Diffusion-weighted (DW)-MRI, ADC, and ADC ratio based on tumor cellularity could contribute to the assessment of cerebellar tumors with promising results as regard type and grade [15]. High-grade tumors appear hyperintense on DW images with low ADC values. High tumor cellularity is probably the major factor of the decreased ADC values in high-grade brain tumors [16]. Also, ADC values correlate with total nuclear area and tumor grade with high-grade tumors having high cellular density and decreased ADC [17].
In the current study, we reported that the pediatric posterior fossa tumor more common in females representing 63.32%, their ages ranged from 2 to 17 years with a mean of 8.7 years, which agreed with Ahmed et al. [18], they included 24 pediatric patients (15 females, 9 males) with CNS tumors, they reported that the pediatric posterior fossa tumor more common in females with a mean age of 6 years (range 1 to 15 years). While Zitouni et al. [14] included 42 patients (23 males, 19 females), they reported that the pediatric posterior fossa tumor more common in males with CNS tumors with a mean age of 7.7 years (range 1 to 18 years).
Among studied patients in the current study, ependymoma was the most common pediatric CNS posterior fossa tumor proved by histopathology representing (40%) of the patients followed by medulloblastoma (33.3%) then pilocytic astrocytoma (26.7%). In contrast to the study done by Zitouni et al. [14], it reported that medulloblastoma was the most common pediatric CNS posterior fossa tumor proved by histopathology representing 42.8% followed by JPA then ependymoma. Also, Rumboldt et al. [11] reported that pilocytic astrocytoma was the most common pediatric CNS posterior fossa tumor proved by histopathology representing 48.5% followed by ependymoma then medulloblastoma.
In the current study, ADC values and ratios were significantly different between the commonest three CNS posterior fossa tumors in children: pilocytic astrocytomas, ependymomas, and medulloblastomas (P < 0.001). We reported ADC values were significantly different between pilocytic astrocytomas (1.43 ± 0.28 × 10−3) (mean ± SD) and medulloblastomas (0.71 ± 0. 21 × 10−3) with a P value < 0.001. Also, there was a significant difference when comparing medulloblastomas (0.71 ± 0. 21 × 10−3) with ependymomas (1.04 × 10−3 ± 0.21) with a P value < 0.001.
These results matched with the study done by Zitouni et al. [14], they found a mean ADC values for JPA, ependymoma and medulloblastoma were 1.42 ± 0.27 × 10−3, 1.03 ± 0.21 × 10−3, and 0.707 ± 0.21 × 10−3 mm2/s respectively (P ≤ 0.001). Also agreed with Ahmed et al. [18], they reported ADC values were significantly different between WHO I (low-grade tumors) (1.49056 ± 0.15 × 10−3mm2/s) (mean ± SD) and WHO IV (high-grade tumors) (0.70352 ± 0.23 × 10−3mm2/s), (P ≤ 0.001). Also, Rumboldt et al. [11] reported that ADC values were significantly different between WHO I (low-grade tumors) (1.24 ± 2.09 × 10−3mm2/s) (mean ± SD), ependymoma (0.97 ± 1.29 × 10−3mm2/s), and WHO IV (high-grade tumors) (0.55 ± 0.63 × 10−3mm2/s), (P ≤ 0.001).
In contrast to the study done by Mohamed et al. [19], they reported significant differences in the ADC values between JPA and medulloblastoma also between ependymoma and medulloblastoma (P = 0.000), but no statistically significant differences between JPA and ependymoma (P > 0.05) were detected. There was no overlap in individual tumor ADC values or ratios between JPA and medulloblastoma.
Among the studied patients, ADC ratio was significantly different between pilocytic astrocytomas (1.95 ± 0.30) (mean ± SD) and medulloblastomas (1.01 ± 0.30) with a P value < 0.001, we also demonstrated a significant difference between pilocytic astrocytoma (1.95 ± 0.30) (mean ± SD) and ependymomas (1.50 ± 0.20) (mean ± SD) with a P value <0.001. Also, we found a significant difference when comparing medulloblastomas (1.01 ± 0.30) with ependymomas (1.50 ± 0.20) with a P value < 0.001.
These findings in JPAs and medulloblastomas are probably secondary to the low cellularity and relatively small nuclear area typically seen in the former tumor types in contradistinction to the densely packed cells and large nuclei characteristic for the latter [11].
These results were agreed with Zitouni et al. [14], they found the mean ADC ratios were 1.95 ± 0.30, 1.50 ± 0.20, and 1.02 ± 0.30 for JPA, ependymoma, and medulloblastoma. While Rumboldt et al. [11] found the mean ADC ratios to be 2.11 ± 0.36, 1.39 ± 0.18, and 0.84 ± 0.14 for JPA, ependymoma, and medulloblastoma.
Yamasaki et al. [20] reported that there was a slight overlap in the result between ependymoma and pilocytic astrocytoma. In the present study, we diagnosed two lesions as ependymoma by ADC value but pathologically proved pilocytic astrocytoma. Also, we diagnosed one lesion as ependymoma by ADC ratio but pathologically proved pilocytic astrocytoma.
Finding in ependymoma could be explained by an overlap between ADC values of the classic type (WHO grade 2, one-half of tumors demonstrating restricted diffusion) and anaplastic type (WHO grade 3, two-thirds of tumors demonstrating restricted diffusion), given the wide histologic and prognostic spectrum of ependymoma, diffusion characteristics of ependymoma also have a wide range overlapping other tumor types [21].
We reported the cutoff ADC ratio of > 1.7 for differentiation of pilocytic astrocytomas from medulloblastoma and ependymomas with sensitivity 87.5% and specificity 93.3%. The cutoff ADC ratio of ≤ 1.2 for differentiation of medulloblastomas from PAs and ependymomas is with sensitivity 100% and specificity 90%. The cutoff ADC value of ≤ 1.6, > 1.2 for differentiation of ependymomas from PAs, and medulloblastomas were 100% with sensitivity 87.8% and specificity 99.5%.
These results agreed with Koral et al. [22], they studied 140 patients (58 medulloblastomas, 10 AT/RT, 51 astrocytomas, 21 ependymomas) and could differentiate astrocytoma from ependymoma with 78% sensitivity and 78% specificity using an ADC ratio ≥ 1.8 and could differentiate embryonal tumors from ependymoma with 87% sensitivity and 83% specificity using an ADC ratio ≤ 1.2. Also, Zitouni et al. [14] differentiated astrocytoma from ependymoma with 85.7% sensitivity and 90% specificity using an ADC ratio ≥ 1.7 and differentiated medulloblastoma from ependymoma with 100% sensitivity and 88.89% specificity using an ADC ratio ≤ 1.18, which agreed with our results. Gimi et al. [15] studied 79 patients (31 JPA, 27 medulloblastoma, 14 ependymoma, 7 AT/RT) and measured the ADC ratios as 2.30, 1.58, 0.97, and 0.83 for astrocytoma, ependymoma, medulloblastoma, and AT/RT. They differentiated astrocytoma from ependymoma with 92% sensitivity and 79% specificity using an ADC ratio ≥ 1.7, and differentiated embryonal tumors (medulloblastoma and AT/RT) from ependymoma with 93% sensitivity and 88% specificity using an ADC ratio ≤ 1.2.