Background

No evidence of survival benefit from radiotherapy as curative treatment for PDAC

Pancreatic ductal adenocarcinoma (PDAC) is one of the most severe malignancies among all solid tumours, with a 5-year survival rate of less than 10% [1, 2]. Most patients with PDAC present with locally advanced pancreatic cancer (LAPC) or metastatic disease that is not suitable for resection [3]. Chemotherapy, radiotherapy, and modern targeted, immunologic therapy exhibit limited efficacy in treating PDAC. Therefore, patients with PDAC usually experience rapid recurrence in the form of locally destructive diseases or distant metastasis [4, 5].

The development of combination chemotherapy consisting of (modified) leucovorin calcium (folinic acid), fluorouracil, irinotecan hydrochloride, oxaliplatin (FOLFIRINOX) [6, 7], and gemcitabine plus nab-paclitaxel (GEM-Nab) [8] has resulted in superior tumour response and survival compared with chemotherapy using single- agent GEM or 5-fluorouracil (5FU) in patients with metastatic or unresectable PDAC. Prospective randomized trials have demonstrated the overall survival (OS) benefit of adjuvant chemotherapy using FOLFIRINOX (54.4 vs. 35.0 months, p = 0.003) [9], GEM plus capecitabine (GEM-Cape; 28.0 vs. 25.5 months, p = 0.032) [10], or GEM plus nab-paclitaxel (41.8 vs. 37.7 months, p = 0.009) [11] compared with using single-agent GEM to treat resected PDAC. For borderline resectable PDAC, neoadjuavant chemotherapy achieves a higher R0 resection rate and survival than does upfront surgery [12,13,14]. A meta-analysis of seven trials with 938 patients revealed significantly improved OS using neoadjuvant therapy (29 vs. 19 months, p = 0.001), especially among patients with borderline resectable PDAC (p = 0.004) [15].

Unlike that of chemotherapy for PDAC, the efficacy of radiotherapy as an adjuvant or curative treatment for PDAC is limited. The results of the European Study Group for Pancreatic Cancer-1 (ESPAC-1) trial led to the omission of radiotherapy from most European adjuvant trials involving resectable PDAC [16]. We conducted a prospective randomised study to evaluate chemo-radiotherapy (CRT) with adjuvant 6-month GEM. The results indicated improved local control (loco-regional recurrence rate of GEM vs. GEM-CRT arms: 54.1% vs. 38.4%, p = 0.056) but no survival benefit (median OS of GEM vs. GEM-CRT: 23.5 vs. 21.5 months, p = 0.73 ) from administering additional CRT to patients with curatively resected PDAC [17]. The results of the Radiation Therapy Oncology Group (RTOG) 0848 study evaluating adjuvant CRT in resected PDAC after adjuvant GEM are highly anticipated [18]. However, the impact of RTOG 0848 may be less relevant because FOLFIRINOX and GEM-Cape have become the standard of care for adjuvant chemotherapy [9, 10]. For borderline resectable PDAC, the PREOPANC-1 study [13, 14] demonstrated long-term survival improvement (median OS: 15.7 vs. 14.3 months, p = 0.025; 5-year survival rate: 20.5% vs. 6.5%) with neoadjuvant GEM-based CRT and improved loco-regional control (p = 0.004) compared with adjuvant GEM alone. The ESPAC-5 [19] and A021501 [20] studies have demonstrated extended survival with neoadjuvant chemotherapy especially using FOLFIRINOX in ESPAC-5 (1-year survival rate: 84% vs. 39% for immediate surgery, p = 0.0028). Despite the high R0 resection and pathologic complete remission rate, neoadjuvant radiotherapy was not associated with favourable survival in either study. For LAPC, the LAP07 study [21] identified better local control (46% vs. 32%, p = 0.03) but no survival benefit (11.9 months vs. 13.6 months, p = 0.09) from the addition of CRT after induction GEM. These results conflict with the report from the Eastern Cooperative Oncology Group trial, which indicated a survival benefit from upfront GEM-based CRT compared with GEM alone (11.1 vs. 9.2 months, p = 0.017) [22]. The conflicting results of the randomized studies concerning borderline resectable and locally advanced PDAC imply a narrow therapeutic window associated with radiotherapy.

Reasons of continued evaluation of radiotherapy for curative PDAC treatment

The role of CRT has been questioned because of controversial clinical trial results. However, CRT remains under careful consideration for PDAC for several reasons: First, the survival outcomes of PDAC remain inferior compared to those of other solid tumours. Novel therapeutic options and modern techniques including stereotactic body radiotherapy (SBRT), magnetic resonance (MR) imaging guided radiotherapy and proton therapy enabled highly conformal and tolerable radiation to be given with solutions for respiratory motion and reduced toxicity to the gastrointestinal area [23, 24]. The Massachusetts General Hospital group demonstrated total neoadjuvant therapy with eight cycles of FOLFIRINOX and losartan, an inhibitor of thrombospondin-1 mediated activation of latent tumour growth factor β (TGFβ), followed by a short or long course of modern radiotherapy for 49 patients with LAPC resulted in a high rate of down-staging and R0 resection in 61% of patients, with a median progression-free survival (PFS) and OS of 17.5 and 31.4 months, respectively [25]. Ablative radiotherapy of 75 Gy in 25 fractions was administered to 119 patients with inoperable PDAC following multiagent induction chemotherapy at Memorial Sloan Kettering Cancer Center. The retrospective analysis revealed safe and durable local control with a median OS of 26.8 months [26]. These studies may influence and inspire current standard approaches. Second, the margin positivity rate and locoregional recurrence rate are high in PDAC, despite radical surgery and intensive systemic chemotherapy [9, 10, 27]. A rapid autopsy study indicated that one-third of patients with PDAC die from local destructive disease without widespread distant metastasis [28]. The efficacy of locoregional control and palliation by radiotherapy has been demonstrated in most studies of PDAC. Jolissaint et al. compared the clinical outcomes of patients with PDAC receiving ablative radiotherapy (n = 104) or surgical resection (n = 105). Despite a selection bias favouring the surgical group, the incidence of locoregional recurrence was similar (16% vs. 21%, p = 0.252) [29]. The excellent locoregional outcomes achieved using modern radiotherapy should be integrated into multimodality treatment of PDAC. Third, the survival benefit of CRT has been demonstrated after exclusion of patients with PDAC with early progression. In the PREOPANC study [14], a significant survival benefit was demonstrated for CRT after long term follow-up (p = 0.025). The steep initial slope of the survival curve, representing early progression, starts to bend and clearly separate from that of patients not receiving CRT after a year from diagnosis, indicating a small difference in median survival (1.4months; 15.7 vs. 14.3 months) between the groups; 5- year survival exhibited a 14% difference (20.5% vs. 6.5%). These results are consistent with the general consensus to prescribe CRT after initial systemic treatment. Accordingly, selecting patients with PDAC with low risk of early disease progression is crucial to translate local control using CRT into a survival benefit.

This review highlights the role of biomarkers in predicting patients with PDAC with low risk of early progression and who are thus suitable for being considered for subsequent radiotherapy with or without concomitant chemotherapy. A biomarker is a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes or pharmacologic responses to therapeutic intervention [30].

Potential biomarkers for identifying patients with PDAC suitable for radiotherapy

Imaging biomarkers

Radiomics, refers to the extraction and analysis of numerous quantitative features from medical images, and it has shown early promise in the analysis of imaging features and in prognostic modeling and outcome analysis [31]. The baseline imaging textural profile of the tumour microenvironment, including vascularity and oxygenation, and tumor heterogeneity was correlated with pathologic and clinical outcomes in resected PDAC (Table 1). Radiomic features derived from textural signals and groupings of pixels of baseline contrast-enhanced computed tomography (CT) in resectable PDAC were demonstrated to predict OS after surgery [32]. The signal intensity multiplied by the contour volume of pancreas was inversely associated with the pathologic lymph node category and correlated with the OS and PFS of patients with resected PDAC [33]. A seven-feature radiomic signature of a contrast-enhanced CT simulation scan could predict locoregional recurrence in patients with PDAC receiving SBRT [34]. Blood perfusion of tumor from CT scans was correlated with fractional tumour cell death in PDAC. The normalised area under the enhancement curve (nAUC) was correlated with OS and response to CRT patients with borderline resectable PDAC and LAPC [35]. These studies demonstrated baseline CT to be a potential tool for predicting the clinical outcomes of PDAC. If further validated, the signature could be used to help select patients who may benefit from neoadjuvant or adjuvant CRT.

Table 1 Studies of potential radiomic biomarkers for PDAC patients considering radiotherapy
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CT imaging profiles after upfront chemotherapy for PDAC are associated with clinical outcomes. A more defined interface response of tumor post chemotherapy was associated with prolonged OS among patients with borderline resectable or locally advanced PDAC [36]. Four radiomic features from simulation CT scans were selected to construct a model to predict resectability in LAPC after neoadjuvant CRT [37]. Radiomic signatures indicating the relationship between tumours and key arteries from CT for radiotherapy treatment planning predicted local control, resectability and OS for borderline resectable and locally advanced PDAC cases after systemic chemotherapy [38, 39]. Patients’ longitudinal radiomic data progress throughout treatment (delta-radiomics) were able to help assess treatment response earlier and more reliably [40]. Yamamoto et al. established a logistic growth pattern of PDAC and defined the Local Advancement Index (LAI) to determine eventual primary tumour size and predict the number of metastases; a smaller LAI value indicates a larger metastatic burden. Radiotherapy after induction chemotherapy improved the survival of patients with larger LAI values [41]. The subgroup of patients with PDAC suitable for consolidative CRT after upfront or induction chemotherapy may be differentiated using potential radiomic parameters developed after chemotherapy.

Furthermore, diffusion-weighted MR quantitative metrics after chemotherapy were demonstrated to indicate response of patients with PDAC to chemotherapy [42]. Collagen molecular imaging using selective MR enhancement of fibrosis with CM-101, a type I collagen-targeted probe, revealed a robust fibrotic response after neoadjuvant therapy of FOLFIRINOX and correlated with improved survival in murine model of PDAC receiving CRT [43]. The preoperative uptake value of fluoro-deoxyglucose positron emission tomography (FDG-PET) and metabolic response to neoadjuvant therapy could predict the OS of patients with PDAC [44,45,46,47,48].

The ability of radiomic signatures to provide superior information for evidence-based clinical decision-making regarding PDAC is promising. To select patients who will benefit from radiotherapy, potential radiomic signatures should be explored in prospective clinical trials and validated through expansion of the available dataset, preferably in a multi-institutional study. Standardisation of radiomic signatures and imaging modalities to reduce inter-observer variability is also necessary.

Histopathologic, liquid and clinical biomarkers

Molecular classifications of PDAC based on genomic, transcriptomic, proteomic and epigenetic data have provided considerable insights into the molecular heterogeneity and aggressive biology of PDAC [49]. Several potential biomarkers have been demonstrated to enable differentiation of the failure patterns in patients with PDAC. (Table 2) SMAD4 gene status and expression have been highly correlated with radiosensitivity and the initial failure site of PDAC in clinical and preclinical studies [28, 50, 51]. In a phase II prospective study of 69 patients with LAPC, a local dominant pattern of progression was identified in patients with intact SAMD4 and not in those with SMAD4 loss (73% vs. 28%, p = 0.016) [52]. A retrospective study of 641 patients with resected PDAC demonstrated that inactivated SMAD4 was strongly associated with metastatic recurrence (hazard ratio (HR) = 4.28, 95% CI = 2.75–6.68). Improved survival with additional radiotherapy was observed only in patients with PDAC with SMAD4 expression (p = 0.002). The investigators concluded that patients with SMAD4 expression benefit more from intensive local control [53]. Whittle et al. further demonstrated that heterozygous mutation of SMAD4 attenuated the metastatic potential of PDAC and increased its proliferation. Loss of the heterozygosity of SMAD4 restored metastatic competency and further increased proliferation– a highly lethal combination. The authors further demonstrated that RUNX3 responded to and interacted with SMAD4 status to regulate the balance between cancer cell division and dissemination, and they suggested that RUNX3 and SMAD4 levels can be used together to inform clinical decision-making for resectable PDAC [54]. Krüppel-like factor 10 (KLF10), a TGFβ early-response gene, has been demonstrated by investigators, including us, to contribute to PDAC radiosensitivity, epithelial - mesenchymal transition, and cancer stemness and progression [55,56,57]. We evaluated potential biomarkers including SMAD4, RUNX3 and KLF10 in tumour tissues from 111 patients with resected PDAC randomised to adjuvant GEM with or without CRT [58]. Loss of both SMAD4 and KLF10 expression in patients with curatively resected PDAC was associated with rapid development of distant metastasis; those who expressed either SMAD4 or KLF10 had a significantly higher chances of benefiting from adjuvant CRT (for patients with KLF10 or SMAD4 expression: GEM–CRT vs. GEM: PFS ∞ vs. 19.8 months; p = 0.026; OS 33 vs. 23 months; p = 0.12) [58]. The tryptophan catabolic enzyme, indoleamine 2,3 dioxygenase-2 (IDO2) has been demonstrated to promote pancreatic tumourigenesis in preclinical studies [59]. An IDO2-deficient genotype correlates with improved PFS for patients with PDAC who received adjuvant radiotherapy (39.0 ± 6.3 vs. 74.1 ± 6.4 months, p = 0.023). Analysis of metabolic profiles from patients with resectable PDAC receiving neoadjuvant therapy demonstrated a significant difference in choline metabolism between those responding favourably and unfavourably. Lower levels of choline and phosphocholine correlated with a low recurrence rate among patients with PDAC receiving neoadjuvant CRT [60]. Genomic profiling using targeted gene sequencing for radiotherapy response prediction was evaluated among 88 patients with cancer receiving local tumour irradiation. Alterations of DNA repair pathways and mutations of CHEK2, MSH2 and NOTCH1 were associated with durable local control using radiotherapy [61]. A radiation sensitivity index (RSI) score for intrinsic tumour radiosensitivity derived from the expression of 10 specific genes (HDAC1, PKCb, RelA, c-Abl, STAT1, AR, Cdk1, c-Jun, SUMO1, and IRF1) and a linear regression algorithm modeled on the surviving fraction at 2 Gy (SF2) of 48 cancer cells were evaluated for 73 patients with PDAC receiving surgery with or without radiotherapy. Among high-risk patients, radiotherapy provided significantly improved survival among radio-sensitive patients compared with radio-resistant patients (p = 0.04). This difference was not observed among low-risk patients [62]. The RSI score was combined with the linear quadratic model to derive a genomic-adjusted radiation dose (GARD) by the same group of investigators to identify the optimum radiotherapy dose at a patient-specific molecular signature level. A high GARD value predicted a strong therapeutic effect of radiotherapy and greater time to first recurrence and OS. GARD independently predicted clinical outcomes for pancreatic cancer, and its use enabled the individualization of radiotherapy dose according to the tumour radiosensitivity [63, 64].

Table 2 Studies of potential tissue biomarker for PDAC patients considering radiotherapy
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Several peripheral blood biomarkers have been demonstrated to determine survival or therapeutic response in PDAC (Table 3). Absolute monocyte count during CRT and changes in the lymphocyte-to-monocyte ratio correlated with OS and PFS among patients with LAPC treated with CRT [65]. The baseline neutrophil-to-lymphocyte ratio (NLR) and NLR dynamics during neoadjuvant chemotherapy were independently associated with pathologic response in resectable PDAC [66]. Despite not being specific to a cancerous condition and a lack of expression in 5 -10% of patients, CA19-9 is the most used tumour marker for monitoring therapy for PDAC. A decrease in the CA19-9 level after neoadjuvant therapy is correlated with improved OS and pathologic major response in PDAC [67,68,69]. We analyzed CA19-9 change during adjuvant chemotherapy among 125 patients with resected PDAC with or without adjuvant radiation. Significant correlations of CA19-9 response with initial failure at distant sites and OS were identified. However, neither postoperative CA19-9 level nor CA19-9 response were helpful in identifying patients who may experience a survival benefit from additional adjuvant CRT [70]. A retrospective analysis reported that a high level of carcinoembryonic antigen but not CA19-9 before neoadjuvant CRT was the most significant predictor of poor survival after surgery for PDAC [71]. Regarding other circulating biomarkers, baseline CC motif chemokine ligand 5 (CCL5) was identified as an independent prognostic biomarker for OS in patients with LAPC in the Selective Chemoradiation in Advanced Localised Pancreatic Cancer (SCALOP) study, which evaluated induction GEM-Cape and CRT [72]. A correlation between CCL5 levels and failure patterns was not identified. Increasing evidence indicates that microRNAs (miRNAs) may serve as diagnostic, predictive and prognostic biomarkers in various cancer entities, including PDAC. The expression of miRNAs was correlated with pancreatic cancer progression and radio-resistance [73]. A four-miRNA molecular signature (miR-29c, miR-125a, miR-155, and mR-200b) was developed to predict risk of locoregional recurrence and OS after PDAC resection. Using the miRNA risk score has potential for identifying patients with PDAC who are most likely to benefit from postoperative CRT [74]. Circulating tumor DNA (ctDNA) is released into the peripheral blood stream during cell death. The presence of ctDNA in patients with PDAC after neoadjuvant therapy indicates recurrence and poor survival [75, 76]. Circulating tumour cells that enter peripheral blood are thought to contribute to metastatic disease with worse survival [77]. In an analysis of the Surveillance, Epidemiology, and End Results database, patients with PDAC with a tumour location over the pancreatic head, stage II/III cancer, T4 cancer, N1 cancer, regional resection, or lymphadenectomy of ≥ 4 lymph nodes were demonstrated to benefit from adjuvant radiotherapy [78, 79]. Several studies have revealed that a combined analysis of radiomic features, clinical parameters, pathology score, and tissue/serum biomarkers improves the prognostic power of clinical outcomes in PDAC [32, 80].

Table 3 Studies of potential peripheral blood biomarkers for PDAC patients considering radiotherapy
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Conclusions

Despite progress in surgical techniques and systemic therapy, the survival outcomes of patients with PDAC remain unsatisfactory. Radiotherapy was a central component of treatment for PDAC. The value of CRT to PDAC has been questioned because of conflicting results of clinical trials. Most studies have been criticised for low patient numbers, poor study design, inappropriate radiation doses or split-course regimens, and poor adherence to the radiation protocol [81,82,83]. However, several prospective trials have demonstrated the efficacy of modern radiation therapy, with an elevated dosage and reduced toxicity to the small bowel, exhibiting a satisfactory safety profile, local control, and prolonged survival for localised PDAC [25, 26]. In addition to the technical improvement of radiotherapy, the development of radiogenomics and the biology of radiotherapy for PDAC may help to optimise the integration of radiotherapy in multimodality PDAC treatment strategies. Because distant metastases are more effectively controlled through modern systemic therapy, local control of the primary site is increasingly critical for patients with PDAC with extended survival [23]. Advances in radiomic, tissue, or peripheral biomarkers may enable superior stratification of patients’ metastatic potential and prediction of those who would most likely benefit from enhanced locoregional therapy. However, studies evaluating the role of potential biomarkers have mostly been retrospective and have demonstrated correlations with survival but not failure patterns. Multi-institutional prospective clinical trials that validate candidate biomarkers in patients with PDAC receiving up-to-date systemic chemotherapy with or without modern radiotherapy are urgently required.

The role of radiotherapy in the curative treatment of PDAC remains unclear. In designing future clinical trials, the exclusion of patients with early distant progression by extended systemic therapy (≥ 4 months) and predictive biomarkers is reasonable. Local control using radiotherapy may yield a survival benefit, especially among patients with PDAC without early distant metastasis.