In the present study, characteristics, treatments, and survival outcomes of patients with PPTID were analyzed based on a patient cohort that was more accurately diagnosed than in the past, including through central pathology review and KBTBD4 mutation status. To evaluate differences between our data and those from earlier reports, we compared our data with the results of a systemic review that included 127 patients with PPTID from 29 studies[12]. In our cohort, median patient age was older (49 vs. 33 years) and the female preponderance was greater (male:female ratio: 1:2.1 vs. 1:1.6). In both cohorts, headache and gait abnormality were the two main symptoms, and the rates of leptomeningeal recurrence were close (67% vs. 62.5%). Those data suggest that female preponderance and leptomeningeal recurrence are major characteristics of PPTID and that the two main symptoms are headache and gait abnormality. With respect to survival, the 5-year PFS was better in our cohort than in the systematic review (64.9% vs. 52.2%), but the 5-year OS was worse (70.5% vs. 84.1%). Those differences in the survival data suggest that our findings might more accurately reflect the progression pattern in PPTID. As mentioned, many patients with PPTID experience hard-to-control leptomeningeal recurrences despite multiple treatments. Thus, theoretically, PFS and OS must be closely, potentially suggesting a more “purely PPTID” cohort in our study than in the systemic review cohort.
Female sex and GTR were independent prognostic factors in our cohort. Maximal safe resection was established as the standard aim of surgery since many reports showed GTR as the better prognostic factor[5, 12, 15]. With respect to female sex, Mallick et al. also reported that factor as prognostic of a better outcome in PPTID[12]. Interestingly, female patients have also been reported to experience significantly better survival in pineoblastoma[17, 18]. Moreover, all pineal parenchymal tumors—including pineocytoma, PPTID, and pineoblastoma—demonstrate a female predominance[10, 12, 19, 20]. Although no specific molecular feature of pineal parenchymal tumors has been reported in female patients, sex might be one of the important factors determining tumor characteristics.
Some earlier reports recommended that patients with a resection less than GTR receive adjuvant radiotherapy[12, 21–23]. However, our study demonstrated an inverse result: radiotherapy was associated with trend toward worse prognosis. We considered that result to have been influenced by treatment bias. At initial treatment, 12 of 13 patients whose resection was less than a GTR received radiotherapy, and two of those who had leptomeningeal dissemination received craniospinal irradiation. At recurrence, six of nine recurrence patients including two who undergone radiotherapy as initial treatment received irradiation. Thus, patients who had residual, recurrent, or disseminated leptomeningeal disease received irradiation aggressively based on earlier reports in the literature that had demonstrated the effectiveness of radiotherapy in PPTID. Survival in our cohort was not inferior to that in previous reports[12]. So, our data might therefore suggest the efficacy of the therapeutic irradiation strategy used in our cohort. Although hard to conduct, prospective randomized controlled trials would be required to prove the efficacy of radiotherapy in PPTID.
In the present study, we found no correlation between WHO tumor grade and survival. WHO grade was previously reported to be an independent predictor of survival in PPTID. Fauchon et al. reported a large survival advantage in grade 2 disease compared with grade 3 disease (5-year OS: 74% vs. 39%)[7]. Lu et al. similarly reported a statistically significantly longer OS duration in grade 2 disease than in grade 3 disease (mean: 108 vs. 44 months)[15]. In contrast, like us, Yu et al. failed to demonstrate that WHO grade is prognostic, and Mellick excluded WHO grade as a candidate prognostic factor[6, 12]. We considered these conflicting reports to be the result of undefined histologic grading criteria. Two major reports concerning the criteria for WHO grading have been published. Sato et al. reported that necrotic appearance, mitosis rate, and immunohistochemical expression of neurofilament protein are useful for determining the WHO grade[24]. In contrast, Jouvet et al. reported that fewer than six mitoses and strong immunopositivity for neurofilaments indicated grade 2 disease, while more than six mitoses or less than six mitoses without strong immunostaining for neurofilaments indicated grade 3 disease[25]. Neither set of criteria was adopted for the WHO classification. As we enter the era of comprehensive tumor molecular data collection, WHO grade might be unimportant factor for tumor prognosis.
In a recent study, comprehensive analysis using next-generation sequencing revealed KBTBD4 mutations in PPTID[10, 13, 14]. Two identified mutations harbor identical somatic small in-frame insertions (p.R313delinsPRR and p.R313_M314insRPP) in the KBTBD4 gene[11]. Although the mechanism by which these mutations drive tumorigenesis in PPTID is currently uncertain, the presence of these mutations is considered useful for distinguishing PPTID from pineoblastoma[10, 13, 14]. In our study, KBTBD4 mutations were analyzed to achieve more accurate diagnoses in our cohort. Liu et al. showed that 74% of patients with PPTID harbor a KBTBD4 mutation, and Uchida et al. showed that 66.7% harbor a mutation[10, 11]. In line with those reports, we found that 80% of our cohort harbored a mutation, a result that we consider to proving the accuracy of the diagnoses in our cohort.
Liu et al. also showed that patients positive for a KBTBD4 mutation were older than those who were mutation-negative (median age: 40 vs. 15 years)[10]. We observed no correlation between age and KBTBD4 mutation status in our cohort (median age: 48 years for patients testing positive vs. 53 years for patients testing negative). But also in our cohort, prognosis tended to be better in patients negative for a KBTBD4 mutation than in patients who were mutation positive (log-rank test: p = 0.089 for PFS, p = 0.181 for OS). However, an evaluation of the 20 KBTBD4 mutation-positive and 5 mutation-negative patients demonstrated treatment bias, in that all mutation-negative patients underwent GTR and received no chemotherapy. Multivariate analysis also dismissed the efficacy of KBTBD4 mutation status as a prognostic factor. Moreover, with respect to other factors including sex, KPS, MRI findings, tumor size, WHO grade, Ki-67 proliferation index and radiotherapy, no statistically significant differences were found between patients with and without KBTBD4 mutation (Supplementary 6). Considering all the foregoing findings, no clinical implications of KBTBD4 mutation status were uncovered in our study. However, Liu et al. hypothesized that KBTBD4 mutation is the disease-defining driver gene for PPTID and a candidate therapeutic target[10]. In the future, KBTBD4 mutation status might not only be accepted as the key factor in diagnosing PPTID, but also as a determiner of clinical status with respect to prognosis and targeted therapy.
Our study has limitations. Its multi-institutional retrospective cohort design could result in bias with respect to choose of treatment strategy. Variation in treatments, including radiotherapy and chemotherapy, at each institution could have affected the survival data. Furthermore, the limited number of patients might explain the absence of the statistical power needed to evaluate the effectiveness of adjuvant therapy in PPTID. The study design also affected that statistical power. Comprehensive molecular analyses were not performed in this study. Although analysis of KBTBD4 mutation status could act as a surrogate in diagnosis, the genetic background of our cohort could have better informed the analysis of the clinical characteristics of PPTID.