Metabolic response as assessed by 18F‐fluorodeoxyglucose positron emission tomography‐computed tomography does not predict outcome in patients with intermediate‐ or high‐risk rhabdomyosarcoma: A report from the Children's Oncology Group Soft Tissue Sarcoma Committee

ABSTRACT Background Strategies to optimize management in rhabdomyosarcoma (RMS) include risk stratification to assign therapy aiming to minimize treatment morbidity yet improve outcomes. This analysis evaluated the relationship between complete metabolic response (CMR) as assessed by 18F‐fluorodeoxyglucose positron emission tomography‐computed tomography (FDG‐PET) imaging and event‐free survival (EFS) in intermediate‐risk (IR) and high‐risk (HR) RMS patients. Methods FDG‐PET imaging characteristics, including assessment of CMR and maximum standard uptake values (SUVmax) of the primary tumor, were evaluated by central review. Institutional reports of SUVmax were used when SUVmax values could not be determined by central review. One hundred and thirty IR and 105 HR patients had FDG‐PET scans submitted for central review or had SUVmax data available from institutional report at any time point. A Cox proportional hazards regression model was used to evaluate the relationship between these parameters and EFS. Results SUVmax at study entry did not correlate with EFS for IR (p = 0.32) or HR (p = 0.86) patients. Compared to patients who did not achieve a CMR, EFS was not superior for IR patients who achieved a CMR at weeks 4 (p = 0.66) or 15 (p = 0.46), nor for HR patients who achieved CMR at week 6 (p = 0.75) or 19 (p = 0.28). Change in SUVmax at week 4 (p = 0.21) or 15 (p = 0.91) for IR patients or at week 6 (p = 0.75) or 19 (p = 0.61) for HR patients did not correlate with EFS. Conclusion Based on these data, FDG‐PET does not appear to predict EFS in IR or HR‐RMS. It remains to be determined whether FDG‐PET has a role in predicting survival outcomes in other RMS subpopulations.


| INTRODUCTION
Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma (STS) in children and adolescents with approximately 350 cases documented in the United States each year. 1 Clinical features present at diagnosis can stratify patients into low-, intermediate-, and high-risk groups to predict outcome and modulate treatment intensity. 2,3 Patients with low-risk and intermediate-risk (IR) RMS have a relatively favorable long-term event-free survival (EFS) of approximately 90% and 60%, respectively, whereas patients with high-risk (HR) disease have poor outcomes with a 5-year EFS of approximately 30%. [4][5][6] Recent studies to improve outcomes in RMS have explored novel ways of stratifying risk to identify patients that may require more intensive or novel therapies. [6][7][8][9][10] Response to induction chemotherapy is a well-established predictor of survival in many pediatric cancers, including acute lymphoblastic leukemia, Hodgkin lymphoma, osteosarcoma, and Ewing sarcoma. [11][12][13][14][15][16] Both radiographic and histologic response have been used to adapt treatment intensity in an overall effort to minimize toxicity for lower risk patients and escalate therapy for higher risk patients. Radiographic response to treatment (as measured by change in tumor size), however, has not proven to be a reliable predictor of outcome in RMS, limiting the ability to identify patients who could benefit from more intensive or novel therapy. [17][18][19] Metabolic activity as assessed by 18 F-FDG-PET/CT (FDG-PET) has recently been shown to improve accuracy of staging in pediatric RMS. 20,21 Preliminary data from a single institution suggested that a complete metabolic response (CMR) as assessed by FDG-PET following radiation therapy for local control may predict local relapse-free survival (LRFS) in pediatric patients with Group III RMS. 22 A follow-up analysis by the same group confirmed these findings in a larger cohort of RMS patients showing that metabolic response by FDG-PET predicted EFS, overall survival, and local tumor control. 23 The predictive value of FDG-PET response, however, has not been evaluated in a prospective multi-institutional RMS clinical trial.
In this analysis, we present the FDG-PET imaging data from two large, prospective Children's Oncology

Lay summary
This manuscript reports the 18

| Patient population
Details of the design, eligibility criteria, treatment, and outcome for ARST0531 (NCT00354835) and ARST08P1 (NCT01055314) have been previously published. 5,7 ARST0531 enrolled newly diagnosed patients with IR RMS defined as patients with non-metastatic (Group I-III) alveolar RMS arising at any site (Stage 1-3) and incompletely excised (Group III) embryonal RMS arising in an unfavorable site (Stage 2-3). Patients received 42 weeks of either VAC alone or alternating cycles of VAC with VI. Only patients with measurable disease at study entry (Clinical Group III) were included in this analysis. Radiation therapy (50.4 Gy in 1.8 Gy fractions for Clinical Group III patients) was given at week 4 of therapy. Patients enrolled on both treatment arms were combined for the purpose of this FDG-PET analysis, as there was no statistical difference in EFS between the two treatment arms. 5 FDG-PET imaging was optional for all patients and was recommended prior to chemotherapy, at week 4, and at week 15. ARST08P1 enrolled newly diagnosed patients with HR RMS defined as patients with metastatic (Stage 4/ Group IV) alveolar or embryonal RMS. Patients were treated with a multi-agent chemotherapy backbone which included two cycles of VI (Weeks 1-6), followed by six cycles of alternating interval compressed vincristine, doxorubicin, and cyclophosphamide (VDC) and ifosfamide and etoposide (IE) (weeks [7][8][9][10][11][12][13][14][15][16][17][18][19]. Two VI cycles were repeated at weeks 20-25 and weeks 47-51. Interval-compressed VDC/IE cycles were administered again at weeks 28 through 34 followed by four cycles of VAC administered every 3 weeks during weeks 35-46. Patients were enrolled in sequential pilot cohorts to receive the described chemotherapy backbone in conjunction with cixutumumab or temozolomide with the primary aim to evaluate the feasibility of the combination. 7 Radiation therapy was administered at week 20 to the primary tumor as well as to sites of metastatic disease at the discretion of the treating institution. FDG-PET imaging was performed prior to chemotherapy, at week 6, and week 19 (prior to radiation therapy) if clinically indicated and available at the treating institution.

| Measurement of response and definition of endpoints
We evaluated both FDG-PET tumor response and baseline maximum standard uptake values. Two nuclear imaging physicians (MTP and BLS) centrally reviewed FDG-PET response. Response was classified according to European Organization for Research and Treatment of Cancer criteria. 24 CMR was defined as complete resolution of abnormal FDG uptake within the tumor region defined on baseline scan. SUVmax values were assessed by central review or institutional report if central review was not available. SUVmax was determined by manually drawing a region of interest over the area of FDG activity corresponding to the tumor in question. Quality assurance of the treating institution included documentation of: blood glucose level before injection, injected 18F-FDG dose, time of injection, time that image acquisition began, patient weight and height for determination of SUV, and assurance that the DICOM header was intact. Both transverse CT files and transverse 18 F-FDG-PET attenuation corrected files were required.

| Statistical analysis methods
EFS was defined as the time from study enrollment to disease progression, disease recurrence, occurrence of a second malignant neoplasm, or death from any cause. EFS for patients who did not experience disease progression or death was censored at the subject's last contact date. Follow-up is current as of 31 December 2018. The Kaplan-Meier method was utilized to estimate the EFS. 25 EFS was compared between groups using the log-rank test. 26 A Cox proportional hazards regression model was used to evaluate SUVmax and ratio of SUVmax. 27 Software SAS 9.4 ® , was used for the analysis.

| Patient demographics and disease characteristics
Demographic data for the evaluated patient cohort are shown in Table 1 for ARST0531 and ARST08P1. Figure 1 shows the disposition of patients enrolled on ARST0531 and ARST08P1 and the distribution of available FDG-PET imaging data.

| Relationship between complete metabolic response and EFS
Of the 26 IR RMS patients who had FDG-PET imaging available for review at week 4 following one cycle of chemotherapy, a CMR was achieved in three (11.5%). No significant improvement in 3-year EFS was seen in patients who achieved a CMR compared to those who did not at week 4 (3-year EFS 67% vs 52%, p = 0.66). Of the 51 patients who had FDG-PET imaging available for review at week 15 following five cycles of chemotherapy and local radiotherapy, CMR  Figure 2).
Of the 70 HR RMS patients who had FDG-PET imaging available for review at week 6, a CMR was achieved in 24 (34.3%). No significant improvement in 3-year EFS was seen in patients who achieved a CMR compared to those who did not at week 6 (3-year EFS 16% vs 21%, p = 0.75). Of the 69 patients who had FDG-PET imaging available for review at week 19, CMR was achieved in 50 (72.5%). No significant improvement in 3-year EFS was seen in patients who achieved a CMR compared to those who did not at week 19 (3-year EFS 13% vs 31%, p = 0.28; Table 2; Figures 2 and 3).
To determine if change in SUVmax during therapy was associated with EFS, a ratio of SUVmax during therapy to SUVmax at study entry (SUVmax0)

| DISCUSSION
This is the first prospective cooperative group study evaluating whether FDG-PET imaging predicts outcome in RMS patients. We evaluated SUVmax at diagnosis, change in SUVmax with therapy, and CMR during treatment (both early CMR following 4 and 6 weeks of chemotherapy in IR RMS and HR RMS, respectively, and late CMR following 15 and 19 weeks of chemotherapy in IR RMS and HR RMS, respectively); none of these parameters predicted EFS in either IR or HR RMS. We conclude that, at the timepoints we used, FDG-PET is not a useful tool to predict outcome for either IR or HR RMS. Metabolic assessment of tumors using FDG-PET evaluation remains a relatively novel modality for assessing response and how best to incorporate this imaging modality into treatment paradigms is an active area of research. Multiple studies have documented FDG-PET response to be a predictor of outcome in several malignancies including Hodgkin lymphoma, 28 renal cell carcinoma, 29 non-small cell lung cancer, 30 Ewing sarcoma, 31 and osteosarcoma. 32-34 A potential prognostic relevance for metabolic response as assessed by FDG-PET in pediatric RMS is supported by the adult STS literature. 35,36 Adult patients with high-grade STS who attain an early metabolic response as defined by a greater than 26% drop in SUVmax after one cycle of chemotherapy have F I G U R E 2 EFS by CMR vs less than CMR for IR and HR RMS Patients EFS for patients by complete metabolic response vs less than complete metabolic response at Week 4 on ARST0531 EFS for patients by complete metabolic response vs less than complete metabolic response at Week 15 on ARST0531 EFS for patients by complete metabolic response vs less than complete metabolic response at Week 6 on ARST08P1 EFS for patients by complete metabolic response vs less than complete metabolic response at Week 19 on ARST08P1 a significantly improved overall survival (OS). Late metabolic response following chemotherapy significantly correlates with OS in univariate but not multivariate analysis. 36 Moreover, several studies in the STS patient population have shown SUVmax at diagnosis to be a significant predictor of outcome, although these have been limited by small sample size and the inclusion of several differing histologic subtypes and tumor primary sites. [36][37][38] Since RMS is uncommon in adults, it has rarely been represented in STS studies conducted evaluating the utility of FDG-PET. 39 The value of FDG-PET imaging for patients with RMS, thus, remains an open question. In osteosarcoma, the most common primary malignant sarcoma of bone in both the pediatric and adult population, FDG-PET has been shown to be a potential predictor of histologic response following neoadjuvant chemotherapy, a well-documented surrogate for outcome in this disease. [31][32][33][40][41][42] While our results do not support the use of FDG-PET to predict outcome in IR or HR RMS, several studies have documented the value of FDG-PET imaging for disease staging. For example, FDG-PET has been found to have increased sensitivity and specificity compared to conventional imaging including 99m Technetium methylene diphosphanate bone scintigraphy in the identification of distant RMS metastases, particularly bone and distant nodal metastases. 20,21 A retrospective evaluation of FDG-PET showed improved sensitivity and specificity in identifying nodal metastases in RMS when compared with conventional imaging. 43 This finding was confirmed in a recent prospective study. 44 This study, however, documented a low concordance rate between FDG-PET and pathology after tissue biopsy with nodal tissue sampling and thus, tissue sampling should remain the gold standard when defining   44 At present, the data clearly support a role for FDG-PET imaging in the initial staging evaluation of RMS. Our data contrast with prior single institution reports that supported a role for using CMR to predict EFS in patients with Group III RMS following 15 weeks of chemotherapy and radiation therapy for local control. 22,23 The reasons for this discrepancy remain unclear. The prior studies were performed at a single institution where the timing of the FDG-PET imaging and techniques used could be well controlled, as opposed to our cooperative group study which relied on imaging performed at multiple institutions. Furthermore, the ARST0531 study requested FDG-PET imaging at Week 4 immediately prior to radiotherapy and at Week 15, approximately 6 weeks after completion of radiotherapy. Both of these timepoints are significantly earlier than the above single institution studies that delivered radiation therapy at week 15.
This study has several limitations, most notably the small sample size of the patient population available for this analysis-particularly the small number of paired baseline and follow-up studies at each timepoint. FDG-PET imaging was not a requirement for enrollment on either study, and there may have been bias introduced by selective submission of imaging studies, although we could detect no significant differences in the clinical features of the population with and without FDG-PET imaging available for review (data not shown). Several limitations similarly exist regarding the collection of the FDG-PET data. While the guidelines for FDG-PET imaging were specific in both protocols, precise standardization of imaging equipment was not practical due to the large number of institutions involved. 45 Furthermore, while several FDG-PET imaging variables and their relationship to outcome were evaluated, more advanced parameters such as metabolic tumor volume and total lesion glycolysis that were beyond the scope of this analysis were not evaluated although could be considered for future analyses. 46 Finally, FDG-PET imaging was performed at diagnosis, week 4 and week 15 of therapy in ARST0531, and at diagnosis, week 6, and week 19 in ARST08P1. It remains possible that FDG-PET response later in therapy, immediately following local control, or at the conclusion of all planned treatment could potentially be prognostic.
In conclusion, our analysis does not support using FDG-PET as a predictor for outcome in the IR or HR RMS population. Further prospective studies are needed to determine whether this imaging modality has a role in predicting response to therapy or outcome in other RMS subpopulations, and whether FDG-PET imaging at a later timepoint would be more predictive of outcome. Additional research is needed to identify other methods to measure therapy response that can reliably predict outcome in RMS.

ETHICAL APPROVAL STATEMENT
The planning, conduct, and reporting of this research are in accordance with the Helsinki Declaration as revised in 2013 (www.wma.net/polic ies-post/wma-decla ratio n-of-helsi nkiet hical -princ iples -for-medic al-resea rch-invol ving-human subje cts/). The research was approved by institutional review board as per ICJME guidelines.