Background
Prostate cancer (PC) is the most common tumor entity in males in many developed countries [
1]. Almost 65.000 patients were newly diagnosed with PC in Germany alone in 2011 and every year over 10.000 die from it [
2]. There are several treatment options for localized PC, among them radiation therapy (RT), which may be administered alone or in combination with hormonal therapy [
3]. Definitive RT for PC relies primarily on accurate clinical and radiological tumor staging. To differentiate between local, regional, or systemic disease, initial staging provides the basis for further treatment decisions and enables risk group-adapted treatment. On the one hand staging is of paramount importance to increase the curative chance of patients and on the other to spare them from unnecessary toxicities. Several imaging modalities are used for staging PC such as CT, MRI or bone scintigraphy. Nuclear medicine methods include radioactive marked tracers such as Choline-PET imaging, each limited by a low specificity. PSMA-PET-imaging has been shown to be more sensitive as well as specific for PC staging [
4‐
6].
Prostate-specific membrane antigen (PSMA) is a cell surface protein with high expression in PC cells [
7]. At lower levels it is expressed within various organs such as salivary gland tissues and kidneys, and less so in liver, spleen, bowel and healthy prostate structures [
8‐
10]. On the surface of PC cells it has been shown to be a thousand-fold increased compared to the other mentioned tissues [
7]. In 2012,
68Ga-PSMA was developed as a novel PSMA-ligand and very recent studies show promising results for its usefulness in recurrent prostate cancer or as a staging tool [
5,
9,
11]. Its introduction in diagnosis as well as treatment of PC is one of the most interesting developments in radiation oncology of urologic tumors.
However, the implementation of 68Ga-PSMA-PET-imaging in clinical routine is still only available in a few centers worldwide. Few data is available, especially on the value for staging and the impact on stage adaption during treatment planning for RT. The present work is to evaluate the integration of 68Ga-PSMA-PET-imaging into standard radiation planning of a primary definitive treatment of PC and to determine the influence on staging and on changes in the initially planned treatment concept for definitive RT in PC.
Methods
Between August 2013 and April 2015, 15 patients were planned for definitive RT of the prostate with treatment planning based on CT, MRI and 68Ga-PSMA-PET-imaging at our institution. All patients gave written informed consent for the purpose of anonymized evaluation and publication of their data. All reported investigations were conducted in accordance with the Helsinki Declaration and with national regulations. The retrospective analysis was approved by the Ethics Committee of the Technical University Munich (permit 5665/13).
68Ga-PSMA-PET-imaging was regularly CT based. In one case (patient #3) MRI was selected because of better comparability with preceding images. Pre-treatment
68Ga-PSMA-PET CT or MRI was performed between June 2013 and October 2014 in the staging process. Patient characteristics are shown in Table
1. The procedure of elaboration and application of the
68Ga-PSMA-ligand complex has been described previously [
12‐
14].
Table 1
Patients´ characteristics
Median Age (years, range)
| 74 (59–82) |
Gleason Score
n
(%)
| |
Low risk (≤6) | 5 (33.3) |
Intermediate risk (7) | 4 (26.7) |
High risk (>7) | 6 (40.0) |
Serum PSA (ng/ml)
n
(%)
| |
Low risk (≤10) | 7 (46.7) |
Intermediate risk (10–20) | 3 (20.0) |
High risk (>20) | 5 (33.3) |
Initial tumor stage (clinical examination and CT/MRI),
n
(%)
| |
Biopsy T1c | 7 (46.7) |
Intermediate risk (T2b) | 1 (6.7) |
High risk (≥T2c) | 7 (46.7) |
All cases were discussed in an interdisciplinary panel of experienced radiation oncologists, radiologists, nuclear medicine physicians and urologists and treatment decisions were taken on consensus. Based on histo-pathological Gleason scores, pre-treatment PSA levels and clinical staging patients were assigned to risk groups (low, intermediate and high risk) according to the 2014 National Comprehensive Cancer Network guidelines on prostate cancer [
15].
Retrospectively initial pre-
68Ga-PSMA-PET tumor stages were classified according to the 2010 version AJCC/UICC staging system - including Gleason score, initial PSA levels, TNM stage, as well as a calculation of the Roach formula (risk of lymph node involvement [%] = 2/3 (PSA) + (GS-6) × 10) [
16]. According to internal standard operation procedures of our department and following international guidelines, initial treatment decisions were made without the information obtained by
68Ga-PSMA-PET imaging based on CT/MRI as well as histo-pathological information available including PSA-level [
3,
17]. After PSMA-imaging, all information was reviewed and re-classification was performed with the additional information taken into account. We evaluated the number of cases in which the information obtained by
68Ga-PSMA-PET imaging led to a change in staging and subsequently resulted in a change of the RT concept, such as additional irradiation of the lymph node regions or local dose escalations.
For treatment planning, CT scans with 3 mm slice thickness at full bladder and empty rectum were performed. For all patients presenting tumor stages cT1-cT3a the CTV definition included the prostate and the base of the seminal vesicles. For patients in stadium cT3b the seminal vesicles were included completely in the CTV. To obtain the PTV of the prostate 7 mm were added to the CTV in all directions. Patients with actual lymph node involvement (cN+) or an increased risk of an involvement (risk more than 20 % according to the Roach formula) received an irradiation of the pelvic lymph nodes. We defined the corresponding PTV of the lymph nodes, including the obturatory, internal and external iliac, common iliac and presacral (down to S3) lymph nodes, with a 5 mm margin [
18]. If patients showed an increased uptake in
68Ga-PSMA-PET in defined lymph nodes, a simultaneously integrated boost was performed to the enhancement. The PTV of the suspected lymph node included the morphological correlate of the enhanced lymph node increased by at least 5 mm.
All patients received an intensity-modulated radiotherapy (IMRT): eight patients were treated with Tomotherapy® (Accuray, USA) and seven patients received an IMRT in RapidArc® technique by Varian, USA. The treatment was performed with full bladder and empty rectum under daily image guidance (IGRT).
Discussion
68Ga-PSMA-PET has shown high diagnostic accuracy for patients with PC. The data from the present manuscript show that 68Ga-PSMA-PET imaging had a huge impact on staging. Thus, compared to standard imaging based on pathological parameters, PSA values and imaging with CT and/or MRI only, a substantial impact on tumor staging and re-staging can be expected based on 68Ga-PSMA-PET imaging. Since in definitive RT for PC precise delineation of involved tissue as well as tissue at risk is necessary, and since there is a clear benefit of dose escalations to involved tissue, the diagnostic value of 68Ga-PSMA PET is evident.
Recently, the usefulness and accuracy of
68Ga-PSMA-PET imaging has been described increasingly in the recurrent setting of PC [
5,
11,
19,
20]. For example, Eiber et al. [
5] attested significantly higher detection rates of PC and lymph nodes as previously reported for other imaging modalities. This particularly applied in the range of low PSA-levels (<0.5 ng/mL). In 33 % of cases, the tumor site was exclusively detected by
68Ga-PSMA-PET examination. An additional 25 % of the patients showed lesions that were not detectable by CT. In total, over 50 % of crucial information was identified by
68Ga-PSMA-PET for final diagnosis, as well as staging and treatment decisions. The specificity of
68Ga-PSMA-PET has been demonstrated by the use of PSMA-radioguided surgery as well as for lymph node staging in primary PC [
21,
22]. With the radiotherapy concept changing in a third of all patients we assume that a better sensitivity and specificity may lead to improved radiotherapy concepts in patients having received a
68Ga-PSMA-PET.
All
68Ga-PSMA PET examinations were performed for treatment planning on the premises of
68Ga-PSMA-PET imaging as a “gold standard”. For precise diagnosis, correlation with pathological analyzes is necessary. This is currently being done for the diagnostic value of lymph node identification within a prospective trial at our institution. Though more data is still needed to verify sensitivity and specificity of the
68Ga-PSMA-PET, promising studies have been conducted. A current study by Maurer et al. [
23] showed for primary patients before radical prostatectomy and patients who underwent surgery for lymph node metastases a high histo-pathologically proven sensitivity (75 and 65.9 %) and specificity (98.8 and 98.9 %). CT or MRI imaging sensitivity (41.7 and 43.9 %) and specificity (85.5 and 85.4 %) was clearly lower. Another recent study by Giesel et al. confirms these findings [
24].
It should also be mentioned that PSMA-negative PC seems to be rare, but false negative cases have been reported in the literature [
8,
20,
25]. Whether this approach is correct will be verified by long-term PSA-levels. At our institution PSA-levels are monitored every 3 months after radiation therapy.
Nevertheless, in previous studies other imaging modalities such as CT and MRI showed similar performances in PC nodal staging by indirectly assessing nodal invasion measuring lymph node diameter. Consequently, their sensitivity was proven to be low. Using a 10 mm threshold, the sensitivity was reported to be less than 40 % [
26]. Functional PET imaging, mostly using radiolabeled-choline derivatives, are of limited value and often underestimate the extent of metastatic spread [
27]. Evangelista et al. [
28] evaluated, in a big meta-analysis, a high specificity for
11C-Choline-PET CT of 95 % in primary lymph node staging but a low sensitivity of just 50 %. Husarik et al. [
29] found a sensitivity to detect recurrent disease of 86 % and described these results as rather discouraging, especially in terms of its inability to detect small metastases - recurrent disease was reliably diagnosed in patients with PSA levels of >2 ng/ml. In another study Choline-PET-CT was found to be statistically significantly inferior when compared to
68Ga-PSMA-PET-CT in a recurrent setting as described by Afshar-Oromieh et al. [
24].
Interestingly, there have been studies that support the theory that detection rates increase parallel with elevated PSA levels [
30]. In a primary setting a tailored treatment approach is very important, and with mostly considerably elevated PSA levels,
68Ga-PSMA-PET imaging could prove to be overly effective. Here, the measurement of PSA level alone can at best calculate the risk of lymph node metastases by using the Roach formula [
16]. On a similar note, it has been noted that there is higher expression in lesions with higher Gleason scores [
5,
31]. The Gleason score as well as PSA levels were also taken into account in our study.
One main limitation of this analysis is its small patient number. However, since no data are currently available and 68Ga-PSMA-PET imaging is performed at several centers, the results provide a useful basis for decision-making in radiation oncology. Additionally, the homogenous results in the present “typical“ PC patient population underline the reliability of the reported data. It is important to note that in patients with higher T-stages, as well as higher PSA-values, the impact of 68Ga-PSMA-PET is larger on restating than in other tumor stages, meaning that in those patients a larger amount of up-stagings were observed than for lower tumor stages.
However, other factors such as anti-hormonal treatment might influence staging changes and therefore represent limitations to our study. In both of the down-staging cases anti-hormonal treatment was given either at the time or before
68Ga-PSMA imaging. These factors possibly had an effect on PSMA-image enhancement and subsequent down-staging. In contrast, Afshar-Oromieh et al. [
11] showed that patients taking anti-hormonal medication at the time of the
68Ga-PSMA-PET examination had more often positive PET results than patients not receiving hormonal therapy. Also, in some cases the time span between initial imaging modalities and
68Ga-PSMA-PET was up to a few months (maximum of 3 months - e.g. due to bridging anti-hormonal treatment). Therefore the possibility cannot be excluded that tumor stages in a few cases not only evolved because of the diagnostic tool, but also simply because of time.
In summary, we see great potential for 68Ga-PSMA-PET imaging. On one hand, in the case of down-staging, patients are potentially spared from unnecessary toxicities to surrounding tissues due to smaller radiation fields. For example, radiation doses could be drastically reduced by only treating lymphatic drainage if pelvic enhancements are obtained by 68Ga-PSMA-PET and not based on the calculated lymph node risk according to the Roach formula. In order to standardize this procedure at least PSA-long-term observations have to be conducted. On the other hand, in the case of up-staging, enlarged radiation volumes were its consequence and additional radiation dose (boost) to affected lymph nodes or within the prostate region could potentially translate into improved local control and/or overall survival for our patients. This will be evaluated within a prospective clinical trial at our institution.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SD made substantial contributions to conception and design, acquisition of data, analysis and interpretation of data, involved in drafting the manuscript, revised it critically for important intellectual content, gave final approval of the version to be publish; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. KCS made substantial contributions to conception and design, acquisition of data, analysis and interpretation of data, involved in drafting the manuscript, revised it critically for important intellectual content, gave final approval of the version to be publish; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. KS made substantial contributions to acquisition of data and analysis; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. ME made substantial contributions to acquisition of data, revised it critically for important intellectual content, gave final approval of the version to be publish; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. TM made substantial contributions to acquisition of data, revised it critically for important intellectual content, gave final approval of the version to be publish; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. MS revised it critically for important intellectual content, gave final approval of the version to be publish; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. JEG revised it critically for important intellectual content, gave final approval of the version to be publish; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. SEC revised it critically for important intellectual content, gave final approval of the version to be publish; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. GH made substantial contributions to conception and design; involved in drafting the manuscript, revised it critically for important intellectual content, gave final approval of the version to be publish; agrees to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.