A retrospective study of 33 women treated with radiotherapy after breast-conserving surgery for right-sided breast cancer has found that simple geometric measurements of the target volume, taken from routine planning scans, correlate with the radiation doses delivered to the lung and heart. The work, conducted at the People’s Hospital of Rongchang District in Chongqing, China, suggests that two straightforward dimensions measured on a simulation CT scan could give radiation oncologists an early warning about which patients are likely to receive higher doses to sensitive organs at risk, before a treatment plan is ever calculated.

The research team focused on three anatomical parameters measured at the nipple-center plane of the simulation CT: the transverse diameter (TD) of the target volume, the oblique diameter (OD), and a newly proposed ratio of the two, termed the transverse-to-oblique ratio (TOR). All measurements were performed using built-in tools of the Monaco treatment planning system by a single experienced radiation oncology physicist. To check the reliability of these measurements, ten randomly selected cases were re-measured after a two-week interval by the same operator, allowing intra-observer reproducibility to be assessed.

The patients enrolled between January 2025 and August 2025 ranged in age from 31 to 65 years, with a median age of 47. Twenty-two tumors were classified as stage T1 and eleven as T2, while 31 patients were lymph-node negative and two had limited nodal involvement. Every patient received the same radiotherapy regimen: a prescription dose of 43.5 Gy in 15 fractions to the planning target volume of the whole breast, together with a simultaneous integrated boost of 49.5 Gy in 15 fractions to the boosted target volume. All plans were created with volumetric modulated arc therapy (VMAT) using the Elekta Monaco 6.0 planning system, and each plan was normalized so that the prescribed dose covered 95 percent of the target volume.

For each patient, the study recorded a panel of dosimetric indicators for the organs at risk: the volume of the affected lung receiving 20 Gy (V20Gy) and 5 Gy (V5Gy) and its mean dose; the mean dose and V5Gy of the heart; the mean doses to the right coronary artery (RCA) and the left anterior descending artery (LAD); the V5Gy of the contralateral lung; and the dose to the contralateral breast. Across the cohort, the average anatomical values were a transverse diameter of 4.96 centimeters, an oblique diameter of 15.92 centimeters, and a TOR of 0.31. Average dosimetric outcomes included an affected-lung V20Gy of 14.77 percent, a V5Gy of 31.39 percent, and a mean lung dose of 8.12 Gy, while the heart received a mean dose of 1.46 Gy and the RCA a mean of 2.67 Gy.

The statistical approach used bivariate Pearson correlation analysis to test the relationships between the three geometric parameters and the dosimetric indicators. Because this involved 66 pairwise correlation tests, the researchers applied a Benjamini-Hochberg false discovery rate correction to guard against spurious findings, defining significance as an adjusted q-value below 0.05. Within this framework, both the oblique diameter and the transverse diameter showed significant positive correlations with V20Gy, V5Gy, and mean dose of the affected lung, with correlation coefficients falling between 0.5 and 0.7.

The cardiac findings pointed in a consistent direction. The transverse diameter was significantly correlated with the mean heart dose (R = 0.58), and showed positive correlations with the mean doses to the RCA (R = 0.45) and the LAD (R = 0.42). The TOR, the shape ratio introduced for the first time in this study, correlated significantly with the mean heart dose (R = 0.47) and with the LAD mean dose (R = 0.47). Its correlation with the RCA mean dose (R = 0.35) did not survive the false discovery rate correction, illustrating how cautious interpretation of multiple comparisons matters in a dataset of this size. Interestingly, no correlation emerged between the TOR and any of the lung dose parameters; the authors suggest this may be because lung irradiated volume depends mainly on the overall size of the target rather than its spatial shape, or because the small sample could not detect a weak association.

From linear regression fits, the team derived exploratory reference thresholds. When the oblique diameter exceeded 19.56 centimeters, or the transverse diameter exceeded 8.00 centimeters, the fitted values for the affected lung exceeded a V20Gy of 20 percent, a V5Gy of 40 percent, and a mean dose of 10 Gy. When the transverse diameter exceeded 8.00 centimeters, the fitted heart mean dose exceeded 2 Gy, the LAD mean dose exceeded 2.4 Gy, and the RCA mean dose exceeded 3.5 Gy. A TOR above 0.47 was associated with a fitted heart mean dose above 2 Gy. The authors are explicit that these cut-offs were generated from linear observations of correlation analyses rather than optimized with receiver operating characteristic analysis, and that they cannot be directly used for clinical risk stratification without prospective and external validation.

The clinical context gives these numbers weight. Postoperative radiotherapy after breast-conserving surgery is a standard treatment for early-stage breast cancer, with established evidence showing local control and survival outcomes comparable to radical surgery. But the heart, lungs, and contralateral breast sit close to the treatment field, and radiation-induced lung injury and radiation-induced heart disease are among the most feared late complications, particularly as patient survival extends over decades. Prior research cited by the authors has estimated that each additional Gy of mean heart dose raises the long-term risk of cardiac damage by about 7.4 percent, and mean doses to the coronary arteries have been linked to the incidence of coronary artery disease. In this cohort, the cardiac dosimetric parameters remained well below guideline dose limits, but the correlations indicate that patients with wider or differently shaped target volumes accumulate more cardiac and pulmonary dose even within a uniform planning protocol.

Not every organ responded to geometry. The mean dose to the contralateral breast showed no correlation with any of the target volume parameters, which the authors attribute to its location in the opposite side of the chest: the dose it receives comes primarily from scattered radiation and beam leakage rather than direct beam paths, so it depends on technique rather than on the size or shape of the target. This implies that contralateral breast protection is best addressed through gantry angle optimization and dose-shading structures rather than through the geometric parameters studied here.

The study’s limitations are considerable and the authors state them plainly. It is a single-center retrospective investigation with only 33 patients; only correlative analyses were performed, and no validated clinical prediction model was established. Inter-observer variability in the anatomical measurements was not assessed, since all measurements came from a single physicist, which may introduce measurement bias. The exploratory thresholds derive exclusively from this one dataset. The practical takeaway is therefore modest but useful: patients whose target volumes exceed these dimensional reference values may warrant closer attention to cardiopulmonary dose during planning, and techniques such as deep inspiration breath hold, which expands lung volume and displaces the heart away from the target, may be considered for such patients. Larger, multicenter, prospective studies will be needed to confirm the correlations, refine the thresholds, and determine whether simple measurements on a planning scan can genuinely serve as an early dosimetric warning system in the radiotherapy clinic.

Subject of Research: Correlations between breast target volume anatomical parameters and organ-at-risk radiation doses after breast-conserving surgery

Article Title: Dosimetric impact of target volume anatomical parameters on radiotherapy after right-sided breast-conserving surgery

Article References: Zou, L., Deng, Z., Liao, X., Duan, L., & Liu, X. (2026). Dosimetric impact of target volume anatomical parameters on radiotherapy after right-sided breast-conserving surgery. Frontiers in Oncology, 16, Article 1944304. https://doi.org/10.3389/fonc.2026.1944304

Image Credits: AI Generated

DOI: 10.3389/fonc.2026.1944304

Keywords: radiotherapy, breast cancer, breast-conserving surgery, dosimetry, organs at risk, VMAT, cardiac dose, lung dose, anatomical parameters, retrospective study, treatment planning, radiation oncology