Introduction

11C-Choline positron emission tomography (PET)/CT is an emerging highly sensitive technique for restaging patients with suspected recurrent prostate cancer (PC) after primary treatment: in patients who had undergone radical treatment for clinically localized PC, an increasing serum prostate-specific antigen (PSA) level typically anticipates the development of evident metastasis by some years in almost 30–40% of cases [1]. In this setting 11C-choline PET/CT has gained an important role because it proved to be more sensitive than conventional imaging (CI) such as transrectal ultrasound (TRUS), endorectal MR, abdominal-pelvic contrast-enhanced CT/MR and bone scan [2–11]. Radiotherapy with curative intent is limited to patients with local relapse only, while in cases of systemic spread androgen deprivation therapy (ADT) is given: luteinizing hormone-releasing hormone (LH-RH) analogs or bicalutamide 150 mg/day. Intermittent ADT has been proposed as a potential alternative to continuous therapy, in order to delay the time to hormone-refractory disease, to minimize the side effects and to reduce the costs of prolonged therapy, respectively [12, 13].

An unresolved matter in the literature is the potential influence of ADT in patients who undergo 11C-choline PET/CT. Giovacchini et al. [14] evaluated six PC patients before and after ADT (bicalutamide 150 mg/day; median treatment of 4 months) and reported a significant influence of ADT on 11C-choline PET/CT uptake in patients with PC at presentation; a significant (p < .05) negative correlation was detected between maximum standardized uptake value (SUVmax) and ADT both at univariate (r 2 = 0.24) and multivariate (r 2 = 0.48) analyses. However, there are no recommendations in the literature about the question of whether ADT should be discontinued before 11C-choline PET/CT. The aim of our study was to investigate this issue.

Materials and methods

Patients

Between September 2003 and September 2010, 1,451 patients were submitted to 11C-choline PET/CT for PC staging before primary treatment (n = 163) or restaging because of biochemical PSA relapse after primary therapy (n = 1,288) in our centres. We identified 14 patients who fulfilled the following criteria: (a) patient submitted to at least two 11C-choline PET/CT scans within 12 months in the setting of tumour restaging; (b) the first 11C-choline PET/CT before commencing ADT and the second 11C-choline PET/CT after 6 months of ADT administration to assess the effectiveness of therapy; (c) confirmation of 11C-choline PET/CT results by biopsy and/or subsequent clinical and imaging follow-up; and (d) availability of complete clinical and pathological data for each patient.

Radiopharmaceuticals

11C-Choline was synthesized according to the solid-phase method as described by Pascali et al. [15] using a commercial synthesis module (TRACERlab, GE Healthcare). 11CO2 produced by a PETtrace cyclotron (GE Healthcare) was converted into 11CH3I by the conventional LiAlH4/HI reaction. 11CH3I was used for the N-methylation of dimethylaminoethanol (60 μl) placed directly on a solid-phase support (C18 Sep-Pak Light, Waters). After a washing step with ethanol and water, 11C-choline retained on a cation exchange resin (Sep-Pak Accell Plus CM, Waters) was eluted with saline, sterilized by a 0.22-μm filter and collected in a final volume of 8 ml.

Radiochemical purity was evaluated by means of a high-performance liquid chromatography radio-detector equipped with a reversed-phase column, and the concentration of organic solvents was measured by gas chromatography. Endotoxin content was measured by the conventional lysosomal acid lipase method (Cambrex Bioscience).

Imaging protocol

All 11C-choline PET/CT scans were accomplished using a dedicated hybrid PET/CT (Discovery STE or Discovery LS, GE Medical Systems, Waukesha, WI, USA). CT attenuation correction acquisition parameters were: 140 kV, 90 mA, 0.8 s tube rotation and 5 mm thickness. The patients fasted at least 6 h before the PET acquisition and received an intravenous injection of 370–555 MBq of 11C-choline. Starting 5 min after injection according to the 11C-choline kinetics results reported by previous papers [16, 17], emission data were acquired at 5–6 bed positions from the base of the skull through the mid-thigh for 5 min at each position; in order to minimize the possible presence of tracer in the urinary pathways, patients were asked to void immediately before being scanned, and the scan started from the pelvis. CT images were used for both attenuation correction of emission data and image fusion.

Image analysis

All 11C-choline PET/CT images were analysed with dedicated software (eNTEGRA, GE Medical Systems, Waukesha, WI, USA) that allowed review of PET, CT and fused image data. PET images were first assessed visually using transaxial, sagittal and coronal views and interpreted by consensus by two experienced nuclear medicine physicians aware of clinical data. At visual inspection any focal uptake of 11C-choline higher than surrounding background was considered suspicious for malignancy. The SUVmax was measured and taken into account but no absolute cutoff value was used for the diagnosis.

PET/CT findings were considered as positive if they were confirmed by any one of the following: (a) presence of malignant PC cells at biopsy for local relapse; (b) confirmation of the same lesion by any imaging procedure performed within 6 months including bone scan, TRUS, abdominal-pelvic contrast-enhanced CT/MR; (c) any 11C-choline-positive lesion which disappeared in a follow-up PET/CT after prolonged ADT administration; and (d) increase of the number and extension of 11C-choline-positive lesions in the follow-up PET/CT.

Statistical analysis

Data were expressed as mean, median and standard deviation (SD). Serum PSA levels measured before and after ADT therapy were compared by the Wilcoxon signed rank test. A p value < .05 was considered statistically significant.

Results

Table 1 shows the characteristics of the patient population: all patients underwent the first 11C-choline PET/CT prior to starting ADT; the mean PSA was 17.0 ± 44.1 ng/ml (median 5.5, range 0.25–170). After at least 6 months of ADT, the PSA value significantly decreased compared to baseline (mean PSA = 2.4 ± 3.1 ng/ml, median 0.55, range 0.01–8.4) (p = .025). Of note, before starting ADT in 13 of 14 patients 11C-choline PET/CT showed metastatic spread. Instead, after at least 6 months of ADT, nine patients presented a negative 11C-choline PET/CT and PSA values significantly decreased (Table 1). On the other hand, three patients showed a rising PSA value during ADT and 11C-choline PET/CT demonstrated a progression of disease. One patient showed both a stable PSA value and PET/CT result. Finally, only one patient did not demonstrate a good correlation between PSA value and PET result (patient 9) in whom a decrease in PSA value and a progression of the disease was observed. However, in this case both PSA values, before and after ADT, were high (9.1 and 7.6 ng/ml, respectively; time interval between the two assays = year). These data indicate the presence of a relationship between PSA values and 11C-choline PET/CT results in ADT responders. An example is shown in Fig. 1.

Table 1 Characteristics of the patient population
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Fig. 1
figure 1

Patient 13 (see Table 1 indicating patient characteristics). a Maximum intensity projection (MIP) image (left) and fused PET/CT image (right) of 11C-choline PET/CT scan performed after discontinuation of ADT (PSA 12.1 ng/ml). Increased 11C-choline uptake in multiple lymph nodes is observed in the MIP image: a large and hot lymph node (SUVmax = 8) is evident in the left iliac chain. b MIP image (left) and fused PET/CT image (right) of 11C-choline PET/CT scan performed 6 months after ADT administration. A complete response is evident. PSA dropped down to 0.01 ng/ml

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Discussion

In the last few years PET/CT with 11C-choline has emerged as a useful method for the detection of PC [18–22]. However, the background of the increased uptake of choline in human PC is not completely understood. Two hypotheses have been suggested: the first one is based on an increased cell proliferation in tumours; the second one is based on an upregulation of choline kinase [23]. Nowadays there is conflicting evidence whether or not 11C-choline uptake is correlated with proliferation. A study by Breeuwsma et al. [24] published in 2005 about 18 PC patients who underwent radical prostatectomy (RP) showed that 11C-choline does not correlate with cell proliferation (Ki-67 labelling) in vivo. On the other hand, in the same year, another in vitro study by Al-Saeedi et al. [25] affirmed that choline incorporation into PC tumour cells is correlated with proliferation. In addition, the authors suggested the hypothesis that choline uptake could change before and after therapy and consequently it may be indicative of tumour response to therapy. Again, in another study by the same authors [26], it was found that the increased choline content and choline kinase activity in the growing cells is related to cell proliferation that may be involved in membrane synthesis and signalling.

Another unresolved matter in the literature is the potential influence of ADT such as LH-RH analogues or bicalutamide in patients who undergo 11C-choline PET/CT. In two recent studies [21, 22], the administration of ADT proved to be a statistically significant parameter in predicting a positive 11C-choline PET/CT scan at univariate analysis but it was not proven to be an independent predictive factor at multivariate analysis. The authors concluded that ADT did not seem to modify the results of 11C-choline PET/CT exam on a patient basis. However, it should be noted that in the two studies cited above, the number of metastases and their intensity of 11C-choline uptake were not specifically compared in the single patient both before and after ATD administration. Moreover, in a recent study by Giovacchini et al. [14], the authors reported a significant influence of ADT on 11C-choline PET/CT uptake in patients with PC at initial staging and they also discussed the possible mechanisms involved. ATD seems able to induce atrophy of glandular cells, both normal and malignant cells. A time-dependent loss of the prostatic metabolites was observed during ATD and it can be associated with downregulation of the expression of several genes, also genes involved in lipid metabolism. And, finally, ADT could influence the cell cycle inducing an arrest, but this possibility is still uncertain.

These preliminary results suggest that ADT significantly reduces 11C-choline uptake in androgen-sensitive PC patients. Based on these results, it may be suggested that the withdrawal of ADT before the execution of 11C-choline PET/CT could increase the detection rate and the intensity of 11C-choline uptake in metastatic lesions, therefore increasing the sensitivity of the 11C-choline PET/CT scan. Our study could also add a step in the controversial scenario about the correlation of choline uptake with cell proliferation suggesting a possible relationship between a significant reduction in proliferation as a consequence of therapy response detected by 11C-choline PET/CT

In the present study, for the first time, to the best of our knowledge, the effect of ADT on 11C-choline PET/CT uptake in distant metastases was evaluated in the same patient both before and after ADT administration. It clearly emerged that ADT is able to significantly modify the uptake of 11C-choline in the same lesions after ADT administration. It is worth noting that the major effect of ADT on 11C-choline PET/CT was recorded in patients with non-hormone-resistant PC, similarly to the effect of ADT on PSA. Finally, we want to underline that the main limitation of our study is the limited patient population: our preliminary data need to be confirmed in a larger series of patients.