When cancer strikes both breasts at once, radiation oncologists face one of the most geometrically awkward challenges in their field: how to bathe two curved targets on either side of the chest with a lethal dose of radiation while threading the beams past the heart, the lungs, and the coronary arteries that keep a patient alive long after the tumor is gone. A new dosimetric study from China now offers the most detailed head-to-head comparison yet of the three leading techniques for this scenario, and its verdict is refreshingly nuanced: there is no single winner, but rather a trade-off between precision, cardiac protection, and time on the treatment table.
The study, conducted at The First Affiliated Hospital of Zhengzhou University in collaboration with Xinxiang Medical College, enrolled 30 patients with synchronous bilateral breast cancer—a rare form of the disease in which tumors appear in both breasts simultaneously, accounting for roughly 0.4 to 2.8 percent of new breast cancer diagnoses. All patients underwent surgery between March 2022 and March 2025, with ages ranging from 32 to 68 and a median of 45 years. Twenty-seven of them had modified radical mastectomies, while three underwent breast-conserving surgery. For each patient, the team generated three separate radiotherapy plans delivering a prescription dose of 50 Gy in 25 fractions: one using volumetric modulated arc therapy, or VMAT, planned on the Monaco treatment planning system, and two using helical tomotherapy, distinguished by the width of the fan beam—5 centimeters (HT-5) or 2.5 centimeters (HT-2.5)—swept in a spiral around the patient.
To appreciate what the researchers measured, it helps to understand the physics of these delivery methods. VMAT uses a conventional linear accelerator whose gantry rotates around the patient while the beam intensity is continuously modulated through a moving multi-leaf collimator—essentially a set of tungsten fingers that sculpt the radiation in real time. Helical tomotherapy, by contrast, works like a CT scanner in reverse: a narrow fan beam rotates while the couch slides the patient through the bore, painting dose in helical strips. The narrower 2.5 cm field width allows finer longitudinal resolution but requires many more rotations, which is why beam-on time balloons. The tomotherapy plans also differed in blocking mode—whether the machine’s binary collimator shuttered the beam completely outside the target region (“Complete block”) or merely attenuated it directionally.
Each technique was evaluated using the standard vocabulary of radiation dosimetry. For the planning target volume (PTV), the team computed the conformity index (CI), which quantifies how tightly the prescribed dose hugs the target—the closer to 1, the better—and the homogeneity index (HI), which penalizes dose variation within the target, with values closer to 0 indicating a more uniform dose. For organs at risk, they recorded V5, V10, V20, and V30—the percentage of an organ’s volume receiving at least 5, 10, 20, or 30 Gy—along with mean doses for the lungs, heart, and the left anterior descending coronary artery (LAD), the vessel whose irradiation is most strongly linked to later coronary artery disease in breast cancer survivors.
The results, published in BioMedical Engineering OnLine, reveal a striking pattern of complementary strengths. All three techniques met clinical requirements for target coverage, with no significant differences in mean dose, D95% coverage, or homogeneity. But VMAT was decisively the most conformal, achieving the highest conformity index with statistical significance over both tomotherapy plans. In plain terms, VMAT’s dose cloud wrapped around the bilateral targets more tightly, spilling less into surrounding tissue at the high-dose end.
The lung story was more intricate. At the low-dose end—V5, the volume of lung receiving 5 Gy or more—tomotherapy clearly won. HT-5 achieved a whole-lung V5 of 52.61 ± 4.90 percent and HT-2.5 reached 50.23 ± 4.43 percent, both significantly lower than VMAT’s 57.24 ± 5.37 percent. This matters because low-dose radiation spread across large lung volumes is considered a critical predictor of radiation pneumonitis, the inflammatory lung complication that can follow thoracic radiotherapy. The finding also confirmed the team’s hypothesis that the narrow 2.5 cm fan beam would offer the greatest low-dose lung sparing, likely because the tightly collimated spiral delivery minimizes the lateral scatter of dose into uninvolved lung. However, the advantage flipped at intermediate doses: VMAT delivered significantly lower V10 and V20 values than either tomotherapy configuration. Mean lung doses were statistically indistinguishable between VMAT (10.65 ± 0.69 Gy) and HT-2.5 (10.59 ± 1.09 Gy), while HT-5 was slightly higher at 11.55 ± 1.26 Gy.
The heart told a similar tale of trade-offs. Both tomotherapy plans reduced the mean heart dose and the V5 compared with VMAT: HT-5 achieved a mean heart dose of 5.71 ± 1.07 Gy and HT-2.5 reached 5.40 ± 0.94 Gy, against 6.37 ± 0.95 Gy for VMAT—differences that were statistically significant. Heart V5 values were also dramatically lower with tomotherapy, at 24.25 ± 8.25 percent for HT-5 and 21.88 ± 7.37 percent for HT-2.5, compared with a striking 49.52 ± 13.33 percent for VMAT. Yet when it came to the high-dose metric V30, VMAT was significantly superior, keeping the volume of heart receiving 30 Gy well below the 5 percent clinical constraint for all techniques. All three mean heart doses fell comfortably within accepted clinical limits, and the authors note their values compare favorably with prior literature—a reference study by Fiorentino and colleagues reported a mean heart dose of 8.3 ± 3.3 Gy for VMAT in this setting, with no radiation-induced cardiac toxicity at two-year follow-up.
Perhaps the most clinically consequential finding concerned the LAD. This vessel, which runs along the front of the heart and supplies much of the left ventricle, has accumulated the strongest dose-response evidence linking radiation exposure to coronary events in long-term breast cancer survivors. Here VMAT shone: its mean LAD dose of 16.25 ± 5.50 Gy was significantly lower than 18.97 ± 7.17 Gy for HT-5 and 18.10 ± 6.59 Gy for HT-2.5. The authors attribute their overall low cardiac doses in part to meticulous optimization—their VMAT plans used a single isocenter with three arcs spanning 150° to 210°, dose calculations based on a Monte Carlo photon algorithm with 0.5 percent statistical uncertainty, and a 3 mm dose grid. The right coronary artery was deliberately not contoured, they explain, because delineating such small vessels on non-contrast planning CT suffers from substantial inter-observer variability, and existing evidence for RCA-specific dose risk remains limited.
Then there is the matter of time. VMAT delivered its dose in an average beam-on time of just 4.5 ± 0.5 minutes. HT-5 took 8.1 ± 0.3 minutes, and HT-2.5 required a lengthy 14.8 ± 0.6 minutes—consistent with an earlier report of approximately 11 minutes for 2.5 cm tomotherapy in bilateral breast cases. Shorter treatment sessions reduce patient discomfort, minimize the chance of intra-fraction motion blurring the carefully sculpted dose, and improve throughput in busy departments treating some 2,000 breast cancer patients a year, as the Zhengzhou center does.
The study’s methodology deserves attention for its rigor. Three dosimetrists with more than a decade of experience each generated the plans, minimizing individual skill as a confounder. Organs were contoured per ICRU Reports 50 and 62, and plan quality was assessed according to ICRU Report 83, with the conformity index defined as the ratio of target volume within the reference isodose to target volume, multiplied by the ratio of that volume to the total reference-isodose volume. Rather than relying on simple paired comparisons, the researchers analyzed their data with linear mixed-effects models using restricted maximum likelihood estimation in R version 4.3.2, treating treatment group as a fixed effect and individual patient variation as a random effect—a statistical approach that properly accounts for the fact that all three plans were computed on the same thirty anatomies.
The authors are careful to place their findings in a broader technological context. Proton therapy, specifically intensity-modulated proton therapy, has shown the most favorable balance of target coverage and organ sparing in smaller comparative studies, though access and cost remain formidable barriers. Deep inspiration breath hold—a technique in which patients hold their breath during delivery to push the heart away from the chest wall—can further improve VMAT’s cardiac profile. Flattening filter-free beams, which deliver higher dose rates and reduce head scatter, have also been shown to shorten treatment times and improve lung protection in this population. The findings also echo prior work by Phurailatpam and colleagues, who similarly found that tomotherapy controlled low-dose spillage better while incurring higher intermediate-dose volumes.
Where does this leave the radiation oncologist choosing a plan for a woman with cancer in both breasts? The authors’ conclusion is pragmatic. VMAT is the natural choice when conformity, LAD protection, and short treatment times are paramount. Tomotherapy earns its place when cardiac sparing at low doses is the priority, and the study specifically endorses the Complete block mode with the 5 cm field width as a recommendable option for synchronous bilateral breast cancer, balancing the lung and heart benefits against a tolerable 8-minute delivery. The narrower 2.5 cm configuration, while offering the best low-dose lung profile, may be harder to justify given its threefold increase in beam-on time. As the authors themselves caution, statistically significant differences in dosimetric parameters do not always translate into clinically meaningful outcomes—the true test will be whether these measured differences in V5 and mean cardiac doses translate into fewer cases of pneumonitis and coronary disease years down the line. Until such long-term data arrive, this study provides radiation oncologists with exactly what they have lacked: a quantified map of the trade-offs, so that the choice of beams can be tailored to the patient rather than the machine.
Subject of Research: Dosimetric comparison of VMAT and two helical tomotherapy regimens (5 cm and 2.5 cm field widths) for adjuvant radiotherapy in patients with synchronous bilateral breast cancer, evaluating target coverage and sparing of lungs, heart, and the left anterior descending coronary artery.
Subject of Research: Medicine
Article Title: Dosimetric comparison of three different radiotherapy regimens for bilateral breast cancer
Article References: Kong, F., Xie, L., Wang, B., Wang, H., Li, Y., Dong, M., Shen, S., & Yu, Y. (2026). Dosimetric comparison of three different radiotherapy regimens for bilateral breast cancer. BioMedical Engineering OnLine, 25(1), Article 97. https://doi.org/10.1186/s12938-026-01574-x
Image Credits: AI Generated
DOI: 10.1186/s12938-026-01574-x
Keywords: Bilateral breast cancer, VMAT, Helical tomotherapy, Dosimetric comparison, LAD, Radiation pneumonitis, Heart sparing, Conformity index, Beam-on time, Radiotherapy planning
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Nathaniel Bowman. (September 8, 2026). Comparing three radiotherapy plans for treating bilateral breast cancer. Scienmag. https://scienmag.com/comparing-three-radiotherapy-plans-for-treating-bilateral-breast-cancer/
Nathaniel Bowman. “Comparing three radiotherapy plans for treating bilateral breast cancer.” Scienmag, 8 September 2026, https://scienmag.com/comparing-three-radiotherapy-plans-for-treating-bilateral-breast-cancer/. Accessed 8 September 2026.
Nathaniel Bowman. “Comparing three radiotherapy plans for treating bilateral breast cancer.” Scienmag. September 8, 2026. https://scienmag.com/comparing-three-radiotherapy-plans-for-treating-bilateral-breast-cancer/
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