Hospital Volume for Rectal Cancer Resection Plays a Pivotal Role in Improving Outcomes: A Population-based Analysis.

OBJECTIVE: Assessing the association between hospital surgical volume (SV) and outcomes after rectal cancer surgery (RCS), using national population-based data. SUMMARY BACKGROUND DATA: For RCS, the association of higher SV with improved short- and/or long-term outcomes remains controversial. METHODS: National cancer registry data and administrative data were used to select patients diagnosed with stage I-III rectal cancer in 2009-2018 and who underwent RCS. The average annual SV of RCS was categorised as low (<15; LV), medium (15-29; MV) or high (≥30; HV). The association between SV and 90-day and 1-year excess postoperative mortality (90DPM and 1YEPM) and 5-year observed survival (5YOS) was evaluated. RESULTS: From the 11,519 patients , RCS was performed in LV (4,088; 36%), MV (2,795; 24%) or HV (4,636; 40%) hospitals. Observed 90DPM was significantly better in HV (2.3% 95%CI[1.9,2.8]) than in LV (3.7% 95%CI[3.2,4.4]) and MV (3.5% 95%CI[2.9,4.3]) with adjusted OR 1.4, P<0.0001. Continuous regression analysis showed significantly higher 90DPM in annual SV <35 compared to ≥35 (OR 1.6 95%CI[1.21,2.11]; P=0.0009). Observed 1YEPM was significantly better in HV (2.9% 95%CI[2.2,3.6]) compared to LV (4.7% 95%CI [3.9,5.6]) with adjusted excess HR 1.31 95%CI[1.00,1.73] and P=0.05, and to MV (5.0% 95%CI[4.0,6.1]) with adjusted excess HR 1.45 95%CI[1.09,1.94] and P=0.01. The 5YOS was significantly better in HV (75.9% 95%CI[74.6,77.2]) than in LV (70.3% 95%CI[68.8,71.8]) and MV (71.5% 95%CI[69.7,73.2]) with adjusted HR 1.4 in both LV and MV versus HV, P≤0.003. CONCLUSIONS: This population-based study identified robustly superior outcomes at 90-days, 1-year and 5-years after RCS in hospitals with higher volumes.

Towards a European prospective data registry for particle therapy.

The evidence for the value of particle therapy (PT) is still sparse. While randomized trials remain a cornerstone for robust comparisons with photon-based radiotherapy, data registries collecting real-world data can play a crucial role in building evidence for new developments. This Perspective describes how the European Particle Therapy Network (EPTN) is actively working on establishing a prospective data registry encompassing all patients undergoing PT in European centers. Several obstacles and hurdles are discussed, for instance harmonization of nomenclature and structure of technical and dosimetric data and data protection issues. A preferred approach is the adoption of a federated data registry model with transparent and agile governance to meet European requirements for data protection, transfer, and processing. Funding of the registry, especially for operation after the initial setup process, remains a major challenge.

Progression-directed Therapy in Oligoprogressive Castration-resistant Prostate Cancer: Final Results from the Prospective, Single-arm, Phase 2 MEDCARE Trial.

BACKGROUND AND OBJECTIVE: Next-line systemic treatment (NEST) is the standard of care for patients presenting with progressive metastatic castration-resistant prostate cancer (mCRPC). Progression-directed therapy (PDT), defined as a lesion-directed approach in patients with a limited number of progressive and/or new lesions, could postpone the need for NEST in these patients with so-called oligoprogressive mCRPC. Our aim was to investigate the feasibility of postponing NEST initiation in oligoprogressive mCRPC by using PDT. METHODS: MEDCARE was a prospective, single-arm, nonrandomized phase 2 trial. Eligible patients had oligoprogressive mCRPC and were treated with PDT while their ongoing systemic therapy was continued. The primary endpoint was NEST-free survival (NEST-FS). Secondary endpoints were prostate-specific antigen response, clinical progression-free survival (cPFS), prostate cancer-specific survival (PCSS), overall survival (OS), and PDT-induced toxicity. KEY FINDINGS AND LIMITATIONS: Twenty patients underwent PDT for 38 oligoprogressive lesions. At median follow-up of 28 mo, median NEST-FS was 17 mo and the 2-yr NEST-FS rate was 35%. Median PCSS and median OS were not reached. The PCSS and OS rates at 2 yr were 80% and 70%, respectively. The 2-yr local control rate was 95%. No patient experienced early or late grade ≥3 toxicity. NEST-FS was longer for patients who received PDT to all lesions visible on 18F-PSMA positron emission tomography/computed tomography (30 vs 13 mo; p = 0.002). CONCLUSIONS AND CLINICAL IMPLICATIONS: This single-center, single-arm, phase 2 trial demonstrated that PDT in oligoprogressive mCRPC resulted in median NEST-FS of 17 mo without any early or late grade ≥3 toxicity. PATIENT SUMMARY: For patients with metastatic prostate cancer no longer responding to hormone therapy, we investigated radiotherapy targeted at progressive cancer lesions while continuing their ongoing systemic treatment. The results show that this targeted therapy had very low toxicity and delayed the need to start a new line of systemic treatment by 17 months.

European clinical practice guidelines for the definition, diagnosis, and treatment of oligometastatic esophagogastric cancer (OMEC-4).

INTRODUCTION: The OligoMetastatic Esophagogastric Cancer (OMEC) project aims to provide clinical practice guidelines for the definition, diagnosis, and treatment of esophagogastric oligometastatic disease (OMD). METHODS: Guidelines were developed according to AGREE II and GRADE principles. Guidelines were based on a systematic review (OMEC-1), clinical case discussions (OMEC-2), and a Delphi consensus study (OMEC-3) by 49 European expert centers for esophagogastric cancer. OMEC identified patients for whom the term OMD is considered or could be considered. Disease-free interval (DFI) was defined as the time between primary tumor treatment and detection of OMD. RESULTS: Moderate to high quality of evidence was found (i.e. 1 randomized and 4 non-randomized phase II trials) resulting in moderate recommendations. OMD is considered in esophagogastric cancer patients with 1 organ with ≤ 3 metastases or 1 involved extra-regional lymph node station. In addition, OMD continues to be considered in patients with OMD without progression in number of metastases after systemic therapy. 18F-FDG PET/CT imaging is recommended for baseline staging and for restaging after systemic therapy when local treatment is considered. For patients with synchronous OMD or metachronous OMD and a DFI ≤ 2 years, recommended treatment consists of systemic therapy followed by restaging to assess suitability for local treatment. For patients with metachronous OMD and DFI > 2 years, upfront local treatment is additionally recommended. DISCUSSION: These multidisciplinary European clinical practice guidelines for the uniform definition, diagnosis and treatment of esophagogastric OMD can be used to standardize inclusion criteria in future clinical trials and to reduce variation in treatment.

Clinical implications of dose to functional lung volumes in the trimodality treatment of esophageal cancer.

BACKGROUND: Trimodality treatment, i.e., neoadjuvant chemoradiotherapy (nCRT) followed by surgery, for locally advanced esophageal cancer (EC) improves overall survival but also increases the risk of postoperative pulmonary complications. Here, we tried to identify a relation between dose to functional lung volumes (FLV) as determined by 4D-CT scans in EC patients and treatment-related lung toxicity. MATERIALS AND METHODS: All patients with EC undergoing trimodality treatment between 2017 and 2022 in UZ Leuven and scanned with 4D-CT-simulation were selected. FLVs were determined based on Jacobian determinants of deformable image registration between maximum inspiration and expiration phases. Dose/volume parameters of the anatomical lung volume (ALV) and FLV were compared between patients with versus without postoperative pulmonary complications. Results of pre- and post-nCRT pulmonary function tests (PFTs) were collected and compared in relation to radiation dose. RESULTS: Twelve out of 51 EC patients developed postoperative pulmonary complications. ALV was smaller while FLV10Gy and FLV20Gy were larger in patients with complications (respectively 3141 ± 858mL vs 3601 ± 635mL, p = 0.025; 360 ± 216mL vs 264 ± 139mL, p = 0.038; 166 ± 106mL vs 118 ± 63mL, p = 0.030). No differences in ALV dose-volume parameters were detected. Baseline FEV1 and TLC were significantly lower in patients with complications (respectively 90 ± 17%pred vs 102 ± 20%pred, p = 0.033 and 93 ± 17%pred vs 110 ± 13%pred, p = 0.001), though no other PFTs were significantly different between both groups. DLCO was the only PFT that had a meaningful decrease after nCRT (85 ± 17%pred vs 68 ± 15%pred, p < 0.001) but was not related to dose to ALV/FLV. CONCLUSION: Small ALV and increasing FLV exposed to intermediate (10 to 20 Gy) dose are associated to postoperative pulmonary complications. Changes of DLCO occur during nCRT but do not seem to be related to radiation dose to ALV or FLV. This information could attribute towards toxicity risk prediction and reduction strategies for EC.

The Feasibility of Quality Assurance in the TOPGEAR International Phase 3 Clinical Trial of Neoadjuvant Chemoradiation Therapy for Gastric Cancer (an Intergroup Trial of the AGITG/TROG/NHMRC CTC/EORTC/CCTG).

PURPOSE: The TOPGEAR phase 3 trial hypothesized that adding preoperative chemoradiation therapy (CRT) to perioperative chemotherapy will improve survival in patients with gastric cancer. Owing to the complexity of gastric irradiation, a comprehensive radiation therapy quality assurance (RTQA) program was implemented. Our objective is to describe the RTQA methods and outcomes. METHODS AND MATERIALS: RTQA was undertaken in real time before treatment for the first 5 patients randomized to CRT from each center. Once acceptable quality was achieved, RTQA was completed for one-third of subsequent cases. RTQA consisted of evaluating (1) clinical target volume and organ-at-risk contouring and (2) radiation therapy planning parameters. Protocol violations between high- (20+ patients enrolled) and low-volume centers were compared using the Fisher exact test. RESULTS: TOPGEAR enrolled 574 patients, of whom 286 were randomized to receive preoperative CRT and 203 (71%) were included for RTQA. Of these, 67 (33%) and 136 (67%) patients were from high- and low-volume centers, respectively. The initial RTQA pass rate was 72%. In total, 28% of cases required resubmission. In total, 200 of 203 cases (99%) passed RTQA before treatment. Cases from low-volume centers required resubmission more often (44/136 [33%] vs 13/67 [18%]; P = .078). There was no change in the proportion of cases requiring resubmission over time. Most cases requiring resubmission had multiple protocol violations. At least 1 aspect of the clinical target volume had to be adjusted in all cases. Inadequate coverage of the duodenum was most common (53% major violation, 25% minor violation). For the remaining cases, the resubmission process was triggered secondary to poor contour/plan quality. CONCLUSIONS: In a large multicenter trial, RTQA is feasible and effective in achieving high-quality treatment plans. Ongoing education should be performed to ensure consistent quality during the entire study period.

"Scan-(pre)Plan-Treat" Workflow for Bone Metastases Using the Ethos Therapy System: A Single-Center, In Silico Experience.

Purpose: To report on the accuracy of automated delineation, treatment plan quality, and duration of an in-silico "scan-(pre)plan-treat" (SPT) workflow for vertebral bone metastases using a 1 × 8 Gy regimen. Method and Materials: The cloud-based emulator system of the Ethos therapy system was used to adapt an organ-at-risk-sparing preplan created on the diagnostic CT to the anatomy-of-the-day using the cone beam CT made before treatment. Results: SPT using the Ethos emulator system resulted in relatively good coverage of the PTV and acceptable dose to the OAR. Delivery time and plan homogeneity was the best for 7-field IMRT plan template. Conclusions: A SPT workflow formula results in a highly conformal treatment delivery while maintaining an acceptable timeframe for the patient on the treatment couch.

From once-weekly to semi-weekly whole prostate gland stereotactic radiotherapy with focal boosting: Primary endpoint analysis of the multicenter phase II hypo-FLAME 2.0 trial.

BACKGROUND AND PURPOSE: The hypo-FLAME trial showed that once-weekly (QW) focal boosted prostate stereotactic body radiotherapy (SBRT) is associated with acceptable acute genitourinary (GU) and gastrointestinal (GI) toxicity. Currently, we investigated the safety of reducing the overall treatment time (OTT) of focal boosted prostate SBRT from 29 to 15 days. MATERIAL AND METHODS: Patients with intermediate- and high-risk prostate cancer were treated with SBRT delivering 35 Gy in 5 fractions to the whole prostate gland with an iso-toxic boost up to 50 Gy to the intraprostatic lesion(s) in a semi-weekly (BIW) schedule. The primary endpoint was radiation-induced acute toxicity (CTCAE v5.0). Changes in quality of life (QoL) were examined in terms of proportions achieving a minimal clinically important change (MCIC). Finally, acute toxicity and QoL scores of the BIW schedule were compared with the results of the prior QW hypo-FLAME schedule (n = 100). RESULTS: Between August 2020 and February 2022, 124 patients were enrolled and treated BIW. No grade ≥3 GU or GI toxicity was observed. The 90-days cumulative incidence of grade 2 GU and GI toxicity rates were 47.5% and 7.4%, respectively. Patients treated QW scored significant less grade 2 GU toxicity (34.0%, p = 0.01). No significant differences in acute GI toxicity were observed. Furthermore, patients treated QW had a superior acute bowel and urinary QoL. CONCLUSION: Semi-weekly prostate SBRT with iso-toxic focal boosting is associated with acceptable acute GU and GI toxicity. Based on the comparison between the QW and BIW schedule, patients should be counselled regarding the short-term advantages of a more protracted schedule. Registration number ClinicalTrials.gov: NCT04045717.

Definition, diagnosis and treatment of oligometastatic oesophagogastric cancer: A Delphi consensus study in Europe.

BACKGROUND: Local treatment improves the outcomes for oligometastatic disease (OMD, i.e. an intermediate state between locoregional and widespread disseminated disease). However, consensus about the definition, diagnosis and treatment of oligometastatic oesophagogastric cancer is lacking. The aim of this study was to develop a multidisciplinary European consensus statement on the definition, diagnosis and treatment of oligometastatic oesophagogastric cancer. METHODS: In total, 65 specialists in the multidisciplinary treatment for oesophagogastric cancer from 49 expert centres across 16 European countries were requested to participate in this Delphi study. The consensus finding process consisted of a starting meeting, 2 online Delphi questionnaire rounds and an online consensus meeting. Input for Delphi questionnaires consisted of (1) a systematic review on definitions of oligometastatic oesophagogastric cancer and (2) a discussion of real-life clinical cases by multidisciplinary teams. Experts were asked to score each statement on a 5-point Likert scale. The agreement was scored to be either absent/poor (<50%), fair (50%-75%) or consensus (≥75%). RESULTS: A total of 48 experts participated in the starting meeting, both Delphi rounds, and the consensus meeting (overall response rate: 71%). OMD was considered in patients with metastatic oesophagogastric cancer limited to 1 organ with ≤3 metastases or 1 extra-regional lymph node station (consensus). In addition, OMD was considered in patients without progression at restaging after systemic therapy (consensus). For patients with synchronous or metachronous OMD with a disease-free interval ≤2 years, systemic therapy followed by restaging to consider local treatment was considered as treatment (consensus). For metachronous OMD with a disease-free interval >2 years, either upfront local treatment or systemic treatment followed by restaging was considered as treatment (fair agreement). CONCLUSION: The OMEC project has resulted in a multidisciplinary European consensus statement for the definition, diagnosis and treatment of oligometastatic oesophagogastric adenocarcinoma and squamous cell cancer. This can be used to standardise inclusion criteria for future clinical trials.

Development of a Patient Decision Aid for Rectal Cancer Patients with Clinical Complete Response after Neo-Adjuvant Treatment.

Surgery is the primary component of curative treatment for patients with rectal cancer. However, patients with a clinical complete response (cCR) after neo-adjuvant treatment may avoid the morbidity and mortality of radical surgery. An organ-sparing strategy could be an oncological equivalent alternative. Therefore, shared decision making between the patient and the healthcare professional (HCP) should take place. This can be facilitated by a patient decision aid (PtDA). In this study, we developed a PtDA based on a literature review and the key elements of the Ottawa Decision Support Framework. Additionally, a qualitative study was performed to review and evaluate the PtDA by both HCPs and former rectal cancer patients by a Delphi procedure and semi-structured interviews, respectively. A strong consensus was reached after the first round (I-CVI 0.85-1). Eleven patients were interviewed and most of them indicated that using a PtDA in clinical practice would be of added value in the decision making. Patients indicated that their decisional needs are centered on the impact of side effects on their quality of life and the outcome of the different options. The PtDA was modified taking into account the remarks of patients and HCPs and a second Delphi round was held. The second round again showed a strong consensus (I-CVI 0.87-1).

Genomic Classifiers in Personalized Prostate Cancer Radiation Therapy Approaches: A Systematic Review and Future Perspectives Based on International Consensus.

Current risk-stratification systems for prostate cancer (PCa) do not sufficiently reflect the disease heterogeneity. Genomic classifiers (GC) enable improved risk stratification after surgery, but less data exist for patients treated with definitive radiation therapy (RT) or RT in oligo-/metastatic disease stages. To guide future perspectives of GCs for RT, we conducted (1) a systematic review on the evidence of GCs for patients treated with RT and (2) a survey of experts using the Delphi method, addressing the role of GCs in personalized treatments to identify relevant fields of future clinical and translational research. We performed a systematic review and screened ongoing clinical trials on ClinicalTrials.gov. Based on these results, a multidisciplinary international team of experts received an adapted Delphi method survey. Thirty-one and 30 experts answered round 1 and round 2, respectively. Questions with ≥75% agreement were considered relevant and included in the qualitative synthesis. Evidence for GCs as predictive biomarkers is mainly available to the postoperative RT setting. Validation of GCs as prognostic markers in the definitive RT setting is emerging. Experts used GCs in patients with PCa with extensive metastases (30%), in postoperative settings (27%), and in newly diagnosed PCa (23%). Forty-seven percent of experts do not currently use GCs in clinical practice. Expert consensus demonstrates that GCs are promising tools to improve risk-stratification in primary and oligo-/metastatic patients in addition to existing classifications. Experts were convinced that GCs might guide treatment decisions in terms of RT-field definition and intensification/deintensification in various disease stages. This work confirms the value of GCs and the promising evidence of GC utility in the setting of RT. Additional studies of GCs as prognostic biomarkers are anticipated and form the basis for future studies addressing predictive capabilities of GCs to optimize RT and systemic therapy. The expert consensus points out future directions for GC research in the management of PCa.

A Population-Based Study Using Belgian Cancer Registry Data Supports Centralization of Esophageal Cancer Surgery in Belgium.

BACKGROUND: Esophageal cancer surgery outcomes benefit from higher hospital volumes. Despite the evidence, organization of national health care often is complex and depends on various factors. The volume-outcome results of this population-based study supported national health policy measures regarding concentration of esophageal resections in Belgium. METHODS: The Belgian Cancer Registry (BCR) database was linked to administrative data on cancer treatment. All Belgian patients with newly diagnosed esophageal cancer in 2008-2018 undergoing resection were allocated to the hospital at which surgery was performed. The study assessed hospital volume association with 90-day mortality and 5-year overall survival, classifying average annual hospital volume of resections as low (LV, <6), medium (MV, 6-19), or high (HV, ≥20) and as a continuous covariate in the regression models. RESULTS: The study included 4156 patients who had surgery in 79 hospitals (2 HV hospitals [37% of all surgeries], 12 MV hospitals [30% of all surgeries], and 65 LV hospitals [33% of all surgeries]). Adjusted 90-day mortality in HV hospitals was lower than in LV hospitals (odds ratio [OR], 0.37; 95% CI, 0.21-0.65; p = 0.001). Case-mix adjusted 5-year survival was superior in HV versus LV (hazard ratio [HR], 0.43; 95% CI, 0.31-0.60; p < 0.001). The continuous model demonstrated a lower 90-day mortality (OR, 0.40; 95% CI, 0.23-0.71; p = 0.002) and a superior 5-year survival (HR, 0.45; 95% CI, 0.33-0.63; p < 0.001) in hospitals with volumes of 40 or more resections annually. CONCLUSION: Population-based data from the BCR confirmed a strong volume-outcome association for esophageal resections. Improved 5-year survival in centers with annual volumes of 20 or more resections was driven mainly by the achievement of superior 90-day mortality. These findings supported centralization of esophageal resections in Belgium.

Ultrahigh-Dose-Rate Proton Irradiation Elicits Reduced Toxicity in Zebrafish Embryos.

Purpose: Recently, ultrahigh-dose-rate radiation therapy (UHDR-RT) has emerged as a promising strategy to increase the benefit/risk ratio of external RT. Extensive work is on the way to characterize the physical and biological parameters that control the so-called "Flash" effect. However, this healthy/tumor differential effect is observable in in vivo models, which thereby drastically limits the amount of work that is achievable in a timely manner. Methods and Materials: In this study, zebrafish embryos were used to compare the effect of UHDR irradiation (8-9 kGy/s) to conventional RT dose rate (0.2 Gy/s) with a 68 MeV proton beam. Viability, body length, spine curvature, and pericardial edema were measured 4 days postirradiation. Results: We show that body length is significantly greater after UHDR-RT compared with conventional RT by 180 µm at 30 Gy and 90 µm at 40 Gy, while pericardial edema is only reduced at 30 Gy. No differences were obtained in terms of survival or spine curvature. Conclusions: Zebrafish embryo length appears as a robust endpoint, and we anticipate that this model will substantially fasten the study of UHDR proton-beam parameters necessary for "Flash."

Study protocol for the OligoMetastatic Esophagogastric Cancer (OMEC) project: A multidisciplinary European consensus project on the definition and treatment for oligometastatic esophagogastric cancer.

BACKGROUND: A uniform definition and treatment for oligometastatic esophagogastric cancer is currently lacking. However, a comprehensive definition of oligometastatic esophagogastric cancer is necessary to initiate studies on local treatment strategies (e.g. metastasectomy or stereotactic radiotherapy) and new systemic therapy agents in this group of patients. For this purpose, the OligoMetastatic Esophagogastric Cancer (OMEC) project was established. The OMEC-project aims to develop a multidisciplinary European consensus statement on the definition, diagnosis, and treatment for oligometastatic esophagogastric cancer and provide a framework for prospective studies to improve outcomes of these patients. METHODS: The OMEC-project consists of five studies, including 1) a systematic review on definitions and outcomes of oligometastatic esophagogastric cancer; 2) real-life clinical scenario discussions in multidisciplinary expert teams to determine the variation in the definition and treatment strategies; 3) Delphi consensus process through a starting meeting, two Delphi questionnaire rounds, and a consensus meeting; 4) publication of a multidisciplinary European consensus statement; and 5) a prospective clinical trial in patients with oligometastatic esophagogastric cancer. DISCUSSION: The OMEC project aims to establish a multidisciplinary European consensus statement for oligometastatic esophagogastric cancer and aims to initiate a prospective clinical trial to improve outcomes for these patients. Recommendations from OMEC can be used to update the relevant guidelines on treatment for patients with (oligometastatic) esophagogastric cancer.

The use of electronic Patient Reported Outcomes in follow-up after palliative radiotherapy: A survey study in Belgium.

Objective: Electronic Patient-Reported Outcome Measures (ePROMs) could be used to monitor patients' symptoms after treatment. However, ePROM implementation in clinical practice has been challenging, especially in (palliative) radiation oncology. The aim of this study was to explore the opinions of healthcare providers (HCP) active in radiation oncology in Belgium on the use of ePROMs for symptom follow-up after palliative radiotherapy. Methods: An anonymous online survey was conducted with different HCP in radiation oncology in Belgium. Participants were recruited through several professional organizations with approximately 390 members actively working in the field of radiation oncology. The survey used was a self-developed questionnaire, based on existing literature on implementation of (e)PROMs in cancer care, our previous research on this topic as well as our personal experience in the field of oncology and palliative care. Results: Of the 128 respondents, 26% had experience with ePROMs in clinical practice. Eighty-four percent considered ePROMs beneficial for patients' health and symptom knowledge, symptom self-management and active participation in care. ePROMs could help HCP to focus on detection of relevant symptoms and improve their management. Almost 75% were willing to implement and use ePROMs. Assigning ePROM introduction and follow-up to a dedicated person, such as a nurse navigator, was suggested to promote ePROM implementation and use in clinical practice. Conclusion: Despite limited experience with ePROMs in clinical care for palliative radiotherapy patients, the majority of respondents is willing to implement and use ePROMs for this particular patient population. Innovation: This is one of the first studies specifically focusing on experiences and opinions of HCP in radiation oncology on the use of ePROMs for symptom follow-up in palliative radiotherapy. HCP should be actively involved in implementation of ePROMs after palliative radiotherapy, to translate their vision of their ideals in practice.

High weekly integral dose and larger fraction size increase risk of fatigue and worsening of functional outcomes following radiotherapy for localized prostate cancer.

Introduction: We hypothesized that increasing the pelvic integral dose (ID) and a higher dose per fraction correlate with worsening fatigue and functional outcomes in localized prostate cancer (PCa) patients treated with external beam radiotherapy (EBRT). Methods: The study design was a retrospective analysis of two prospective observational cohorts, REQUITE (development, n=543) and DUE-01 (validation, n=228). Data were available for comorbidities, medication, androgen deprivation therapy, previous surgeries, smoking, age, and body mass index. The ID was calculated as the product of the mean body dose and body volume. The weekly ID accounted for differences in fractionation. The worsening (end of radiotherapy versus baseline) of European Organisation for Research and Treatment of Cancer EORTC) Quality of Life Questionnaire (QLQ)-C30 scores in physical/role/social functioning and fatigue symptom scales were evaluated, and two outcome measures were defined as worsening in ≥2 (WS2) or ≥3 (WS3) scales, respectively. The weekly ID and clinical risk factors were tested in multivariable logistic regression analysis. Results: In REQUITE, WS2 was seen in 28% and WS3 in 16% of patients. The median weekly ID was 13.1 L·Gy/week [interquartile (IQ) range 10.2-19.3]. The weekly ID, diabetes, the use of intensity-modulated radiotherapy, and the dose per fraction were significantly associated with WS2 [AUC (area under the receiver operating characteristics curve) =0.59; 95% CI 0.55-0.63] and WS3 (AUC=0.60; 95% CI 0.55-0.64). The prevalence of WS2 (15.3%) and WS3 (6.1%) was lower in DUE-01, but the median weekly ID was higher (15.8 L·Gy/week; IQ range 13.2-19.3). The model for WS2 was validated with reduced discrimination (AUC=0.52 95% CI 0.47-0.61), The AUC for WS3 was 0.58. Conclusion: Increasing the weekly ID and the dose per fraction lead to the worsening of fatigue and functional outcomes in patients with localized PCa treated with EBRT.