Dynamic changes in 18F-borono-L-phenylalanine uptake in unresectable, advanced, or recurrent squamous cell carcinoma of the head and neck and malignant melanoma during boron neutron capture therapy patient selection

Background We evaluated dynamic changes in 18F–borono-L-phenylalanine (18F–BPA) uptake in unresectable, advanced, or recurrent squamous cell carcinoma of the head and neck (SCC) and malignant melanoma (MM) during boron neutron capture therapy (BNCT) patient selection. Methods Dynamic changes in the maximum standardized uptake value (SUVmax), tumor-to-normal tissue ratio (TNR), and tumor-to-blood pool ratio (TBR) for 18F–BPA were evaluated in 20 patients with SCC and 8 patients with MM. Results SUVmax in SCC tumors decreased significantly from 30 to 120 min. There was a non-statistically significant decrease in SUVmax for SCC tumors from 30 to 60 min and from 60 to 120 min. Patients with MM had nonsignificant SUVmax changes in 18F–BPA uptake on delayed imaging. Nonsignificant 18F–BPA TNR and TBR changes were seen in patients with SCC and MM. Conclusions Dynamic changes in SUVmax for 18F–BPA uptake had a washout pattern in SCC and a persistent pattern in MM. Dynamic 18F–BPA -PET studies should be performed to investigate the pharmacokinetics of 18F–BPA in humans and select appropriate candidates who may benefit from BNCT.


Background
Boron neutron capture therapy (BNCT) has been used for various types of intractable cancers, including glioblastoma, head and neck tumors, and melanoma [1][2][3][4][5][6]. This type of radiation therapy is based on nuclear reactions between neutrons and boron-10 ( 10 B). After a targeted tumor contains a considerable concentration of 10 B, the region to be treated is exposed to thermal neutrons. The nuclear reactions between these neutrons and 10 B produce alpha particles and 7 Li in a very short range (<10 μm) that should kill the cell. The success of BNCT depends on sufficient accumulation of 10 B in tumor cells relative to adjacent tissues [5,6]. Therefore, it is necessary to assess 10 B concentration in tumor tissue before BNCT is performed [7].
Positron emission tomography (PET) using 18 F-borono-L-phenylalanine ( 18 F-BPA) has been used to screen for appropriate candidates who can benefit from BNCT [2,3,[8][9][10][11]. Before BNCT, the 10 B concentration in tumor tissue is estimated by measuring the tumor-to-normal tissue ratio (TNR) and the tumor-to-blood pool ratio (TBR) with 18 F-BPA PET imaging [2,3,12,13]. Hanaoka et al. demonstrated a significant positive correlation between levels of BPA and 18 F-BPA accumulation in an animal model [14]. 10 B accumulation is not consistent across patients; it is reported to also depend on tumor type [15,16]. Thus, knowledge of the dynamic changes in 10 B accumulation by tumor type is critical for performing BNCT. However, there is still limited information in the literature regarding dynamic changes in 18 F-BPA uptake in various tumor types in humans. The purpose of this study was to examine the dynamic changes in the maximum standardized uptake value (SUVmax) of 18 F-BPA in squamous cell carcinoma of the head and neck (SCC) and malignant melanoma (MM). TNR and TBR of 18 F-BPA in SCC and MM were also evaluated.

General
The study protocol was approved by the institutional review board and independent ethics committee of our hospital. All patients provided written informed consent before inclusion in the trial.

Patients and PET/CT protocol
This study included 20 patients with SCC and 8 patients with MM who underwent 18 F-BPA PET/CT from March 2012 to August 2016. Patients had histologically confirmed malignant tumors and an Eastern Cooperative Oncology Group performance status of 0-1. We defined adequate organ function for patients with unresectable cancer on the basis of the normal range observed by our hospital laboratory. Adequate organ function was determined by neutrophil count ≥1500 /μL, platelet count ≥75,000 /μL, hemoglobin ≥9.0 g/dL, serum bilirubin ≤1.5 mg/dL, aspartate transaminase (AST) ≤ 100 IU/L, alanine aminotransferase (ALT) ≤ 100 IU/L, serum creatinine ≤1.5 mg/dL, and baseline left ventricular ejection fraction >60%. The main exclusion criteria were congestive heart failure, uncontrolled angina pectoris, arrhythmia, symptomatic infectious disease, severe bleeding, pulmonary fibrosis, obstructive bowel disease or severe diarrhea, and symptomatic pleural or pericardial effusion. This study was approved by the ethics committees of our institution.
Dynamic changes in 18 F-BPA uptake were evaluated in 20 patients with SCC and 8 patients with MM. PET images were acquired using a Discovery 600 scanner (GE Healthcare, Milwaukee, WI, USA). PET images were reconstructed as using a 3D ordered-subset expectation maximization algorithm. PET image evaluation and quantification of SUV were performed using AW Volume Share 4.5 software. SUV was defined as regional radioactivity divided by injected radioactivity normalized to body weight. PET/CT images were taken 30, 60, and 120 min after 18 F-BPA injection (4.0 MBq/kg of body weight). Regions of interest (ROIs) were drawn on the reconstructed PET images. Tumor SUVmax in ROIs was defined as the area of highest activity. ROIs were also drawn around normal tissue surrounding the tumor to calculate the TNR for 18 F-BPA and the blood pool in order to calculate the TBR for 18 F-BPA. The retention index (RI) was defined as the difference in SUVmax between early and delayed 18 F-BPA PET imaging, expressed as a percentage of the initial uptake (RI = (SUVdelayed − SUVearly)/SUVearly × 100%). The difference in SUVmax and RI were calculated to evaluate the change in tracer levels in malignant lesions at 30, 60 and 120 min after 18 F-BPA injection. Quantitative values above zero were defined as increased SUVmax and values below zero were defined as decreased SUVmax.

Statistical analysis
SUVmax, TNR, and TBR were analyzed using paired oneway ANOVA. The paired t-test was used to determine the significance of differences in dynamic SUVmax values, TNR, and TBR. P < 0.05 was considered to indicate a statistically significant difference. For statistical analysis, JMP software (version 11.0, SAS Institute, Inc., Cary, NC, USA) was used.

Results
Patient characteristics are summarized in Table 1. SUVmax, TNR, and TBR for 18 F-BPA in SCC and MM are summarized in Table 2. Only SUVmax showed Abbreviation: SD Standard deviation significant differences between 30 and 120 min in patients with SCC. Figure 1 is a box plot of SUVmax for tumors at 30, 60, and 120 min after injection. SUVmax in SCC tumors decreased significantly from 30 to 120 min, but the decrease was not statistically significant from 30 to 60 min and from 60 to 120 min. All 20 patients with SCC had gradual decreases in SUVmax from 30 to 120 min ( Table  2). On the other hand. Nonsignificant 18 F-BPA differences on delayed imaging were seen in patients with MM (Fig. 1,  Tables 2 and 3). In contrast to patients with SCC, not all patients with MM had decreases in SUVmax from 30 to 60 min, 60 to 120 min, and 30 to 120 min.
Nonsignificant TNR and TBR for 18 F-BPA were seen on delayed imaging in both patient groups (Table 2). Representative 18 F-BPA PET images are shown in Figs. 2 and 3.

Discussion
The aim of this study was to examine dynamic 18 F-BPA changes in SUVmax in SCC and MM as part of the patient selection process for BNCT. In SCC, dynamic changes in SUVmax for 18 F-BPA uptake had a washout pattern, compared with a persistent pattern of 18 F-BPA uptake in MM. 18 F-BPA was developed to predict 10 B accumulation in tumors and normal tissues with PET [18]. Studies have shown that there are a variety of amino acid transporters, such as Systems L, A, ASC, and B [19,20]. System L is the primary contributor to 18 F-BPA uptake, which is correlated with total L-amino acid transporter (LAT) expression, more specifically LAT1 and LAT4. Many tumors overexpress LAT1 or LAT4 [21][22][23]. Previous studies have shown that the expression of amino acid transporters in tumors varies widely, and it sometimes reflects proliferation speed and malignancy [24]. However, reasons for differences in dynamic changes in 18 F-BPA uptake between SCC and MM remain uncertain. It is unclear whether 18 F-BPA undergoes metabolic transformation, although metabolic transformation of L-phenylalanine has been reported [25]. LAT and the metabolic transformation of 18 F-BPA may contribute to dynamic changes in 18 F-BPA accumulation in tumors. Further studies with more participants and evaluation of processes involved in 18 F-BPA metabolic transformation are needed to resolve this question.
In clinical BNCT, 18 F-BPA accumulation was measured about 1 h after 18 F-BPA administration [26][27][28][29]. However, the number of dynamic studies of 18 F-BPA uptake in humans is limited. Therefore, we focused on dynamic 18 F-BPA uptake in humans. Our study showed that SUVmax for 18 F-BPA uptake in SCC has a washout pattern. It is very important to realize that some tumor histological types may have a washout pattern. 18 F-BPA uptake in different tumor types may be vary with extended distribution time in 18 F-BPA PET imaging. Further dynamic 18 F-BPA -PET studies should be performed to determine who are appropriate candidates that can benefit from BNCT.
In this study, we did not evaluate the pharmacokinetics of BPA or the BPA-fructose complex because we focused on dynamic accumulation of 18 F-BPA in human tumors. Hanaoka et al. showed a positive association between the levels of BPA and 18 F-BPA accumulation in a rat model [14]. However, the biodistribution of 18 F-BPA in animals and humans is different [30]. In addition, metabolic  The present study had some limitations. Two different tumor types were examined in our study. Our 18 F-BPA findings for SCC were consistent with previous studies [28,31]. However, the characteristics of dynamic 18 F-BPA accumulation in radioresistant head and neck carcinomas,  such as mucoepidermoid carcinomas and adenoid cystic carcinomas, is unknown [3]. Various intractable cancers that can be treated with BNCT represent a wide spectrum of histopathological backgrounds. Further studies involving more patients, each representing a specific pathological entity, are therefore needed.

Conclusions
Dynamic changes in SUVmax for 18 F-BPA uptake in SCC has a washout pattern, while 18 F-BPA uptake in MM has a persistent pattern. Dynamic 18 F-BPA -PET studies should be performed as part of a human pharmacokinetic study of 18 F-BPA and to select appropriate candidates who may benefit from BNCT.