## 1 Introduction  
Breast cancer is the most common cancer diagnosed in women worldwide, comprising \(\sim\)30% of all diagnosed cancer cases in women in 2021 [1]. Although most diagnosed, breast cancer has high treatment potential, especially if detected in the earlier stages [2]. Current breast cancer screening methods include mammography, ultrasound, and MRI [3]. However, these methods have limitations with regard to diagnostic sensitivity/specificity [4]. Nonetheless, confirmatory diagnostics for breast cancer is undertaken through biopsies, and breast cancer staging is performed using pathological grading as well as imaging with ultrasound and CT/PET scans [5]. After breast cancer diagnostics, breast cancer treatment regimens are implemented. Stage 0 breast cancer (ductal carcinoma in situ, DCIS) is treated with surgery (mastectomy or breast-conserving surgery) with or without radiation therapy. However, although most cases of DCIS do not progress, there is no method to distinguish tumors that will remain indolent from ones that will become aggressive. Therefore, in many cases, DCIS lesions may be overtreated [6]. For stage I-III breast cancer, localized therapies are usually implemented and include surgery followed by radiation therapy. Systemic therapies, including chemotherapules and other drugs (hormone suppressive therapy and Her-2 targeted therapy), are also used before (neoadjuvant) and/or after (adjuvant) surgery. Stage IV breast cancer (metastatic) is generally treated with systemic therapies, and in some cases, surgery and radiation may also be beneficial [7]. The efficacy of treatments are usually monitored using CT scans and bone scans or PET scans every 3-6 months after treatment. However, CT scans and/or PET scans present significant challenges for monitoring treatment efficacy. It can be difficult to differentiate tumor progression from pseudoprogression or infection on CT scans [8]. PET scans are expensive and are often challenging to get reimbursed by insurance providers. Many of these imaging techniques require the administration of iodinated contrast for accurate monitoring and interpretation, which can be nephrotoxic. There is a definite need for a less time-consuming and less invasive test to monitor breast cancer during treatment. Therefore, identifying alternative methods for monitoring breast cancer treatment efficacy is of significant interest. An accurate and noninvasive assay to diagnose breast cancer, monitor tumor progression, and determine treatment efficacy over time could decrease overdiagnosis/overtreatment and aid in patient decision making during the treatment process. Previous investigations have shown the potential of volatile organic compounds (VOCs) as biomarkers of breast cancer, along with other diseases [9; 10; 11; 12; 13; 14]. VOCs present a noninvasive opportunity for disease detection, as they are differentially expressed in biological samples in the presence of disease [15; 16; 17]. Several groups have employed solid-phase microextraction (SPME) coupled with gas chromatography-mass spectrometry (GC-MS) to profile VOCs for breast cancer biomarker discovery [16; 18; 19; 20]. Through instrumental and chemometric analyses, VOCs have been previously shown to be biomarkers for breast cancer in urine. One study published by [PERSON] et al. reports a human urinary VOC biosignature that can discriminate patients with breast cancer from healthy patients with 100% accuracy by using unsupervised multivariate statistical analysis on a panel of six VOCs [21]. Further investigation by the same group identified a panel of ten urinary VOCs that can differentiate breast cancer from healthy controls with more than 90% accuracy [22]. Both studies report carbonyls (ketones and aldehydes), terpen, and sulfur-containing VOCs as potential biomarkers for breast cancer in urine. A study published by the current authors explored utilizing murine urinary VOCs to track mammary tumor progression for three weeks after tumor injection.