Evaluation of Fluoride-18-Labeled Boronophenylalanine-Positron Emission Tomography Imaging for the Assessment of Boron Neutron Capture Therapy in Patients with Recurrent Head and Neck Squamous Cell Carcinoma

Introduction: A role for fluoride-18-labeled boronophenylalanine positron emission tomography (18FBPA-PET) in boron neutron capture therapy (BNCT) has not been fully elucidated. We investigated the role of 18FBPA-PET in BNCT for recurrent head and neck squamous cell carcinoma (HNSCC) patients. Materials and methods: 18FBPA-PET images were obtained from 10 histologically verified recurrent HNSCC patients who received BNCT. The intratumoral accumulation of 18FBPA was calculated as the ratio of maximum and minimum radioactivity counts to that of normal tissue (Tmax/N and Tmin/N ratios, respectively). The percentage volume at which the radioactivity count ratio was >2.5 compared to normal tissue was also calculated (Vo2.5). Moreover, mean and minimum irradiation dose to the tumor was calculated. We investigated which parameters could predict the treatment effect of BNCT. Results: Treatment effects of local lesions were as follows: complete remission (CR) in 5 cases and non-CR in 5 cases. Only Tmin/N ratio showed a significant difference between the CR and non-CR groups (P=0.008). Conclusion: The Tmin/N ratio of 18FBPA-PET can predict the treatment effect of BNCT for recurrent HNSCC.


Introduction
Boron neutron capture therapy (BNCT) is a type of high linear energy transfer radiation therapy. High irradiation doses can be selectively delivered to tumor cells without causing serious damage to surrounding normal tissue, due to high accumulation of 10 B in the tumor compared to adjacent normal tissue. The results of clinical trials of BNCT that used L-10 B-para-boronophenylalanine (L-BPA) as a boron delivery agent have been reported for the treatment of recurrent head and neck cancers [1][2][3][4].
The actual irradiation dose to the tumor during BNCT is determined by the intratumoral 10 B concentration and the neutron flux to the tumor [5,6]. Since a fairly constant neutron flux is achieved in the irradiation field, the tumor irradiation dose can be largely determined from the intratumoral 10 B concentration. Thus, estimation of 10 B concentrations is required. Less invasive methods for measuring 10 B concentration did not exist 2 decades ago. Therefore, using surgical specimens from malignant melanomas, Mishima and Fukuda et al. measured 10 B concentrations directly to estimate the tumor/normal tissue ratio (T/N ratio) [7,8].
In 1998, Imahori et al. reported a less invasive method for estimating intratumoral 10 B concentrations using positron emission tomography (PET) [9] and calculation of the T/N ratio of 10 Bcontaining compounds using PET has since become the standard BNCT planning method. The tumor irradiation dose is calculated from the neutron flux and the T/N ratio of 18 FBPA-PET ( Figure 1). Clinically, a 18 FBPA-PET T/N ratio of ≥ 2.5 is generally considered a requirement for use of BNCT in head and neck cancer [2,10]. This value was established to ensure that tumors with a depth ≥ 6 cm will receive >20 Gray equivalent (GyE), which is necessary for tumor control, while surrounding normal tissue (primarily skin) receives a tolerable irradiation dose [11].
Although the utility of the T/N ratio has been comprehensively reported in some clinical studies, actual intratumoral accumulation of 18 FBPA can fluctuate depending on cell density and tumor cell activity [12] and this value is inhomogeneous in tumor tissue. Moreover, radiosensitivity differs for each pathologic tumor type. In this study, we investigated whether 18 FBPA accumulation that has potentially inhomogeneous distribution in tumors can predict the treatment effect of BNCT for head and neck squamous cell carcinoma (HNSCC).

Study design
This study was a retrospective, single-institution review of HNSCC patients treated with BNCT.

Patients
We previously reported 20 recurrent head and neck cancer patients treated with BNCT [2]. We selected 10 of these 20 patients with squamous cell carcinoma for the present analysis; the remaining 10 patients with other pathologies were excluded, as their tumors had different degrees of radiosensitivity. Characteristics of the selected 10 cases are presented in Table 1. Patients included 8 men and 2 women, with a median age at time of treatment of 62.5 years (range, 39 to 77 years). Primary lesion sites were the oropharynx in 5 cases, tongue in 2 cases, and nasal ala, oral floor and larynx in 1 case each. All patients were undergoing third-line or later treatment, and had already received 40-66 Gy (median, 62.5 Gy) of radiotherapy prior to BNCT. The clinical stage was rN2a in 5 cases, rT4 in 3 cases and rT2 in 2 cases at the time of treatment based on the Union for International Cancer Control Disease Stage Classification (VIIth Edition).

Parameters
We placed a rectangular region of interest (ROI) (size: 4 pixels, 1 pixel=2 × 2 mm) on tumor and normal tissue. The left ventricle was designated as normal tissue. The contralateral cervical artery was also substituted for normal tissue in patients whose PET scans only evaluated the head and neck regions. The highest ROI count was defined as the T max , while the lowest was defined as the T min . The ratios of the T max or T min to the ROI count of normal tissue were calculated as the T max /N and T min /N ratios, respectively. The percentage intratumor volume with a T/N ratio ≥ 2.5 was defined as Vo2.5. All ROIs were drawn by the same physician, who has >13 years of experience in PET analysis.
Mean and minimum irradiation doses to the tumor were calculated by the BNCT planning system (mean tumor dose and minimum tumor dose, respectively).

Examinations
Patients were divided into two groups based on local treatment effect: the complete remission (CR) and the non-CR groups. The T max /N ratio, T min /N ratio, Vo2.5, mean tumor dose and minimum tumor dose were compared between the CR and non-CR groups.

Statistical analysis
Student's t test was used for statistical analysis. A P-value <0.05 was considered statistically significant.

Results
All results are shown in Table 2. Of the 10 patients, recurrences consisted of local recurrence in 5 patients (2 of whom also had distant metastasis), regional recurrence in 2 patients and distant metastasis in 1 patient. Based on local treatment effect, 5 patients were assigned to the CR group and the remaining 5 were assigned to the non-CR group. Survival times ranged from 3 to 20 months; all patients died within 20 months. The 1 year survival rate was 20% and median survival was 10.5 months. Causes of death were local recurrence in 5 cases, distant metastasis in 3 cases and other (carotid blowout syndrome [CBS]) in 2 cases. (We have previously reported these 2 CBS deaths [17]  In the CR group, the T max /N ratio ranged from 2.6 to 4.2 (median, 3.8) and the T min /N ratio ranged from 2.3 to 3.5 (median, 2.6). Vo2.5 ranged from 78% to 100% (median, 100%). Mean tumor dose ranged from 38.2 to 62.9 (median, 53.6) GyE and minimum tumor dose ranged from 20.1 to 29.3 (median, 25.3) GyE.
Of these five parameters, a statistically significant difference between the CR and non-CR groups was only noted for the T min /N ratio (Figure 2, P=0.008).

Discussion
In our institute, the BNCT irradiation dose for head and neck cancers is determined using the T/N ratio of 18 FBPA-PET in the following manner: (1) skin dose ≤ 18 GyE and (2) tumor dose ≥ 20 GyE. Biologically, the effect of the boron neutron capture reaction does not differ much in normal tissue between individuals [18], but largely differs in tumor tissues. As shown in this manuscript, the only parameter that predicted treatment effect was the T min /N ratio (P=0.008); none of the other parameters appeared to be associated with treatment effect. We consider that in patients with high T min /N ratios, a sufficient dose could be administered to the large tumor volume, so satisfactory local treatment effect was achieved. In contrast, in patients with low T min /N ratios, the volume to which a sufficient irradiation dose was not delivered was large, residual tumor cells survived and the patients failed to achieve a CR. For example, the tumor disappeared 10 weeks after BNCT in a patient with a high T min /N ratio (3.5) and a high T max /N ratio (5; Case 9, Figure 3). In contrast, the tumor remained 12 weeks after BNCT in a patient with a low T min /N ratio (1.2). The tumor irradiation dose was calculated as 32.7 GyE based on the T max /N value of 5 at the actual BNCT planning and 18 FBPA accumulation varied largely in the tumor in this patient (Case 2, Figure 4).   The actual tumor irradiation dose is less than the calculated dose, based on the value of the T max /N ratio. In particular, in cases in which the difference between the T max /N and T min /N ratios is large, large discrepancies in BNCT treatment effects may exist. In fact, the calculated irradiation dose in the area with the lowest 18 FBPA accumulation was 25-85% of the highest in the 5 non-CR cases. Of 5 non-CR patients, the total amount of neutron flux was determined by the normal skin irradiation dose in 1 patient and the tumor irradiation dose in 4 patients. In those 4 patients, the tumor irradiation dose was calculated to be 12.4-32.7 GyE using the T max /N ratio for actual BNCT and the skin irradiation dose was 4-9 GyE. The minimum tumor irradiation dose was only 7.8-11.4 GyE based on our simulation using the T min /N ratio. Considering the skin irradiation dose was much lower than the possible tolerated dose, a higher treatment effect could be obtained if the T min /N ratio had been used for dose calculation.
Some issues regarding the use of 18 18 FBPA was taken up into cells via the same amino acid transporter L-system as BPA in human glioblastoma cells [19]. 18 FBPA and BPA showed almost identical pharmacokinetics in the body in the experimental study [20]. Thus, we have used 18 FBPA-PET in BNCT planning as an indicator of 10 B concentration. Second, it is difficult to reflect the inhomogeneous 10 B intratumoral concentration with the current BNCT dose planning system (SERA [5] and JCDS [6]). We anticipate the development of a BNCT planning system that can support inhomogeneous 18 FBPA accumulation in the tumor. Alternatively, we propose use of the T min /N ratio rather than the T max /N ratio to increase the treatment effect of BNCT.
The source of neutrons in Japan has been the long-employed nuclear reactors; however, BNCT accelerators that can provide an alternative to nuclear reactors are currently under development [21]. Detailed analysis of previous clinical cases is required to properly administer next-generation BNCT using accelerators. We hope such endeavors will lead to the development of highly effective head and neck cancer treatment. This is the first study in which the results of 18 FBPA-PET imaging have been shown to predict a treatment effect of BNCT against recurrent HNSCC.

Conclusion
The T min /N ratio of 18 FBPA-PET is a useful parameter for predicting the BNCT treatment effect in patients with recurrent HNSCC.