1 INTRODUCTION

Oral malignancies, although of various histological types, are mostly squamous cell carcinomas. Furthermore, 6% to 10% of oral malignancies are diagnosed as nonsquamous cell carcinomas.1 The standard treatment for oral nonsquamous cell carcinomas is surgery.2, 3 For inoperable cases, photon radiation therapy (RT) is usually applied. However, nonsquamous cell carcinomas are known to be relatively radio-resistant, and therefore the role of photon RT for inoperable oral nonsquamous cell carcinomas is limited.4, 5

Carbon-ions (C-ions) provide higher linear energy transfer (LET) and large relative biological effectiveness (RBE) compared with photons, and thus offer a greater possibility of tumor control.6 Four carbon-ion facilities were functioning in Japan by the end of 2014 (Hospital of the National Institute of Radiological Sciences, Hyogo Ion Beam Medical Center, Gunma University Heavy Ion Medical Center, SAGA HIMAT Foundation). A retrospective multicenter study to evaluate the clinical outcomes of C-ion RT for head and neck malignancies was performed (Japan Carbon-Ion Radiation Oncology Study Group [J-CROS] Study: 1402 HN). The clinical outcomes of the respective major histological types of head and neck malignancies have already been reported.7-11 The 5-year local control and overall survival (OS) rates among patients with adenoid cystic carcinoma, mucosal melanoma, and adenocarcinoma of 68% and 74%, 72.3% and 44.6%, and 79.3% and 60.4%, respectively, were also reported.8, 9, 11

Oral malignancies are in close proximity to risk organs, such as oral mucosa and jawbone, compared with other head and neck malignancies. To clarify the efficacy and safety of C-ion RT for oral nonsquamous cell carcinomas, subanalysis based on primary sites could prove to be clinically useful. Herein, we report the results for a subgroup of patients with oral nonsquamous cell carcinomas using the data of J-CROS 1402 HN.

2 PATIENTS AND METHODS

J-CROS 1402 HN was designed as a retrospective, multicenter study conducted in four carbon-ion facilities in Japan (Hospital of the National Institute of Radiological Sciences, Hyogo Ion Beam Medical Center, Gunma University Heavy Ion Medical Center, SAGA-HIMAT Foundation). It was approved by the institutional review boards of the respective institutes and was conducted in accordance with the Declaration of Helsinki. This trial is registered with UMIN-CTR (http://www.umin.ac.jp/ctr/index-j.htm), identification number UMIN000024473.

Patients suffering from head and neck malignancies and treated with C-ion RT between November 2003 and December 2014 were included. Detailed inclusion criteria have been described previously.7-11 Patients with previous irradiation to the head and neck were excluded.

The survey included 908 eligible patients. Among them, 76 patients with oral nonsquamous cell carcinomas were enrolled in the study. All tumors were classified by the TNM staging system, seventh edition (International Union Against Cancer, 2009).

3 RESULTS

3.2 Local control and survival

Median follow-up time was 31.1 months (range, 3-118 months). All 7 patients (9.2%) with local recurrence developed it within the PTV. The median interval between C-ion RT and local recurrence was 20 months (range: 7-47 months). In these patients, histologies were 4 with ACC, 1 with acinic cell carcinoma, 1 with spindle cell carcinoma, and 1 with malignant melanoma. Regarding T classification, 1 case was T3, 2 cases were T4a, and 4 cases were T4b. Regarding the prescribed dose, 3 cases received 64 Gy (RBE) in 16 fractions, 2 cases 57.6 Gy (RBE) in 16 fractions, 1 case 65 Gy (RBE) in 26 fractions, and 1 case 70.4 Gy (RBE) in 26 fractions. Regional recurrence and distant metastasis developed in 13 (17.1%) and 17 patients (22.4%), respectively. Of the 13 regional recurrence cases after C-ion RT, 12 had cervical lymph node metastasis, but none had been found to have lymph node metastasis prior to C-ion RT. The median interval between C-ion RT and regional recurrence was 18 months (range: 1-41 months). Of these patients, histologies were mucosal melanoma in 8, ACC in 4, and spindle cell carcinoma in 1. As for T classification, 6 cases were T3, 3 cases were T4a, and 4 cases were T4b. Three cases developed regional + distant metastasis. The most common sites of distant metastasis were lung (n = 7) and brain (n = 3). Nineteen patients died from their disease, and 3 from unrelated causes.

Figure 1A shows the cumulative local control, PFS, and OS of all patients. Cumulative 3- and 5-year local control rates were 86.8% (95% confidence interval [CI], 76.6 - 97.0) and 82.9% (95% CI, 70.5 - 95.2), 3- and 5-year PFS rates were 63.1% (95% CI, 51.1 - 75.1) and 49.9% (95% CI, 35.9 - 63.9), and the 3- and 5-year OS rates were 78.4% (95% CI, 67.3 - 89.5) and 59.5% (95% CI, 44.6 - 74.4), respectively.

Figure 1B shows the Kaplan-Meier survival curves for local control according to histological subtype. The 3- and 5-year local control rates of patients with salivary gland carcinoma (SGC) were 85.0% (95% CI, 70.8 - 99.1) and 78.4% (95% CI, 60.5 - 96.4), respectively. Both 3- and 5-year local control rates of patients with mucosal melanoma were 93.3% (95% CI, 80.7 - 100.0).

The Kaplan-Meier survival curves for OS by histological subtype are shown in Figure 1C. The 3- and 5-year OS rates of patients with SGC were 90.8% (95% CI, 80.7 - 100.0) and 71.7% (95% CI, 53.1 - 90.3), and those of patients with mucosal melanoma were 62.0% (95% CI, 40.8 - 83.3) and 43.9% (95% CI, 20.9 - 66.8), respectively.

4 DISCUSSION

The standard treatment for resectable oral nonsquamous cell carcinoma is surgical treatment, and effective therapeutic results have been reported. The standard treatment option for unresectable or inoperable oral nonsquamous cell carcinoma is photon RT,2, 3, 12 although nonsquamous cell carcinomas are mostly radioresistant and associated with poor outcomes.4, 5 To date, the effectiveness of photon RT for oral nonsquamous cell carcinoma has not been extensively studied. In the meantime, C-ion RT has already been proven as a promising treatment for radioresistant tumors,7-11, 13, 14 and its therapeutic effects for oral nonsquamous cell carcinomas have been reported.13, 14 Although several studies have examined the clinical results of photon RT or C-ion RT for oral nonsquamous cell carcinomas, most published series were designed retrospectively, consisting of small sample sizes from single institutes. To the best of our knowledge, this is the first multicenter study, and is the largest analysis of photon RT or C-ion RT for oral nonsquamous cell carcinoma. In this multicenter study, C-ion RT demonstrated promising outcomes with acceptable toxicities in the treatment of oral nonsquamous cell carcinomas.

Intraoral minor SGCs are predominantly treated with surgery as well as adjuvant RT if deemed necessary, and the reported 5-year rates of local control and survival are 73.8-83.1% and 71.8-73%, respectively.15-17 However, surgical resection is often difficult for intraoral minor SGCs cases with extraoral extension (eg, into the intracranial space or masticator space). As adjuvant or definitive treatment for intraoral minor SGCs, in spite of the still limited data regarding clinical outcomes, photon RT has become a widely used potion. Yorozu et al4 reported that the 5-year local control and OS rates were 54% and 63%, respectively, among 12 patients with intraoral minor SGCs treated with photon RT, although 3 had undergone the RT postoperatively. As for brachytherapy, Huang et al18 reported the clinical results of ¹²⁵I brachytherapy alone in 38 patients with recurrent or locally advanced oral and maxillofacial adenoid cystic carcinoma (including 12 with adenoid cystic carcinoma of the oral cavity). The 5-year local control and OS rates were 59% and 65%, respectively. Fast neutron therapy, which has a similar RBE to C-ion RT,6 has been reported to have therapeutic effects on intraoral minor SGC, which is known to be radioresistant. Douglas et al19 reported a series of 151 patients with adenoid cystic carcinoma of the head and neck who were treated with fast neutron therapy. In their study, 5-year locoregional control and OS were 68% and 83%, respectively, among 26 patients with intraoral tumors. In the present multicenter study, C-ion RT was found to be an effective treatment for intraoral minor SGCs, presenting a result similar to fast neutron therapy, with 3-year local control and OS of 85.0% and 90.8%, respectively. Our findings highlight the possibility of C-ion RT being a viable treatment option for intraoral minor SGCs, and especially for inoperable tumors.

The rare disease of mucosal melanoma of the oral cavity has consistently been associated with a poor outcome. Surveillance, epidemiology, and end results data from the period of 1973-2012 have indicated a 5-year OS rate of approximately 25% among patients with oral mucosal melanoma.20 Complete surgical resection with clear margins is the pillar of oral mucosal melanoma management, and optimal results might be expected. However, it is frequently difficult for oral mucosal melanoma to keep a tumor-free surgical margin of 1-2 cm, which is generally required and accepted for cutaneous melanoma, as the extent of histopathologically assessed tumor expanse is usually greater than that of the gross disease, and it might include the pharynx and paranasal sinus. In the case of surgical therapy, Lopez-Graniel et al21 reported a 5-year OS rate of 6.6% for 15 oral mucosal melanoma patients, and Tanaka et al22 reported a 5-year OS rate of 15.4% for 13 primary oral mucosal melanoma patients. However, for inoperable cases, Wushou et al5 reported a 3-year OS rate of 0% in 21 patients treated with photon RT alone. The therapeutic effect of fast neutron therapy on primary mucosal melanomas of the head and neck of 14 patients (including 3 with oral mucosal melanomas) as reported by Liao23 resulted in 5-year local control and OS rates of 66% and 21%, respectively. However, their report did not describe the outcomes of the patients in detail. In comparison, the current study showed 3-year local control and OS of patients with oral mucosal melanoma to be 93.3% and 62.0%, respectively. Thus, C-ion RT attained superior local control and positive survival benefits compared with the historical data of photon RT.

A retrospective multicenter study of C-ion RT for head and neck mucosal melanoma has already shown overall survival benefit with use of DTIC-based concurrent chemotherapy. In this study, concurrent chemotherapy was administered for 17 mucosal melanoma patients but there was no significant improvement in overall survival. It has recently been reported that ipilimumab (cytotoxic T-lymphocyte-associated antigen-4 checkpoint inhibitor) and nivolumab (programmed death-1 checkpoint inhibitor) offer complementary activities against melanoma. According to Karlsson et al checkpoint inhibitors outperform immunotherapy or chemotherapy in terms of survival and tumor response in patients with stage III/IV unresectable cutaneous melanoma.24 Thus C-ion RT in combination with immunotherapy may improve treatment outcomes in patients with oral mucosal melanoma.

ORN is one of the critical complications in patients with oral malignancies who are treated with RT. Kuhnt et al23 reported that 13.6% of 256 patients with oral cancer developed severe ORN that required extensive surgical intervention after photon RT. Maor et al25 reported that, among 68 patients with advanced head and neck cancer (including 21 with oral cancer) who were treated with fast neutron therapy, ≥ grade 3 late toxicity was observed in 39.7%. In their study, grade 4 ORN occurred in 4 patients (5.9%). Liao et al26 reported that 14.3% of 14 patients with mucosal melanoma of the head and neck (including 3 oral mucosal melanomas) developed severe ORN requiring extensive surgical intervention after fast neutron therapy. However, in those reports of fast neutron therapy, the incidence of ORN in patients with oral malignancies was not described in detail. Chang et al27 reported that a radiation dose >70 Gy was associated with an increased risk for ORN. In our study, 64.0 Gy (RBE) in 16 fractions at 4 fractions per week was the most commonly prescribed dose. If the predetermined RBE value was correct and the linear-quadratic model could be applied to C-ion RT, 64 Gy (RBE) in 16 fractions would be equivalent to 89.6 Gy (RBE) at a fractionation of 2 Gy (RBE) per fraction with a 3-Gy α/β value. Although this dose was definitely high compared with the standard dose for oral cancer used in photon RT, the incidence of grade 3 ORN requiring surgical intervention was 11.8%, which was similar to the reported photon RT data. C-ion RT with its dose-localization properties may reduce the irradiated volume of the jaw compared with photon RT.

Sasahara et al28 concluded, in regard to 63 head and neck tumors treated with C-ion RT, that the risk factors for ORN of the maxilla included the presence of teeth within the PTV and a maxillary volume receiving >50Gy (RBE) in a 16-fraction protocol. In addition, oral health conditions (periodontal pocket depth, dental plaque score, alveolar bone loss level, and radiographic periodontal status) post radiation treatment were risk factors for ORN.29 To reduce the risk of ORN, in case of a questionable prognosis within the PTV, total tooth extraction before starting RT was recommended.30 Moreover, a custom-made mouthpiece and spacer might reduce the volume of the jawbone that would be exposed to high-dose irradiation.31

In this study, T4 classification was found to be the significant risk factor for local control in multivariate analysis. Although the prescribed dose was not a significant risk factor of local control, a dose less than 64 Gy in 16 fractions (EQD2^{α/β 10} = 74.7 Gy [RBE]) was used in 4 cases of 7 cases who developed local recurrence. Of these 4 cases, 2 cases received 57.6 Gy (RBE) in 16 fractions (EQD2^{α/β 10} = 65.3Gy [RBE]), 1 case received 65 Gy (RBE) in 26 fractions (EQD2^{α/β 10} = 67.7Gy [RBE]) and 1 case received 70.4 Gy (RBE) in 26 fractions (EQD2^{α/β 10} = 74.6Gy [RBE]). Consideration of prescribed doses and fractionation could improve clinical outcome of treatment. However, increasing the prescribed dose beyond 64 Gy (RBE) in 16 fractions could be difficult in terms of safety and tolerability given the risk of osteradionecrosis and other acute and chronic toxicities.

This study has two limitations. First, it was based on retrospective data, and several doses and fractionations of C-ion RT were included. However, no significant differences in tumor control, toxicity, or OS were noted on the basis of the dose fractionations. An integrated treatment schedule using 16 fractions over 4 weeks with a total dose of 57.6 or 64.0 Gy (RBE) has been followed in a multicenter prospective registry study of Japan since April 2016. The second limitation was the short follow-up period (median, 31.1 months). Successive follow-up will be necessary to confirm the long-term efficacy and the incidence rate of late toxicity.

In conclusion, a multicenter study in Japan showed that C-ion RT offered promising local control and outcomes in a large cohort of patients with locally advanced oral nonsquamous cell carcinomas. In the case of patients with inoperable, locally advanced oral nonsquamous cell carcinomas, C-ion RT might be considered as a viable option. Prospective multicenter studies with longer follow-up periods will be necessary to validate these findings.

CONFLICT OF INTEREST

The authors declare that they have no conflicts of interests.