Cervical cancer is the second most common cancer among women in the world, and a leading cause of cancer mortality, affecting mainly the under deserved populations of sub-Saharan Africa, Central and Latin America, and South-Central Asia (Ferlay et al., 2010a,b). Clinical staging of cervical cancer is based on the International Federation of Gynaecology and Obstetrics (FIGO) system, which was revised in 2009 (Pecorelli et al., 2009). This staging system is based on physical examination and inspection with scarce radiographic evaluation, aiming to be easily introduced in non-developed nations with limited access to imaging studies. However, compared with surgical staging, clinical examinations alone can under-stage cervical cancer in 20–30% of stage IB and up to 64% of stage IIIB patients (Lagasse et al., 1980). Improvements in tumor staging by imaging modalities, such as computed tomography (CT), magnetic resonance imaging (MRI), and fluorine-18-labeled fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) can significantly improve treatment decisions and the accuracy of highly precise radiotherapy.

Locally advanced cervical cancer is treated with chemo-radiotherapy (CRT), which has shown to improve local control and survival. Nevertheless, increasingly more radio- and chemo-resistant tumors still recur. New research strategies have focused on the development of tumor biomarkers aiming to combine CRT with new molecular targets. In this setting FDG-PET/CT and other molecular tracers might help to identify more aggressive tumors.

The aim of this article is to review the evidence and illustrate the role of FDG-PET/CT in the pre-treatment evaluation, disease delineation, and treatment response, with a particular focus on new and emerging metabolic tracers that could eventually performed better as biomarkers of tumor response to therapy.

One third of patients with locally advanced cervical cancer will have disease recurrence, usually within 2 years of completing treatment. Predictors of disease recurrence include clinical stage, lymph node status at diagnosis, and tumor response after treatment.

After CRT as definitive treatment of locally advanced cervical cancer there is sufficient evidence to support the use of PET/CT for the assessment of treatment response. The presence of FDG activity (either persistent or new) can predict survival outcome. A study in which FDG-PET/CT was performed 3 months after completion of treatment showed that a metabolic response was predictive of long-term survival, with a 3-year survival rate of 78% in patients with a complete metabolic response, 33% in patients with a partial metabolic response, and 0% in those with progressive disease (Schwarz et al., 2007). Multivariate analysis in that study showed that post-treatment response and lymph node status at diagnosis were the only accurate predictors of progression-free survival.

Mayr et al. (2002) used MRI scans to evaluate tumor regression at 40–50 Gy of external beam RT combined with chemotherapy in 34 cervical cancer patients. Regression to less than 20% of residual tumor volume resulted in a cumulative incidence of local recurrence of 9.5 vs. 77% in patients with more than 20% residual volume (p < 0.001).

Standardized surveillance programs have proposed the use of routine physical examination and patient’s symptoms education to facilitate early disease detection. However, studies have reported better overall survival in patients with asymptomatic disease recurrence (Bodurka-Bevers et al., 2000). In that setting, the use of FDG-PET/CT in a selected group of patients could potentially lead to a salvage curative therapy of local or oligometastatic disease (Brooks et al., 2009). In a study performed by Mittra et al. (2009), 30 women with locally advanced tumors who had undergone FDG-PET/CT during the surveillance period were evaluated. FDG-PET/CT facilitated the detection of local and distant metastasis, with a sensitivity of 93–96% and a specificity of 93–95%. Seventy-one percent of the scans performed in symptomatic patients showed true-positive findings against 44% in asymptomatic patients. This could have significant implications for the use of salvage radiotherapy (Figures 3A–C). Stereotactic radiosurgery has been evaluated in several retrospective studies of metastatic gynecological malignancies and has demonstrated activity at various doses and schedules. Particularly in patients with small tumor burden at recurrence and good performance status, the use of stereotactic body radiation therapy (SBRT) to treat FDG-PET avid para-aortic disease has shown a 4-year local control rate of 67.4%, with low incidence of G3-4 complications (Choi et al., 2009; Kunos et al., 2012a,b). More prospective studies are needed to confirm the role of molecular imaging as a routine examination during the follow-up of these patients (Elit et al., 2010).

The rapid evolution of radiotherapy now makes it possible to deliver HDRs to tumors located near normal structures with explicitly sculpted dose sparing of the normal tissues. Anatomical images have historically been used; however, they lack sensitivity for defining tumor extent, and the capacity to evaluate the biology of the tumor and normal tissue. In this context, the use of anatomical images associated with biological images is essential. Biological images allow mapping of molecular distributions and their surrogates, and can be used to guide external beam radiotherapy. For example, Ma et al. (2011) has shown important tumor volume discrepancies between FDG-PET and MRI probably due to the important geometrical changes in the position of the cervix and corpus uteri as well as variations in bladder and rectal filling. Chan et al. (2008) studied the internal movement of the tumor, cervix, and uterus using weekly cine-MRIs and a point of interest analysis (POI). The fundus POI drifted 1.5 cm caudally during CRT, and the cervical canal 1 cm.

As previously stated, pathological uptake of FDG-PET may modify treatment strategy, either by extending the radiation volumes to the para-aortic area, or by modifying the dose to the affected lymph nodes (Figures 4A–C). Esthappan et al. (2004) proposed dose escalation to 59.4 Gy to the positive para-aortic lymph node and 50.4 Gy to the para-aortic region using CT/PET-guided IMRT. In a series of 208 patients with cervix cancer, lymph nodes were scored as either positive or negative for abnormal FDG uptake PET and lymph node status by CT was classified as <1 cm (negative) or >1 cm (positive) (Grigsby et al., 2004). All enlarged lymph nodes detected by CT were PET positive. No patient underwent lymph node dissection. The dose to pelvic lymph nodes was dependent on PET and CT findings: PET negative nodes, <1 cm, 66.8 Gy, and 0/76 failures; PET positive nodes, <1 cm, 66.8 Gy, and 3/89 failures; 1.1 to <2 cm, 66.9 Gy, and 0/21 failures; 2.1 to <3 cm, 69.4 Gy, and 2/15 failures; and 3.1 to <4 cm, 74.1 Gy, and 0/5 failures. The risk of isolated nodal failure was <2%. Nevertheless, most of the patients with para-aortic positive lymph nodes failed at distant sites. The use of higher doses of radiotherapy might only help to increase loco-regional control. For instance the GOG protocol 125 has studied the feasibility of administering chemotherapy and extended field radiotherapy, and showed that in patients with positive para-aortic lymph nodes the combined treatment achieves 33% of progression-free interval at 3 years, this supports the idea that the treatment of para-aortic nodes is important but that better systemic treatments are needed to avoid distant metastasis (Varia et al., 1998).

Not only can FDG-PET/CT drive tumor dose painting with IMRT, but it might also help to limit hematological toxicity. In locally advanced cervical cancer treated with CRT, both modalities are myelosuppressive (Green et al., 2001; Bachtiary et al., 2005; Vale et al., 2010). Identifying active bone marrow sub-regions with FDG-PET might facilitate bone marrow sparing and improve tolerance to chemotherapy (Mell et al., 2006). In a recent study reported by Rose et al. (2012), a strong correlation was observed between radiation dose-volume histogram on the active area of the bone marrow identified by FDG-PET, and the development of hematological toxicity. IMRT can reduce the dose to bone marrow sub-regions identified by FDG-PET/CT: the mean functional bone marrow V10 (volume of bone marrow receiving ≥10 Gy), and V20 (volume receiving ≥20 Gy) has been shown to be significantly less with total bone marrow sparing IMRT (Liang et al., 2012).

This has important implications in the development of new therapeutic strategies to treat cervical cancer. A recently published trial identified a survival advantage in patients with locally advanced cervical cancer treated with concurrent gemcitabine, cisplatin, and pelvic radiation with adjuvant gemcitabine and cisplatin compared with concurrent cisplatin and pelvic radiation alone. In this study, more than Grade-3 hematological toxicity occurred in 72% of the experimental arm and was a frequent cause of treatment discontinuation (Duenas-Gonzalez et al., 2011). Several research groups are now focusing on the implementation of phase III trials looking at the potential benefits of adjuvant chemotherapy (NCT01414608). Consequently, reducing radiation-induced bone marrow damage is essential.

The use of image-guided brachytherapy has become standard in our clinic as well as many other cancer centers. MRI-guided brachytherapy is the method most frequently used, allowing an accurate tumor delineation and dose optimization. Recommendations have been published to avoid inter-observer variability in the delineation of tumors and organs at risk as well as a reliable definition of target volumes with a common language among centers (Haie-Meder et al., 2005; Potter et al., 2006).

A few studies have assessed the role of FDG-PET-guided brachytherapy. Malyapa et al. (2002) Compared two-dimensional (2D) treatment planning orthogonal radiography-based brachytherapy with 3D treatment planning based on FDG-PET in 11 patients with cervical cancer. The patients underwent two PETs: a first one to visualize the tumor and a second one with the FDG placed inside the tandem and ovoid applicators to visualize the treatment source positions for 3D treatment planning. The authors concluded that this technique was feasible and accurate relative to 2D treatment planning. Lin et al. (2007) conducted a dosimetric study comparing intracavitary brachytherapy using a standard plan with a PET-defined tumor volume in 11 patients undergoing intracavitary treatments. The coverage of the target isodose surface for the first implant with and without optimization was 73 and 68%, respectively (p = 0.21). For the mid and final implant, the coverage was 83 and 70% (p = 0.02). The dose to point A was significantly higher with the optimized plans for both the first implant (p = 0.02) and the mid and last implants (p = 0.008). The dose to the 2 and 5 cm³ of bladder or rectum were not significantly different. The authors concluded that FDG-PET-based treatment planning improved tumor dose coverage without significantly increasing doses to the bladder and rectum. A recent publication by Nam et al. (2012) confirms these results; they evaluated the feasibility of FDG-PET/CT conformal brachytherapy in 12 patients with cervical cancer. Brachytherapy was performed at 41.4 Gy, and the prescribed dose to point A was 4 Gy. The median dose that encompassed 95% of the target volume (D95) of the CTV was 3.23 Gy for point A-2D-based plan vs. 3.99 Gy for the FDG-PET/CT optimized plan. They concluded that PET/CT conformal brachytherapy was feasible and target coverage was better than conventional point A plans.

The most extensively studied biological predictor of response to radiotherapy is hypoxia. Hypoxic cells are more resistant to killing by ionizing radiation and chemotherapy (Brown and Giaccia, 1998).

In general, cervical cancer hypoxia has been associated with more malignant phenotypes (Hockel et al., 1999), higher rates of metastatic disease (Lyng et al., 2000; Fyles et al., 2002, 2006), and higher recurrence rates regardless of whether treatment is RT or surgery (Hockel et al., 1996). Hypoxia coupled with abnormal angiogenesis will provoke impaired tumor perfusion and high interstitial fluid pressure (IFP) which has been further linked with worst outcome (Milosevic et al., 2004).

Several hypoxic tracers suitable for PET have received special attention. Fluoromisonidazole (18-FMISO) is the hypoxia tracer most extensively studied (Rasey et al., 1987, 1989). However, its major disadvantages refer to its slow clearance kinetics and its high lipophilicity. Another PET tracer under study is ¹⁸F-fluoroazomycin-arabinoside (¹⁸FAZA). The feasibility of ¹⁸FAZA was evaluated recently in patients with advanced cervical cancer in a study performed by Schuetz et al. (2010). Fifteen consecutive patients with locally advanced cervical cancer were treated with CRT. ¹⁸FAZA-PET scans were performed before, during and after external beam therapy and image-guided brachytherapy. Five patients had visually identifiable tumors on ¹⁸FAZA-PET scans performed prior to therapy, and four patients before brachytherapy. One of five PET positive patients had incomplete remission 3 months after RT, and one had regional recurrence. Four of ten PET negative patients developed distant metastases. The authors concluded that ¹⁸FAZA-PET imaging is feasible, however, its predictive and prognostic value in cervical cancer remains to be clarified.

One of the most promising agents currently under study is ⁶⁰Cu-labeled diacetyl-bis (N⁴-methylthiosemicarbazone) (⁶⁰Cu-ATSM). In a preliminary study by Dehdashti et al. (2008), 38 women with locally advanced cervical cancer were evaluated before the initiation of definitive CRT. ⁶⁰Cu-ATSM uptake was evaluated semi quantitatively as the tumor-to-muscle activity ratio (T/M). A log-rang test determined that the T/M cut-off uptake value of >3.5 was significantly associated with worst outcome. Higher uptake of ⁶⁰Cu-ATSM has been shown to correlate with other biomarkers of tumor hypoxia such as vascular endothelial growth factor receptor (VEGF), epidermal growth factor receptor (EGFR), cycloxygenase-2, and carbonic anhydrase-IV (Grigsby et al., 2007).

Most clinical copper-ATSM studies have used the agent labeled with the short-lived positron-emitting radionuclide of copper, ⁶⁰Cu (half-life, 0.395 h; β1-decay, 92.5%; electron capture, 7.5%; Dehdashti et al., 2003). To enable copper-ATSM to be translated for use in PET centers that do not have an in-house cyclotron, copper-ATSM labeled with one of the longer-lived positron-emitting nuclides, ⁶⁴Cu (half-life, 12.7 h; β1-decay, 17.4%; β2-decay, 38.5%; electron capture, 43%) or ⁶¹Cu (half-life, 3.33 h; β1-decay, 62%; electron capture, 38%), is required. The longer half-lives of ⁶⁴Cu and ⁶¹Cu allow for production at a regional center and distribution to PET facilities in a fashion similar to that for ¹⁸F-labeled radiopharmaceuticals (Blower et al., 1996).

⁶⁴Cu-labeled diacetyl-di(N(4)-methylthiosemicarbazone) (⁶⁴Cu-ATSM) has also been studied in cervical cancer and comparisons with ⁶⁰Cu-ATSM showed better image quality due to reduced noise. Furthermore the pattern and magnitude of tumor uptake of ⁶⁰Cu-ATSM and ⁶⁴Cu-ATSM were similar (Lewis et al., 2008). A multicentre, prospective, phase II study is currently recruiting patients to define the role of pre-therapy ⁶⁴Cu-ATSM in predicting prognosis and determining the behavior of locally advanced cervical cancer (NCT00794339).

The development of new PET tracers targeting hypoxic response is essential because we are now in the era of rationally designed molecularly targeted therapies combined with CRT, which poses a significant challenge not only in evaluating mixed toxicity profiles but also in the evaluation of tumor response. New molecular targets may work by mechanisms unlikely to cause tumor regression, and there remains an important need to develop biomarkers to provide early evidence of drug activity not only in the tumor but also its vasculature.

Targeting the angiogenic pathway is an increasingly important therapeutic strategy for cervix cancer, and recent phase II studies have shown encouraging results (Townsley et al., 2011; Schefter et al., 2012). The choice of agents and combinations is dependent on understanding the biology of cancer and the availability of anticancer agents and their toxicities. Integrin α_vβ₃ is up-regulated in both tumor cells and angiogenic endothelial cells, making it an attractive therapeutic target. In recent studies in cervix cancer patients the expression of β₃ integrins, had a significant prognostic impact on outcome according to univariate and multivariate analyses (Gruber et al., 2005). In another study the expression of α_vβ₆ in cervix cancer correlated with different clinico-pathological parameters and with worse overall and disease-free survival. Over expression of α_vβ₆ in cervical squamous carcinomas is an unfavorable prognostic factor. This might reflect an increased capacity of α_vβ₆-expressing tumor cells to migrate in a fibronectin-rich extra cellular matrix (ECM) and/or to activate TGF-β₁ at the tumor/stroma interface, both of which processes may contribute to cervical cancer progression (Hazelbag et al., 2007).

Tumor-associated vessels express integrin α_vβ₃ (Brooks et al., 1994a,b). It is possible that increased expression of integrins α_vβ₃ and α_vβ₅ allow angiogenic endothelial cells to bind provisional matrix proteins such as vitronectin, fibrinogen, von willebrand factor, osteopontin and fibronectin that are deposited in the tumor microenvironment. These adhesive interactions could provide survival cues and/or traction for invading endothelial cells. Through genetic deletion, or treatment with integrin antagonists, several additional integrins have been identified as crucial for angiogenesis, including α₁β₁, α₂β₁, α₄β₁, α₅β₁, α₆β₁, α₉β₁, and α₆β₄ (Avraamides et al., 2008).

Cilengitide (EMD 121974, manufactured by Merck KGaA, Darmstadt, Germany) is an investigational cyclic arginine–glycine–aspartic acid (RGD) containing pentapeptide sequence that selectively inhibits the α_vβ_3/5 integrins (Dechantsreiter et al., 1999). Cilengitide is the first integrin inhibitor to reach phase III clinical trials in glioblastoma, another highly vascularized cancer (Reardon et al., 2008a,b, 2011; Maurer et al., 2009; Stupp et al., 2010). Cilengitide is now being tested in phase II studies in patients with lung, pancreas, head and neck, and prostate cancer in combination with chemotherapy, radiotherapy, and other molecular targeted agents (Beekman et al., 2006; Friess et al., 2006; Vermorken et al., 2011; Alva et al., 2012).

As a result, better vascular imaging techniques are being developed to monitor responsiveness to treatment. In particular, considerable effort has been expended on characterizing integrin antagonists for their ability to specifically deliver diagnostic agents to tumor cells and associated blood vessels. ⁶⁸Ga-NODAGA-RGD is one of them, composed of one pentacyclic motif (RGDyK) and the ⁶⁸Galium-chelating reagent NODAGA. CycloRGD-NODAGA peptide is labeled with ⁶⁸Ga eluted from a ⁶⁸Ge/⁶⁸Ga generator directly on site (GMP) so as to form the ⁶⁸Ga-NODAGA-RGD that will be administrated to the patient. Dosimetry of ⁶⁸Ga-NODAGA-RGD PET/CT has been extrapolated from mice (Buchegger et al., 2011), and this radiopharmaceutical agent is in clinical use in our institution in a Swissmedic-approved study (NCT01608516). Our group is now evaluating the possibility of embarking on a phase I–II study to evaluate toxicity and efficacy of cilengitide combined with CRT in locally advanced cervical cancer.

There is a high level of evidence that FDG-PET/CT plays an essential role in the primary evaluation of cervical carcinoma, particularly in evaluating lymph nodal status and distant metastases, contributing to precise tumor staging and changes in therapeutic attitudes.

In surgical staged patients the diagnostic performance of FDG-PET/CT has shown a sensitivity of >80%, a specificity of >90% for detecting lymph node metastasis.

Positron emission tomography – computed tomography has gained importance in determining prognosis, assessing treatment response and evaluation of disease recurrence. The use of FDG-PET/CT is important to accurately define radiotherapy volumes, spare active bone marrow from high doses of radiation, and deliver more precise brachytherapy. Despite improved survival with the use of CRT, loco-regional control still constitutes a major problem, and other treatments are necessary to improve effectiveness. Advances in the understanding of the tumor microenvironment such as hypoxia, and angiogenesis, open the window to implement new molecular targeted approaches. Advances in biological images like PET/CT have a tremendous impact on the evaluation of treatment response to new therapeutic strategies.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.