Histopathology and Clinical Implication of Treatment-related Image Changes After Surgical Resection of Brain Metastases Previously Treated with Gamma Knife Radiosurgery

The true pathology and clinical implication of treatment-related image changes (TRICs) after stereotactic radiosurgery (SRS) for brain metastases (BM) have not been established. This study compared the surgical pathology and outcomes of intracranial metastatic lesions featured as TRICs or progressive disease (PD) in advanced magnetic resonance imaging (MRI). METHODS A total of 86 patients who underwent surgical resection of brain metastases previously treated with gamma knife radiosurgery (GKS) from 2009 to 2019 were retrospectively reviewed and classied by MRI ndings and histopathology. RESULTS Among 54 patients with TRICs in preoperative MRI, the histopathology of pure radiation necrosis (RN) was conrmed in 19 patients (TRIC-RN) and mixed or predominant viable tumor cells in 35 patients (TRIC-PD). Thirty-two patients diagnosed with PD exhibited the metastatic histology well correlated with imaging (PD-PD). The TRIC-PD group showed larger prescription isodose volume (9.4 cm 3 ) than the TRIC-RN (4.06 cm 3 , p=0.014) group and shorter time interval from GKS to preoperative MRI diagnosis related to neurological decits (median 4.07 months) than the PD-PD group (median 8.77 months, p=0.004). Signicant differences in progression-free survival were conrmed among the three groups (p<0.001) but not between TRIC-RN and TRIC-PD (post hoc test, p=1.00), whereas no signicant difference was observed in overall survival (p=0.067). radiation necrosis; TRIC treatment-related image whole-brain radiation p = 0.030) but not in the multivariate analysis (p = 0.25). For primary malignancy, breast cancer and other types of cancer exhibited higher risk of tumor recurrence than lung cancer in univariate analyses (overall p = 0.012, HR 2.809 [95% CI 1.317–5.990], p = 0.008 and HR 2.292 [95% CI 1.050–1.883], p = 0.037, respectively).


Introduction
Treatment-related image changes (TRICs) occur months to years after stereotactic radiosurgery (SRS), and the histology of TRICs is usually considered to be radiation necrosis (RN). Symptomatic focal brain necrosis developed in 2-10% of patients who underwent gamma knife radiosurgery (GKS) for brain metastases (BM) [1][2][3], and the overall risk of RN leading to permanent morbidity and additional surgical resections was reported as 4.7-7.0% [4][5][6].
Based on current imaging diagnoses, it is not easy to predict true pathology and clinical outcome of TRICs [7]. Differentiating them from progressive disease (PD) has been a challenge after stereotactic radiosurgery for brain metastasis, due to their similar magnetic resonance imaging (MRI) appearances with enlarged, heterogeneous rim enhancement in the T1-weighted sequence [8]. Discordance between radiologic and histologic diagnoses may lead to the misdiagnosis of local lesions and inadequate determination of the systemic treatment protocols [7,9,10].
To the best of our knowledge, there have been few reports about the true pathology of radiation-changed lesions.
The study retrospectively analyzed the pathologic and clinical outcomes of TRICs and progressive disease (PD) in metastatic tumors that were surgically resected due to clinical worsening after SRS, therefore, aimed to elucidate the clinical implications of TRICs which are not always equivalent to RN.

Study cohort
Between April 2009 and May 2019, the medical records of 211 patients who underwent surgical resection after gamma knife radiosurgery for BMs at our institute were retrospectively reviewed.
Approval was obtained from the SMC Institutional Review Board. Patients with GKS dosimetry reports, advanced MRI sequences to discriminate TRICs and PD, and su cient clinical follow-ups using MRI for at least 6 months after surgery were included. We excluded patients with other surgical pathologies, acute intracranial hemorrhage or abscess, and preoperative GKS as adjuvant therapy. Patients who survived less than 6 months postoperatively were also excluded.

Radiosurgical treatment
Radiosurgery was performed with the Leksell Gamma Knife® Perfexion™ from 2009 to 2015 or the Leksell Gamma Knife® Icon™ (both Elekta AB, Stockholm, Sweden) from April 2016 onwards. The 1.0-mm slices of T1-weighted and 2.0-mm slices of T2-weighted uid-attenuated inversion recovery (FLAIR) contrast-enhanced MR images were obtained and transferred to the Leksell GammaPlan Software version 11.1.1 (Elekta AB). The target volume was de ned as the contrast-enhancing tumor volume, and the dosimetry planning was carried out in accordance with the RTOG (Radiation Therapy Oncology Group) 90 − 05 study guidelines [11,12].
For post-GKS lesions that exhibited an increased extent of T1-enhancement with FLAIR hyperintensity in follow-up MRI, tumor progression was suspected if a solid enhancement pattern, relatively high cerebral blood volume (CBV), and more restricted diffusion were observed. TRICs were preferably diagnosed through a "soap bubble" or "Swiss cheese" sign of enhancement, low or constant CBV, and less restricted diffusion [7,10,[13][14][15].
Indication for surgical resection and postoperative follow-up After GKS, surgical resection was planned if neurological de cits or symptoms of increased intracranial pressure (IICP) presented in patients with Karnofsky performance scale (KPS) score of > 60, and more than 3 months of life expectancy. All outpatients were prescribed oral steroids, and intravenous steroids and mannitol were administered to patients readmitted for urgent surgery to control the IICP and relieve the neurological symptoms and signs related to the lesion mass effect. No patient was treated with bevacizumab for RN. Patients underwent routine outpatient follow-up 1 month after surgery and were further evaluated at 3-month intervals thereafter, with the data reported up to July 2020.

Statistical analysis
Patient demographics and clinical data were summarized using standard descriptive statistics and frequency tabulation. If statistically signi cant results indicated non-equal distribution, post hoc tests were used to identify the signi cant differences between the groups. Overall survival (OS) of patients was calculated from the time of resection until death or last follow-up. Progression-free survival (PFS) was de ned as the time from the surgical resection to the diagnosis of local progression of the resected lesion. Survival probability was estimated using the Kaplan-Meier product-limit method.
Univariable and multivariable analyses were performed using a binary logistic regression model to identify the probability factors for local progression and survival.
Statistical signi cance was determined when the p-value was less than 0.05. SAS version 9.4 (SAS Institute, Cary, NC) was used for all statistical analyses.

Patient classi cation
A total of 86 patients were included in the nal analysis. Based on the radiological and surgical pathology, patients were classi ed into three groups (Fig. 1). For preoperative imaging, TRIC was diagnosed in 54 (62.7%) patients and PD in 32 (37.3%). Among the TRIC patients, the surgical pathology con rmed RN in only 19 of the 54 (35.2%) patients, whereas the other 35 (64.8%) patients revealed mixed (11 of 35, 31.5%) or pure metastatic (24 of 35, 68.5%) histology, represented as the TRIC-RN and TRIC-PD groups, respectively. The 32 cases of MRI-diagnosed tumor progression were all histopathologically con rmed as tumor progression and included in the PD-PD group.

Patient demographics and lesion characteristics
The preoperative patient characteristics and treatment parameters are shown in Table 1. The median age of the entire study population was 58 years (range: 40-81 years). Most metastatic lesions were located in the frontal (40, 46.5%) and parietal (22,25.6%) regions with similar proportions in all three groups (p = 0.754). Sixteen patients underwent fractionated radiosurgery due to prolonged beam-on-time of more than 2 hours. There was a mean isodose of 24 Gy in three fractions for 14 patients, and 18 Gy in two fractions and 30 Gy in four fractions for one patient each.  (3), colorectal cancer (4), melanoma (2), hepatocellular carcinoma (2), cholangiocarcinoma, endometrial carcinoma, thyroid cancer GKS = gamma knife radiosurgery; PD = progressive disease; PIV = prescription isodose volume; RN = radiation necrosis; TRIC = treatment-related image change; WBRT = whole-brain radiation therapy.
There were signi cant differences in GKS history among the three groups (p = 0.005). A much higher percentage of patients (63.1%) in the TRIC-RN group had a history of prior GKS, whereas 9 (25.7%) had it in the TRIC-PD group and 7 (21.6%) in the PD-PD group (Jonckheere-T erpstra test=-2.68, p = 0.007). No signi cant difference was observed for history of whole brain radiation therapy.
No major intraoperative complications were observed. One patient had newly developed hemiplegia in the immediate postoperative status, a predictable complication due to the location of the tumor in the thalamus.

Survival analysis
The median OS for the 86 patients was 21.7 months (95% con dence interval [CI]: 12.9-30.6 months). Kaplan-Meier estimates and median OS of each group are shown in Fig. 2. TRIC-RN patients displayed a tendency toward longer survival than patients of the pathologic PD group: 1-year and 2-year actuarial survival was 82.4% and 75.5% in the TRIC-RN group, respectively, compared to 63.2% and 41.5% in the TRIC-PD and 72.9% and 27.5% in the PD-PD group, respectively, although statistical signi cance was not observed (p = 0.067).
Forty-one patients (47%) exhibited progression of the resected lesion. Local recurrence was con rmed using diagnostic MRI in 7 (36%), 11 (47%), and 23 (71%) patients of the TRIC-RN, TRIC-PD, and PD-PD groups, respectively. The median PFS was obtained only for the PD-PD group (5.4 months [95% CI: 2.7-8.0 months]). There was a signi cant difference in PFS among the three groups (Fig. 3, p < 0.0001). In the post hoc test, the PD-PD group exhibited a shorter time to progression than the other two groups (p = 0.001 vs. TRIC-RN and p = 0.002 vs. TRIC-PD), whereas no difference was observed between the TRIC-RN and TRIC-PD groups (p = 1.0) (Fig. 3).

Prognostic factors for survival and disease progression
In the univariate and multivariate analyses, a signi cant association was observed between the radio-surgical pathologic group and lesion progression (p = 0.0002 and p = 0.0016 in the univariate and multivariate analyses, respectively); there was a higher risk of recurrence in the PD-PD group, with a hazard ratio (HR) of 4.11 (95% CI: 1.73-9.79, p = 0.0002) in the univariate analysis and HR of 5.46 (95% CI: 1.86-16.06, p = 0.002) in the multivariate analysis, compared to that in the TRIC-RN group (HR = 1, reference). The risk of progression was not signi cantly different between the TRIC-RN and TRIC-PD groups (p = 0.535) (

Illustrative case
A 73-year-old male patient with non-small cell lung cancer was diagnosed with a 2.9-cm brain metastatic lesion in the left temporo-occipital junction (Fig. 4, A). GKS with an initial prescription dose of 20 Gy at 50% isodose line (IDL) was performed. Due to suspicion of tumor progression at 11 months after the rst session, a secondary prescription dose of 17 Gy at 50% IDL was administered (Fig. 4, B, IDL of the second GKS [yellow] and the previous session [blue]). After 9 months from the last GKS, the patient visited the emergency room with IICP signs, and MRI showed a lesion suspicious of TRIC with no distinct increase in CBV and absence of restricted diffusion (Fig. 4, C-E). Surgical resection was conducted, and the IICP improved. The pathology was con rmed as a mixture of tumor cells and RN (Fig. 5). Without adjuvant local therapy for the resected lesion, the patient was free of local progression for 15 months (Fig. 4, F).

Discussion
In this study, we investigated the clinical implications of TRIC-PD representing patients with discrepant radiopathology and surgical histopathology. Metastatic lesions featured as TRICs may worsen the neurological de cits and increase the frequency and cost of imaging to differentiate them from tumor progression [16][17][18].
A recent meta-analysis by Chuang et al. [14] investigated perfusion-weighted imaging (PWI) and magnetic resonance spectroscopy (MRS) for differentiating recurrent tumors from necrosis in patients with various brain tumors. Of the 397 patients in 13 studies, 95 patients had BM. The meta-analysis revealed that the relative CBV derived from PWI, as well as MRS metabolite ratios in contrast-enhancing lesions, were signi cantly different in local progression compared to lesions resulting from radiation-related changes. Previous studies also reported variable diagnostic performance of PWI in sensitivity (range: 56-100%), speci city (range: 68-100%), and relative CBV threshold (range: 1.52-2.14) [13,19,20].
MRS appears to provide high speci city for detecting tumor recurrence (almost 100%). However, the relatively low sensitivity (range: 33-50%) and limited application for small-sized lesions (< 2 cm 3 ) are its two major drawbacks [13,21]. The potential bene t of amino acid PET for differentiating pseudoprogression or RN from true disease progression following checkpoint inhibitor treatment of BM has been suggested, although the evidence is preliminary [22].
In our study, most TRIC or PD lesions, which worsened neurological de cits, were usually of large sizes with median volume of 6.41 cm 3 (range: 0.14-62.29 cm 3 ). Thus, we primarily utilized only PWI plus standard MRI protocols for the preoperative imaging to achieve the diagnostic accuracy and time-cost bene t in terms of determining urgent surgical resection.
With regard to clinical characteristics and prognosis, TRIC-PD group was compared to TRIC-RN or PD-PD groups. The TRIC-PD group exhibited a shorter time (median: 4.07 months) than the PD-PD group (median 8.77 months) from the last GKS session to the preoperative imaging diagnosis. Recent studies reported a local control rate in small-to-medium (1-3 cm)-sized BM lesions treated with 18 Gy of 87-93% at 6 months and 49-91% at 12 months. [16,26] According to our results, some cases of early histologic local progression may have been overlooked due to the radiographic RN diagnosis. This would lead to a lower 6-month local control rate.
According to our survival analysis, the TRIC-RN group seems to have a better OS than the TRIC-PD and PD-PD groups, although statistical signi cance was not observed. For the risk of tumor recurrence after surgical resection, only the PD-PD group was signi cantly higher than other two groups in both the univariate and multivariate analyses. These results suggest a relatively benign prognosis of the TRIC-PD group compared to that of the PD-PD group, regardless of the same histologic tumor progression. Rather it shows a benign prognosis as the "pure RN" (TRIC-RN group), especially in aspect of local recurrence.
This study has several limitations. It is a retrospective single-center study with a small cohort. Further radiomicsoriented analysis is required for a more precise differentiation of TRICs and PD based on MR images, in terms of clinical prognosis. We did not include systemic therapies such as chemotherapy and immunotherapy, one of the possible risk factors for RN, due to the heterogeneity of individual therapeutic regimens.
In conclusion, notable histologic heterogeneity of metastatic lesions with TRICs after radiosurgery was observed, represented as the TRIC-PD group in this study. TRICs may be pure RN or more frequently mixed pathology of necrosis and viable tumor tissue. The postoperative clinical prognosis of patients with TRIC-PD was similar to that of the TRIC-RN group, and more indolent than that of the PD-PD group. These ndings suggest that the imaging diagnosis of TRICs in advanced MRI may serve as a prognostic factor, regardless of their histologic heterogeneity.

Declarations
Funding: Not applicable.
Con icts of interest/Competing interests: Figure 1 Patient classi cation. A total of 86 patients were initially classi ed into four groups according to preoperative MRI and surgical pathology. For surgical pathology, only pure radiation necrosis was sorted into "radiation necrosis." Mixed pathology, containing both metastatic tumor and necrotic cells, was classi ed as "metastatic cells." No patient was assigned to the PD-RN group; therefore, the three groups TRIC-RN, TRIC-PD, and PD-PD were nally included in the present study MRI = magnetic resonance imaging; PD = progressive disease; RN = radiation necrosis; TRIC = treatment-related image change Representative histopathologic features of TRIC-PD. Adjacent to a large, con uent necrotic portion (Right), an area with viable tumor cells (Left) was present, rendering a diagnosis of metastatic adenocarcinoma. (hematoxylin and eosin stain, x100, scale bar=500 μm)