentiation 31 was performed in order to investigate the expression of VEGF and the MVD, as previously described (17). A total of three high power fields (magnification, ×200) were randomly selected and the tumor necrosis rate was semi-quantitatively analyzed using Image-Pro Plus 6.0 imaging software (Media Cybernetics, Inc., Rockville, MD, USA). Tumor necrosis rate = necrotic area/field area × 100%.

Figure 1. Flow chart of the study design showing grouping, treatment and imaging time points. (A) DCE-MRI was performed for all rats on day 0 after which they were randomly assigned to treatment and control groups. The rats in the treatment group received 12 mg/kg DTX on day 0. Post-treatment (day 0) and 3, 7, 14 and 21 days after DTX therapy, all rats underwent MRI scanning under anesthesia. Twenty-one days after DTX therapy, tumors with decreased or unchanged longest diameters were categorized into a sensitive group, with increased ones into an insensitive group. Youden's index was obtained by means of retrospective logistic regression analysis of the DCE-MRI parameters at different time points (Tumor size indicates the longest diameter; Youden's index indicates the cut-off value that was the predictive factor for assessing early therapy response at optimal time point in Δ% of parameters; ‘x’ indicates DCE-MRI; ‘■’ indicates the time of DCE-MRI). (B) The rats received the same DCE-MRI and DTX as those in A. According to the above cut-off value at optimal time point, the treatment rats were categorized into sensitive or insensitive groups. Several rats in the control group on day 0 and in every group at a different time point were sacrificed and the tumors were excised for histology (‘▲’ indicates DCE-MRI and histology). % Change in DCE parameter = (DCE parametersubsequent to treatment - DCE parameterday 0)/DCE parameterday 0 × 100%. % change in tumor size = (tumor sizesubsequent to treatment - tumor sizeday 0)/tumor sizeday 0 × 100%. % change in MVD/VEGF expression levels/necrosis rate = (MVD/VEGF expression levels/necrosis ratesubsequent to treatment - MVD/VEGF expression levels/necrosis rateday 0)/MVD/VEGF expression levels/necrosis rateday 0 × 100% (Note: The tumor size indicated the longest diameter of the tumor. MVDday 0, VEGFday 0 and necrosisday 0 were the mean expression levels of the corresponding biomarkers in the control group on day 0). DCE-MRI, dynamic contrast enhanced magnetic resonance imaging; DTX, docetaxel.

Experimental design

The study consisted of training and validation phases (Fig. 1). During the training phase (Fig. 1A), 24 rats were randomly assigned to treatment (n=16) and control (n=8) groups. The rats in the treatment group received 12 mg/kg DTX on day 0, as previously described (19). All rats underwent conventional MRI and DCE-MRI scanning on days 0, 3, 7, 14 and 21 post-treatment. On day 21 following DTX therapy, rats with EOCs were divided into a sensitive group (a tumor with decreased or unchanged tumor size) and an insensitive group (tumor with increased tumor size) according to RECIST guidelines and a previous study (20,21). DCE-MRI parameters and tumor sizes at different time points (days 3, 7, 14 and 21) between the three groups were compared. Youden's index, which indicated the cut-off value that was used as the predictive factor for determining an early response to therapy (the optimal time point for the percentage change in the DCE-MRI parameters), was obtained using logistic regression analysis and receiver operating characteristic (ROC) curve analysis. In the validation phase (Fig. 1B), the experiment was repeated, and a further 101 rats received the same DTX therapy and MRI scanning as those in the training phase. The treatment group was divided into sensitive and insensitive groups at the optimal time point according to the obtained Youden's index. A number of rats in every group at different time points and in the control group on day 0 were sacrificed by cervical dislocation for histopathological and IHC analyses. In the two phases, only rats validated to have EOC (confirmed by autopsy and histopathology) were included in the study. Non-epithelial ovarian tumor types were excluded from the study. The numbers of rats sacrificed with EOC in different groups and at different time points are listed in Table I.

Table I. Numbers of rats with epithelial ovarian cancer in the different groups and at different time points following treatment.

Table I.

Numbers of rats with epithelial ovarian cancer in the different groups and at different time points following treatment.

Group	Day 0	Day 3	Day 7	Day 14	Day 21
Treatment group	56
Sensitive group	35 (0)	35 (8)	27 (8)	19 (10)	9 (9)
Insensitive group	21 (0)	21 (4)	17 (6)	11 (5)	6 (6)
Control group	45 (10)	35 (8)	27 (9)	18 (10)	8 (8)

[i] The number of rats sacrificed for histopathology are indicated in parentheses.

Statistical analysis

Data were analyzed using SPSS 22.0 (IBM Corp., Armonk, NY, USA) and values are presented as the mean ± standard deviation. One-way analysis of variance was used for data analysis between multiple groups, and comparisons between every two groups were performed using Fisher's least significant difference test. Spearman's correlation analysis was used to analyze the correlation between the percentage changes in DCE-MRI parameters and the percentage changes in tumor size, VEGF, MVD and tumor necrosis as follows: A correlation coefficient between 0.75 and 1.00 was considered highly relevant, a coefficient between 0.50 and 0.74 was considered moderately relevant, a coefficient between 0.25 and 0.49 was considered weakly relevant and a coefficient ≤0.249 was not considered relevant as previously described (22). P<0.05 was considered to indicate a statistically significant difference.

Results

Tumor size changes in rat EOC subsequent to DTX therapy

As presented in Fig. 2, on days 0, 3 and 7, the difference in the mean tumor size of EOC among the three groups was not significant (P>0.05). From day 14 onwards following DTX administration, the tumor size of the sensitive group was significantly decreased compared with the insensitive and control groups (P<0.05). As presented in Table II, the percentage change in tumor size was not significantly different between the three groups on day 3 (P>0.05). On day 7, there remained no significant difference in the sensitive group compared with the insensitive group (P>0.05), but there was a significant decrease in the sensitive group compared with the control group (P=0.028). On days 14 and 21, significant differences were observed between the sensitive and insensitive or control groups (P<0.05). However, there was no significant difference between the insensitive and control groups (P>0.05).

Figure 2. Effect of DTX on tumor size in rat EOC. The tumor size values were obtained from conventional MRI. The tumor size values are presented as the mean ± standard deviation. From day 14 after DTX administration, the tumor size of the sensitive group was significantly decreased compared to its insensitive and control counterparts. Tumor size indicated the longest diameter. aP<0.05 vs. control; bP<0.05 vs. insensitive. DTX, docetaxel; EOC, epithelial ovarian cancer; MRI, magnetic resonance imaging.

Table II. The change rate in the tumor size at different time points (%).

Table II.

The change rate in the tumor size at different time points (%).

Group	Day 3	Day 7	Day 14	Day 21
Sensitive	−2.17±5.35	−4.82±5.14a	−10.90±3.05a,b	−30.46±3.05a,b
Insensitive	4.09±6.69	5.53±4.77	5.44±2.95	20.58±4.24
Control	8.53±4.33	14.77±5.63	22.41±8.26	37.49±9.64

{ label (or @symbol) needed for fn[@id='tfn2-or-41-06-3335'] } Tumor size = longest diameter. Data are represented as means ± standard deviation.

a P<0.05 vs. control

b P<0.05 vs. insensitive.

DCE-MRI parameter changes in rat EOC subsequent to DTX therapy

On days 0, 3 and 7, there were no significant differences in the DCE-MRI parameters (Ktrans, kep and IAUC) of the EOC compared between the three groups (P>0.05). On days 14 and 21, there were significant differences obtained in all pairwise comparisons (P<0.05), except for Ktrans, kep and IAUC in the insensitive group compared with the control group (Fig. 3). The percentage changes of the DCE-MRI parameters in the different groups at different time points are summarized in Table III. On days 3, 7, 14 and 21, there were significant differences in the pairwise comparisons between the percentage change of the DCE-MRI parameters (Ktrans, kep and IAUC), except for the comparison of the insensitive group with the control group. The ve and percentage change in the ve lacked significant differences between the three groups at all time points (P>0.05). Figs. 4 and 5 present representative MRI images of the sensitive and insensitive groups respectively.

Figure 3. Effect of DTX on DCE-MRI parameters in rat EOC. Quantitative parameters [(A) Ktrans, (B) kep, (C) IAUC and (D) ve] were achieved with Tissue 4D software and two-compartment (Tofts) modeling (Verio; Siemens Healthineers, Erlangen, Germany). From day 14 after DTX administration, DCE-MRI parameters, Ktrans (A), kep (B), IAUC (C) included, of the sensitive group were significantly decreased compared to its insensitive and control counterparts. Data are represented as means ± standard deviation. aP<0.05 vs. control; bP<0.05 vs. insensitive. DTX, docataxel; DCE-MRI, dynamic contrast enhanced magnetic resonance imaging. Ktrans, volume transfer constant; kep, rate constant; ve, extravascular extracellular space volume ratio; IAUC, initial area under the curve.

Figure 4. Conventional and DCE-MRI of EOC in sensitive rats at different time points. In the right adnexal area, a multilocular cystic-solid EOC (red arrow) progressively became smaller on T2WI (A). On DCE-MRI, ROI 1 and ROI 2 were located in ovarian EOC and muscle, respectively (B); DCE-MRI parameters (Ktrans, kep, ve and IAUC) of EOC in ROI 1 were obtained by quantitatively analyzing software on pseudo-color images (C). DCE-MRI, dynamic contrast enhanced magnetic resonance imaging; EOC, epithelial ovarian cancer; T2WI, T2-weighted imaging; ROI, region of interest; Ktrans, volume transfer constant; kep, rate constant; ve, extravascular extracellular space volume ratio; IAUC, initial area under the curve.

Figure 5. Conventional and DCE-MRI of EOCs in insensitive rat at different time points. In the right adnexal area, a multilocular cystic-solid EOC (red arrow) progressively became larger on T2WI (A). On DCE-MRI, ROI 1 and ROI 2 were located in ovarian EOC and muscle, respectively (B); DCE-MRI parameters (Ktrans, kep, ve and IAUC) of EOC in ROI 1 were obtained by quantitatively analyzing software on pseudo-color images (C). DCE-MRI, dynamic contrast enhanced magnetic resonance imaging; EOC, epithelial ovarian cancer; T2WI, T2-weighted imaging; ROI, region of interest; Ktrans, volume transfer constant; kep, rate constant; ve, extravascular extracellular space volume ratio; IAUC, initial area under the curve.

Table III. Change in dynamic contrast enhanced-magnetic resonance imaging parameters at different time points (%).

Table III.

Change in dynamic contrast enhanced-magnetic resonance imaging parameters at different time points (%).

Time	Group	% Change in volume transfer constant	% Change in rate constant	% Change in extravascular extracellular space volume ratio	% Change in initial area under the curve
Day 3	Sensitive	−20.55±2.40a,b	−22.44±5.65a,b	6.38±9.39	−17.83±4.28a,b
	Insensitive	−3.28±4.28	−0.51±5.50	−0.99±4.92	−1.56±2.55
	Control	6.75±1.70	9.06±7.42	−1.42±8.98	5.08±3.61
Day 7	Sensitive	−29.19±1.92a,b	−23.39±4.04a,b	−9.19±10.12	−29.11±1.93a,b
	Insensitive	−1.56±4.53	−5.57±3.57	−0.67±14.39	−2.93±5.64
	Control	9.87±1.55	9.51±17.05	−3.28±6.71	7.40±3.31
Day 14	Sensitive	−34.04±3.05a,b	−32.88±5.36a,b	3.69±8.03	−33.32±5.71a,b
	Insensitive	1.80±5.46	−2.73±5.81	6.69±8.19	−1.65±4.35
	Control	13.05±1.19	15.89±7.15	−5.85±10.35	12.47±4.68
Day 21	Sensitive	−33.80±4.44a,b	−32.85±7.27a,b	−8.52±7.14	−33.17±6.47a,b
	Insensitive	3.48±6.44	4.48±9.93	3.35±8.09	0.34±8.70
	Control	15.14±1.26	18.92±8.36	−5.29±8.63	11.32±4.62

a P<0.05 vs. control

b P<0.05 vs. insensitive.

ROC evaluation based on DCE-MRI parameters

On days 0, 3 and 7, the Ktrans, kep, ve and IAUC did not achieve a statistically significant difference in the sensitive group compared with the insensitive or control groups, for monitoring and evaluating the response to DTX therapy in EOC. On day 14, Ktrans and kep were able to be used to evaluate the efficacy of DTX in EOC, and the AUCs, sensitivities and specificities of Ktrans and kep were 0.917 and 0.889, 83.3 and 100% and 100 and 66.7%, respectively. On day 21, the AUCs, sensitivities and specificities of the two parameters were 0.917 and 0.917, 83.3 and 100% and 83.3 and 83.3%, respectively. However, as early as day 3, the percentage changes in Ktrans combined with kep were able to be used to evaluate the efficacy of DTX in EOC, and the AUC, sensitivity and specificity were 1, 100 and 100%, respectively. Youden's index revealed a 17.59% reduction in Ktrans and kep.

Effective DTX therapy decreases MVD and VEGF expression in rat EOC

On day 3, the treatment group was divided into the sensitive and insensitive groups, with a ~17.59% reduction in Ktrans and kep. The specimens collected from the rats with EOC are described in Table I. As presented in Figs. 6 and 7 and Table IV, on days 7, 14 and 21, the MVD and VEGF expression levels or the percentage change in the MVD and VEGF expression levels were significantly different between the three groups and pairwise comparisons (P<0.05), except for in the insensitive group compared with the control group (P>0.05).

Figure 6. Effect of DTX on MVD in rat EOC. Images obtained under a microscope (high power field magnification, ×200) on (a) day 0, (b) day 3, (c) day 7, (d) day 14, (e) day 21 post-treatment, presenting the expression of MVD (A), which were observed to have brown microvessels. The MVD decreased in the sensitive group, but increased in the insensitive or control group. On days 7, 14 and 21, MVD expression (B) was significantly different between the three groups and pairwise comparisons, except for insensitive vs. control. Data are represented as means ± standard deviation. aP<0.05 vs. control; bP<0.05 vs. insensitive. DTX, docataxel; MVD, microvessel density; EOC, epithelial ovarian cancer.

Figure 7. Effect of DTX on VEGF expression in rat EOC. Images obtained under a microscope (high power field magnification, ×200) on (a) day 0, (b) day 3, (c) day 7, (d) day 14, (e) day 21 post-treatment, presenting the immunohistochemical expression of VEGF (A), which was observed to have brownish yellow granules in the cytoplasm and intercellular spaces. The expression of VEGF (B) decreased in the sensitive group, but increased in the insensitive or control group. On days 7, 14 and 21, VEGF expression was significantly different between the three groups and pairwise comparisons, except for insensitive vs. control. Data are represented as means ± standard deviation. aP<0.05 vs. control; bP<0.05 vs. insensitive. DTX, docataxel; VEGF, vascular endothelial growth factor; EOC, epithelial ovarian cancer.

Table IV. Changes in MVD and VEGF expression levels at different time points (%).

Table IV.

Changes in MVD and VEGF expression levels at different time points (%).

Day	Group	% Change in the MVD	% Change in VEGF levels
Day 3	Sensitive group	−7.70±6.73	−16.62±15.82
	Insensitive group	−0.32±2.54	−2.03±9.97
	Control group	4.09±9.21	5.99±30.07
Day 7	Sensitive group	−25.82±4.03a,b	−36.59±11.00a,b
	Insensitive group	−1.03±7.66	−3.08±2.42
	Control group	9.27±9.89	13.76±13.79
Day 14	Sensitive group	−26.34±4.10a,b	−39.84±7.50a,b
	Insensitive group	−4.64±2.66	−1.76±8.33
	Control group	12.87±9.41	17.71±17.87
Day 21	Sensitive group	−30.84±5.05a,b	−40.20±9.88a,b
	Insensitive group	−1.27±3.35	−3.13±5.18
	Control group	17.02±8.64	17.93±15.32

a P<0.05 vs. control

b P<0.05 vs. insensitive. MVD, microvessel density; VEGF, vascular endothelial growth factor.

Tumor necrosis in rat EOC following DTX therapy

As presented in Fig. 8 and Table V, the tumor necrosis or the percentage change in necrosis rates lacked significant differences among the three groups on days 3 and 7 (P>0.05). Significant differences were observed between the sensitive and insensitive or control groups (P<0.05), but not between the insensitive and control groups (P>0.05) on days 14 and 21.

Figure 8. Effect of DTX on tumor necrosis in rat EOC. Images obtained under a microscope (high power field magnification, ×200) on (a) day 0, (b) day 3, (c) day 7, (d) day 14 and (e) day 21 post-treatment, presenting the histopathology of (A) EOC specimens, which were observed to have spotty or patchy necrosis scattering. Tumor necrosis increased in the sensitive group over time, but demonstrated no notable change in the insensitive or control group. (B) On days 14 and 21, tumor necrosis rates were significantly different between the three groups, except for the insensitive group vs. the control group. Data are represented as means ± standard deviation. aP<0.05 vs. control; bP<0.05 vs. insensitive. DTX, docataxel; EOC, epithelial ovarian cancer.

Table V. Change in tumor necrosis at different time points (%).

Table V.

Change in tumor necrosis at different time points (%).

Group	Day 3	Day 7	Day 14	Day 21
Sensitive	75.98±36.30	90.21±24.05	177.58±45.00a,b	274.26±20.62a,b
Insensitive	12.95±21.83	27.81±15.42	43.67±10.47	47.78±20.09
Control	16.46±24.71	22.60±16.04	29.81±16.18	40.80±19.99

a P<0.05 vs. control

b P<0.05 vs. insensitive.

Correlations between the percentage change in DCE-MRI parameters and the percentage change in tumor size, MVD, VEGF expression levels and tumor necrosis

As presented in Table VI, the percentage change in Ktrans had a moderately positive correlation with the percentage change in tumor size, MVD and VEGF expression levels (P<0.01) and a moderately negative correlation with the percentage change in tumor necrosis rates. The percentage change in kep was moderately positively correlated with the percentage change in tumor size and MVD; there was a weak positive correlation with the percentage change VEGF expression levels and a moderate negative correlation with the percentage change in tumor necrosis rates (P<0.01). No significant correlation was observed between the percentage change in ve and tumor size, MVD, VEGF expression levels or tumor necrosis rates (P>0.05). The IAUC was weakly positively correlated with the percentage change in tumor size, moderately positively correlated with the percentage change in MVD and VEGF expression levels and weakly negatively correlated with the percentage change in tumor necrosis rates (P<0.01).

Table VI. Correlation between the percentage change in dynamic contrast enhanced-magnetic resonance imaging parameters and the percentage change in tumor size, VEGF levels, MVD and necrosis.

Table VI.

Correlation between the percentage change in dynamic contrast enhanced-magnetic resonance imaging parameters and the percentage change in tumor size, VEGF levels, MVD and necrosis.

	% Change in volume transfer constant		% Change in rate constant		% Change in extravascular extracellular space volume ratio		% Change in initial area under the curve
Tumor characteristics	r-value	P-value	r-value	P-value	r-value	P-value	r-value	P-value
% Change in diameter	0.735	<0.001	0.589	<0.001	−0.060	0.499	0.410	<0.001
% Change in MVD	0.748	<0.001	0.450	0.001	0.126	0.375	0.638	<0.001
% Change in VEGF levels	0.728	<0.001	0.386	0.005	0.239	0.084	0.639	<0.001
% Change in necrosis	−0.554	<0.001	−0.524	<0.001	−0.115	0.417	−0.411	0.001

[i] MVD, microvessel density; VEGF, vascular endothelial growth factor.

Discussion

DTX is a chemotherapy drug that is commonly used to treat EOC, and one of its notable antitumor mechanisms is associated with its inhibitory effect on angiogenesis (3,23). Previous studies on breast, cervical, spine and other cancer types have revealed that quantitative DCE-MRI parameters may be able to detect a treatment response prior to a change in the tumor size (11–15,24). Cebulla et al (15) demonstrated that DCE-MRI may detect cellular and vascular responses to phosphoinositide 3-kinase/mammalian target of rapamycin inhibition in vivo in ovarian cancer xenografts. In the present study, quantitative DCE-MRI was applied to analyze the early treatment response to DTX in induced rat EOC.

The present study demonstrated that there were significantly higher percentage changes in Ktrans, kep and IAUC in the DTX sensitive group compared with its insensitive and control counterparts between day 3 and day 21. However, differences in these parameters themselves were not observed until day 14 or 21. Early during the treatment process (on day 3), the percentage change of Ktrans combined with kep had an AUC of 1 and a sensitivity and specificity of 100 and 100%, respectively (which may be used for detecting the response to DTX), using a cut-off value of a 17.59% reduction in Ktrans and kep. These results illustrated that the percentage change in the DCE-MRI parameters were more effective compared with the DCE-MRI parameters alone in detecting the tumor response to chemotherapeutic agents in individual animals. Therefore, they may be reliable biomarkers for monitoring the tumor response to chemotherapy and for determining individualized therapeutic methods.

For oncologists, a change in tumor size according to guidelines such as RECIST is the most commonly used method of assessing the tumor response to chemotherapy (25). In the present study, the percentage change in the tumor size observed using MRI was significantly higher in the DTX sensitive group compared with its insensitive counterpart from day 7 onwards, and its control counterpart from day 14 onwards. However, neither the tumor size nor its percentage change achieved a perfect result of an AUC of 1, a sensitivity of 100% and a specificity of 100% for the monitoring of the response of EOC to DTX until day 21. These results suggested that the percentage changes in Ktrans, kep and IAUC reflected more effectively and more quickly the efficacy of DTX treatment compared with the percentage change in the size of the tumor. And more importantly, quantitative DCE-MRI parameters (Ktrans, kep and IAUC) were superior to imaging tumor size for early detection of response to DTX chemotherapy in EOCs.

Ktrans and kep reflect the tissue perfusion, vascular permeability and tumor angiogenesis (26–28). Ve indirectly represents the appearance of tumor angiogenesis (29). IAUC is associated with the blood flow, volume and interstitial space of the tumor, and it is the comprehensive reflection of the changes in Ktrans, kep and ve (30). DCE-MRI is being increasingly used in research to detect the treatment response to targeted antiangiogenic agents by demonstrating the occurrence of vascular disruption and changes in the microcirculation (31). A study by Li et al (32) demonstrated that the DCE-MRI parameters Ktrans and kep allowed for the estimation of angiogenesis in breast cancer and predicted breast cancer prognosis. Tumor growth or development is accompanied by angiogenesis. Consequently, tumor growth results in higher Ktrans, kep and IAUC values (33). By contrast, effective DTX treatment blocks neovascularization, which in turn decreases the Ktrans, kep and IAUC values.

As early as the 1970s, Folkman (34) demonstrated that angiogenesis was essential for the survival and sustained growth of solid tumor types and proposed the theory of anti-angiogenic therapy for a tumor. MVD has been accepted as a standard indicator of angiogenesis that is tightly regulated by pro-angiogenic and anti-endothelial growth factors (35). VEGF is one of the most necessary pro-angiogenic growth factors, and it has appeared to be essential in the angiogenic process (36,37). Our previous study demonstrated that Ktrans, kep and IAUC were positively correlated with MVD and VEGF expression, which suggested that changes of Ktrans, kep and IAUC induced by antiangiogenic therapy may reflect changes of MVD and VEGF (17).

When the treatment group of the present study was divided into sensitive and insensitive groups according to a cut-off value of a 17.59% reduction in Ktrans and kep values on day 3, comprehensive histopathology analysis at each time point demonstrated that the MVD and VEGF expression levels in EOC were notably decreased in the sensitive group compared with the insensitive group in addition to the control group on day 7. The results of the present study were in accordance with the results of Ji et al (38) and Zhang et al (39,40). The present results indicated that DTX chemotherapy had inhibitory effects on angiogenesis by decreasing the MVD count and VEGF expression levels, and treatment-induced hemodynamics changes occurred prior to changes in the tumor morphology.

Chen et al (41) demonstrated that Ktrans and IAUC were moderately positively correlated with MVD, while the kep and ve values were not correlated with MVD subsequent to percutaneous ethanol injection in a rabbit VX2 liver tumor. According to a study by Yuan et al (42), the Ktrans value, which reflects the perfusion and permeability of tumor microvessels, was highest in VEGF189-overexpressing tumor types. The results of the present study additionally revealed that changes in the Ktrans, kep and IAUC values were positively correlated with changes in MVD and VEGF. Additionally, dynamic changes in these parameters may noninvasively reflect the expression of tumor biomarkers in vivo.

Histopathological analysis demonstrated more prominent necrosis in the sensitive group compared with the insensitive and control counterparts from day 14 onwards, which was concurrent with, but occurred later compared with, the results of the analysis of the quantitative DCE-MRI parameters. Furthermore, the percentage changes in the Ktrans, kep and IAUC values were positively correlated with that of the necrosis rates. These results demonstrated that the changes in DCE-MRI parameters may help to predict the tumor histopathological responses and monitor the effectiveness of DTX treatment.

However, the present study also had limitations. For example, 3.0 T MRI was used. In order to improve the image resolution, 7.0 T MRI will be applied in future studies. Neither DCE-MRI nor angiogenesis were analyzed regionally, and the vascular variant in the tumor and normal tissue junction was the greatest. Furthermore, the vascular variant may have affected the overall stability of tumor angiogenesis (43).

In conclusion, the results of the present study indicated that the quantitative DCE-MRI parameters were superior to imaging tumor size for the detection of tumor histopathological responses for the early detection of responses to DTX in EOC. Quantitative DCE-MRI parameters may contribute to adjusting the treatment regimen for non-responders sooner in the progression of the disease and improving the prognosis.

Acknowledgements

Not applicable.

Funding

The present study was supported by the Shanghai Municipal Health Bureau Commission of Health and Family Planning (grant no 20134y156).

Availability of data and materials

All data generated or analyzed during this study are included in this published article.

Authors' contributions

JWQ conceived the study concepts and study design, and defined the intellectual content. SJY and SQC performed the data analysis, acquired the data and performed the literature research. SJY edited the manuscript. TKQ was involved in the conception of the study. All authors read and approved the final manuscript and agree to be accountable for all aspects of the research in ensuring that the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Ethics approval and consent to participate

All animal experimental procedures were ethically approved by the Institutional Review Board of Jinshan Hospital of Fudan University (Shanghai, China) and were performed according to the Guide for the Care and Use of Laboratory Animals of the National Science and Technology Committee of China.

Patient consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

References