This observational study was conducted to develop and test a new self-report questionnaire, the Cancer Pathology Acceptance Questionnaire for Breast Cancer (CPAQ-8-BC), specifically adapted for women diagnosed with Breast Cancer (BC). The primary goal is to evaluate if this new tool is a reliable and valid measure for assessing disease acceptance - defined as the psychological ability to acknowledge and experience cancer-related thoughts, emotions, and physical sensations without excessive avoidance or struggle. The study investigates whether the Cancer Pathology Acceptance Questionnaire for Breast Cancer (CPAQ-8-BC) accurately measures two distinct aspects: the psychological receptivity to the illness (Disease Receptivity) and the commitment to activity engagement despite the disease (Activity Engagement). This short questionnaire is intended for use by healthcare providers and rehabilitation specialists to identify patients who struggle with acceptance, potentially guiding personalized psychological therapies, such as Acceptance and Commitment Therapy (ACT), and improving participation in rehabilitation and overall quality of survivorship.

Study Background and Rationale Breast Cancer (BC) remains a major global health challenge that often leads to long-term psychological distress, body image concerns, and fatigue, even after the completion of medical treatment. Disease acceptance has emerged as a key determinant of positive emotional and functional outcomes. Therefore, reliable and valid instruments are necessary to quantify acceptance in the context of Breast Cancer (BC) for both research and clinical practice.

This study addresses the limited availability of disease-specific outcome measures by developing the Cancer Pathology Acceptance Questionnaire for Breast Cancer (CPAQ-8-BC). This instrument is an adaptation of the 8-item Chronic Pain Acceptance Questionnaire (CPAQ-8). The adaptation process involved replacing the words "pain" and "chronic pain" with the terms "tumor" or "disease" to make the instrument context-specific for patients.

Methodology and Participants This is a cross-sectional, observational study approved by the University Ethical Review Committee of Sapienza University of Rome (Protocol No. 53-2023).

Sample Size: A total of 101 women with a confirmed histopathological diagnosis of Breast Cancer (BC) were consecutively enrolled from two inpatient Hospital Breast Units.

Inclusion Criteria: Adult age, confirmed Breast Cancer (BC) diagnosis, and the ability to understand the Italian language.

Data Collection: Participants filled out the self-administered Cancer Pathology Acceptance Questionnaire for Breast Cancer (CPAQ-8-BC), which consists of 8 items rated on a scale of 0 (never true) to 6 (always true). A pilot testing phase was conducted to assess the scale's face validity, appropriateness, acceptability, and feasibility.

Psychometric Evaluation Plan

The primary focus of the psychometric testing is to investigate the Structural Validity, Reliability, and Interpretability of the Cancer Pathology Acceptance Questionnaire for Breast Cancer (CPAQ-8-BC):

• Structural Validity: Confirmatory Factor Analysis (CFA) is performed to test a single-factor structure against a pre-hypothesized two-factor structure consisting of Activity Engagement and Disease Receptivity. Model fit is evaluated using the Comparative Fit Index (CFI), Tucker Lewis Index (TLI), Standardized Root Mean Square Residual (SRMR), and Root Mean Square Error of Approximation (RMSEA).
• Reliability: Internal consistency is assessed using Cronbach's alpha and McDonald's omega. Test-Retest Stability is investigated using the Intraclass Correlation Coefficient (ICC2,1) over a 10-15 day period. The Standard Error of Measurement (SEM) is also calculated.
• Interpretability: The Minimum Detectable Change (MDC) is estimated to determine the smallest change in score that can be considered a true change rather than measurement error.

Clinical Relevance The Cancer Pathology Acceptance Questionnaire for Breast Cancer (CPAQ-8-BC) is intended to serve as a tool for person-oriented evaluations in multidisciplinary rehabilitation settings. Identifying patients with low acceptance levels may help clinicians identify those prone to activity avoidance and poor functional recovery. The instrument is designed to support the tailoring of interventions, such as Acceptance and Commitment Therapy (ACT) or Cognitive-Behavioral Therapy (CBT), by assessing key areas of psychological flexibility.

Inclusion Criteria:

• Adult age
• Confirmed histopathological diagnosis of Breast Cancer (BC)
• Ability to understand the Italian language

Exclusion Criteria:

• Documented peripheral or central neurological disorders
• Systemic illnesses
• Cognitive impairment
• Recent myocardial infarction
• Previous cerebrovascular events

This study is specifically focused on women who have received a diagnosis of Breast Cancer. Participation is limited to individuals who are biologically female, consistent with the typical population affected by this condition.

The study participants were consecutively enrolled from two specific inpatient Hospital Breast Units in Italy. The population consists of women attending these units who had received a confirmed histopathological diagnosis of Breast Cancer (BC). The units served as the primary recruitment sites for the study.