Objectives: To analyze the role of PCI variation (Δ-PCI) before and after neoadjuvant chemotherapy (NACT) in an interval cytoreductive surgery (ICS) setting with the aim to propose a scoring model for predicting both complete cytoreduction and histopathologic response. Methods: A total of 50 consecutive patients who underwent ICS at our institution were prospectively collected between January-2020 and December-2023. PCI was assessed at exploratory surgery and at ICS. The clinical and histopathological response to NACT was determined by Δ-PCI and CRS. A cut-off value for Δ-PCI, to predict complete cytoreduction, histopathological response, and both together, was identified using a receiver operating characteristic (ROC) curve. The Kaplan-Meier test was used to define disease-free survival (DFS) based on the Δ-PCI cut-off value. Results: Complete cytoreduction was achieved in 82% of patients, with a median Δ-PCI score at ICS of 12 (range 7-29). The remaining 18% had a median Δ-PCI score at IDS of 8 (range 4-11). The best predictor of complete cytoreduction, histopathologic response CRS 3, and both was the Δ-PCI score, with an area under the curve (AUC) of 0.85 (0.73-0.96), 0.98 (0.94-1.00) and 0.88 (0.75-0.96), respectively; ROC curve analysis determined a Δ-PCI cut-off of 8, 17 and 15, respectively. Δ-PCI ≥ 15 as a predictor for both complete cytoreduction and histopathologic response CRS 3 with a median DFS of 26 months for Δ-PCI ≥ 15 versus 12 months for Δ-PCI < 15 (p = 0.02). Conclusions: Δ-PCI (cut-off ≥ 15) is a predictive model for complete cytoreduction, histological response CRS 3, and improved DFS.