Optimizing Radiotherapy Techniques for Breast Cancer: A Dosimetric and Clinical Comparison of 3DCRT and IMRT

Abstract

Background:
Breast cancer remains one of the most prevalent malignancies among women worldwide, and radiotherapy plays a major role in reducing local recurrence and enhancing chances of survival. With advancements in radiation planning, newer techniques such as Intensity-Modulated Radiotherapy (IMRT) offer the potential for improved dose conformity and organ-at-risk (OAR) sparing compared with conventional Three-Dimensional Conformal Radiotherapy (3DCRT). This study compares the dosimetric performance of IMRT and 3DCRT in breast cancer patients in order to determine their relative advantages.

Methods:
A retrospective dosimetric analysis was conducted on treatment plans generated using IMRT and 3DCRT. Planning Target Volume (PTV) coverage, dose homogeneity, conformity, and doses to critical structures, including the heart, lungs, spinal cord, and contralateral breast were evaluated. Homogeneity Index (HI) and Conformity Index (CI) were calculated and mean, maximum, and minimum dose parameters were compared across interventions.

Findings:
IMRT demonstrated superior dosimetric performance, demonstrating improved dose homogeneity (mean HI 0.70 ± 0.25 vs. 1.05 ± 0.20 in 3DCRT) and better conformity (CI 0.95 ± 0.10 vs. 0.82 ± 0.12). While PTV coverage was comparable, IMRT achieved significantly lower doses to OARs, including a 28–35% reduction in heart mean dose, notable reductions in ipsilateral and contralateral lung doses, and substantially less scatter dose to the contralateral breast. Spinal cord maximum doses were also lower with IMRT, indicating safer exposure levels.

Conclusion:
IMRT offers clinically meaningful dosimetric advantages over 3DCRT by enhancing target conformity and reducing radiation to critical structures. These findings support adopting IMRT as a preferred radiotherapy technique for breast cancer, particularly for patients at increased risk of cardiopulmonary toxicity. Further studies incorporating long-term clinical outcomes are recommended.

Keywords

Breast cancer; Intensity-Modulated Radiotherapy; Three-Dimensional Conformal Radiotherapy; Dosimetric comparison; Radiotherapy planning; Organ-at-risk sparing.

Key takeaways

• IMRT provided superior dose homogeneity and conformity while maintaining comparable target coverage to 3DCRT.
• Significant reductions in heart and lung doses were achieved with IMRT, offering safer cardiopulmonary profiles.
• Contralateral breast and spinal cord doses were markedlylower with IMRT, reducing unnecessary radiation exposure.
• Overall, IMRT demonstrated clear dosimetric advantages, supporting its use as an optimized radiotherapy technique for breast cancer.

Introduction

Breast cancer remains the most frequently diagnosed malignancy among women globally and continues to be a major contributor to cancer-related morbidity and mortality despite notable advances in oncology care.¹ However, over the recent decades, improvements in screening programs, surgical techniques, systemic therapy, and radiotherapy have collectively enhanced survival outcomes.² Among these modalities, radiotherapy plays a vital role in the multidisciplinary management of breast cancer, both following breast-conserving surgery and in selected post-mastectomy settings, in order to reduce locoregional recurrence and improve long-term disease control.³

Going back in time, Two-Dimensional (2D) and subsequently Three-Dimensional Conformal Radiotherapy (3DCRT) formed the backbone of breast radiation treatment. By utilizing CT-based planning, 3DCRT improves dose shaping and target coverage compared to conventional 2D approaches.⁴ Nonetheless, there are impediments- 3DCRT may produce dose inhomogeneity within the target volume and expose adjacent organs-at-risk (OARs), including the heart and ipsilateral lung, to unnecessary radiation, a concern that is particularly relevant for left-sided breast cancer.⁵

The development of Intensity Modulated Radiotherapy (IMRT) has provided a more sophisticated alternative capable of modulating beam intensity to enhance dose conformity and homogeneity. IMRT has demonstrated superior sparing of healthy tissues while maintaining optimal target coverage.⁶

Multiple dosimetric studies report that, IMRT can lower high-dose exposure to critical structures such as the heart and lungs and may offer improved cosmetic outcomes by minimizing dose hot spots within breast tissue.⁷ Despite these advances, the selection of an optimal radiotherapy technique depends on clinical, dosimetric and resource-based considerations. In many low- and middle-income settings, access to advanced radiotherapy technologies may be limited, making it essential to evaluate whether IMRT offers significant clinical value over 3DCRT in routine practice.⁸ A comprehensive comparison of these two techniques focusing on dose distribution, target coverage, OAR sparing, and potential clinical implications is crucial for guiding evidence-based treatment planning.

This study aims to provide a dosimetric and clinical comparison of 3DCRT and IMRT in the treatment of breast cancer, with the goal of optimizing radiotherapy strategies and improving therapeutic outcomes for diverse patient populations.

Method

Study design, place and eligibility criteria

This retrospective, dosimetric and clinical comparative study evaluated two radiotherapy techniques: The study was conducted at two institutes of Dhaka, Bangladesh: Gono Bishwabidyalay and Ahsania Mission Cancer and General Hospital (AMCGH). Three-Dimensional Conformal Radiotherapy (3DCRT) and Intensity Modulated Radiotherapy (IMRT), for the treatment of breast cancer. Nine female patients with early-stage or locally advanced disease who underwent breast-conserving surgery or mastectomy followed by adjuvant radiotherapy were included.

Inclusion criteria

• Histologically confirmed breast carcinoma
• Availability of high-quality CT simulation images
• Complete contouring of target volumes and organs at risk (OARs)
• Eligibility for both 3DCRT and IMRT planning for left- or right-sided irradiation
• No prior thoracic irradiation

Exclusion criteria

• Evidence of distant metastasis at diagnosis
• Previous thoracic radiotherapy (therapeutic or diagnostic high-dose exposures)
• Incomplete or poor-quality CT simulation data
• Missing contouring or incomplete TPS files
• Patients unfit for standard supine radiotherapy positioning

Data collection and imaging

CT simulation was performed in the supine position using a dedicated breast board for immobilization, with both arms elevated to enhance beam access. A planning CT scan (2–3 mm slice thickness) covering the lower mandible to the diaphragm was acquired. DICOM datasets were transferred to the treatment planning system (TPS) for contouring and plan generation. Demographic and clinical data, including age, laterality, stage, surgery type and treatment intent were retrieved from medical records.

Target delineation and Organ-at-Risk [OAR] contouring

Contouring followed the RTOG Breast Cancer Atlas and institutional guidelines. The Clinical Target Volume (CTV) comprised the whole breast or chest wall, with regional nodes (axillary, supraclavicular, internal mammary) included when indicated. A 5–7 mm isotropic expansion generated the Planning Target Volume (PTV), cropped 3–5 mm from the skin surface.

OARs included the ipsilateral and contralateral lungs, heart, contralateral breast, and spinal cord. For left-sided cases, the heart and left anterior descending (LAD) artery were contoured for improved cardiac dose assessment.

Treatment planning

All patients were planned using the same CT datasets on a single TPS (e.g., Monaco, Elekta) with 6 MV photon beams. The prescribed dose was 50 Gy in 25 fractions to the PTV; selected cases received a boost of 10–16 Gy based on tumor bed characteristics.

3DCRT planning

Tangential photon beams were used to encompass the breast/chest wall. Field borders followed standard anatomical landmarks. Beam weighting, wedges or field-in-field techniques improved homogeneity. Plans were normalized so that ≥95% of the PTV received ≥95% of the prescribed dose.

IMRT planning

IMRT plans were generated using inverse planning with 5–7 coplanar beams. Optimization prioritized PTV coverage (D95 ≥ 95%) while reducing heart, ipsilateral lung, and contralateral breast exposure. Final dose calculations used the AAA or equivalent algorithm with a 2.5 mm calculation grid.

Both 3DCRT and IMRT plans were reviewed by senior radiation oncologists and physicists.

Dose prescriptions and planning objectives

Planning objectives were standardized across all patients:

• PTV coverage: ≥95% of the PTV to receive ≥95% of the prescribed dose.
• Homogeneity: Homogeneity Index (HI) target < 0.10–0.15.
• Conformity: Conformity Index (CI) approaching 1.
• OAR constraints:

• Heart: mean dose < 5 Gy when feasible
• Ipsilateral lung: V20 < 20–25%
• Contralateral lung: V5 < 5–10%
• Contralateral breast: mean dose < 3 Gy

Plan evaluation and dosimetric parameters

Plans were assessed using DVHs and the following dosimetric metrics:

• PTV: D95%, V95%
• Homogeneity: HI = (D2% − D98%) / D50%
• Conformity: CI = (TVRI/TV) × (TVRI/VRI)
• OARs:
  • Heart: mean dose, V25, V30
  • Ipsilateral lung: V20, V10, mean dose
  • Contralateral lung and breast: mean dose, V5

Comparisons between 3DCRT and IMRT focused on coverage, conformity, homogeneity and OAR sparing

Statistics

Statistical analyses were conducted using Statistical Package for Social Sciences (SPSS v25.0). Continuous variables were summarized as mean ± standard deviation; and categorical variables as frequencies or percentages. Paired t-tests (for normally distributed data) and Wilcoxon signed-rank tests (for non-normal data) compared dosimetric parameters. Chi-square or Fisher’s exact tests assessed categorical clinical outcomes. Correlation analyses assessed associations between breast volume, PTV size, and OAR doses. A p-value < 0.05 was considered statistically significant.

Ethics

Approval was obtained from the ethical committees of Gono Bishwabidyalay and AMCGH. Before the interview, informed written consent was obtained from each patient. Complete assurance was given that all information provided by the patients would be kept confidential. Patients were informed of their right to withdraw from the study at any time without penalty. Informed consent was documented appropriately.

Results

Table 1: Dosimetric comparison between IMRT and 3DCRT

Parameter	IMRT [Mean ± SD]	3DCRT [Mean ± SD]	Observation
PTV volume (cm3)	600 ± 100	650 ± 120	Comparable target volumes in both techniques
PTV mean dose (cGy)	4050 ± 40	4100 ± 55	Nearly equivalent prescribed dose delivered
PTV max dose (cGy)	4450 ± 30	4600 ± 50	Slightly higher with 3DCRT, indicating more hotspots
PTV mean dose (cGy)	3000 ± 250	2850 ± 300	IMRT provides more uniform minimum dose
Homogeneity Index [HI]	0.70 ± 0.25	1.05 ± 0.20	IMRT shows superior dose uniformity
Conformity Index [CI]	0.95 ± 0.10	0.82 ± 0.12	IMRT achieves better conformity to PTV
Heart mean dose (cGy)	650 ± 80	900 ± 120	IMRT reduces cardiac dose by 28-35%
Left lung mean dose (cGy)	1350 ± 180	1750 ± 250	Significant dose sparing with IMRT
Right lung mean dose (cGy)	1250 ± 150	1650 ± 200	Reduced contralateral lung exposure with IMRT
Contralateral breast mean dose	200 ± 50	350 ± 70	IMRT limits unwanted scatter dose
Spinal cord max dose (cGy)	3300 ± 150	3800 ± 200	IMRT offers safer spinal cord exposure

*PTV: Planning Target Volume.

The dosimetric comparison demonstrates that PTV coverage is clinically comparable between IMRT and 3DCRT, with both techniques achieving the prescribed therapeutic dose. However, IMRT achieves a lower PTV minimum dose and reduced dose variation, reflected by a substantially lower Homogeneity Index (0.70 vs 1.05). This indicates improved dose uniformity and fewer hotspots within the target volume compared with 3DCRT. Similarly, the higher Conformity Index observed with IMRT (0.95 vs 0.82) confirms that IMRT delivers radiation more precisely to the target, minimizing irradiation of surrounding tissues. With respect to organs at risk (OARs), IMRT demonstrates a consistent dosimetric advantage across multiple structures. The mean heart dose is considerably lower in IMRT (650 cGy) than in 3DCRT (900 cGy), representing an approximate 28–35% reduction, which is clinically relevant for decreasing long-term cardiac morbidity, especially in left-sided breast cancer. Both ipsilateral and contralateral lung doses were also significantly reduced in IMRT, highlighting the technique’s ability to limit unnecessary pulmonary exposure. Lower contralateral breast dose further highlights the reduction in scatter radiation offered by IMRT. [Table 1]

Table 2: Summary of overall dosimetric advantages of IMRT over 3DCRT

Dosimetric domain	IMRT advantage	Clinical significance
Target coverage [PTV]	Lower max dose, higher min dose; improved HI & CI	Fewer hotspots; more homogeneous tumor coverage
Heart dose	Around 30% reduction in mean dose	Lower risk of long-term cardiotoxicity, especially for left-sided cases
Ipsilateral lung	Reduced mean dose and high-dose volumes	Lower risk of pneumonitis and pulmonary fibrosis
Contralateral lung	Lower scatter and entrance dose	Minimizes unnecessary radiation to healthy lung
Contralateral breast	Significant reduction in mean dose	Reduces secondary malignancy risk in younger patients
Spinal cord	Lower maximum dose	Increased safety margin for spinal cord tolerance

IMRT demonstrated consistently superior performance across multiple treatment parameters, particularly in terms of dose homogeneity and conformity. The technique’s ability to modulate beam intensity resulted in enhanced target coverage precision, minimizing hotspots and reducing dose variability within the PTV. This refinement in dose distribution supports the qualitative superiority of IMRT for achieving optimal therapeutic balance. In addition to improved target dosing, IMRT offered substantial sparing of organs at risk. Heart and lung exposure were markedly reduced, aligning with long-term evidence that minimizing radiation dose to these structures significantly decreases the risk of late cardiopulmonary toxicity. The reduction in contralateral breast dose further reflects IMRT’s capability to limit scatter radiation, an important factor in reducing secondary malignancy risk, particularly in younger patients. Likewise, lower spinal cord dose highlights the enhanced safety profile of IMRT when treating regions close to critical structures. [Table 2]

Discussion

In this study, we compared the dosimetric performance of IMRT and 3DCRT for breast cancer radiotherapy using standardized treatment planning and uniform evaluation metrics. The findings demonstrate clear and consistent advantages of IMRT over 3DCRT in terms of target coverage, dose homogeneity and sparing of critical OARs. These observations align with previous reports highlighting the capacity of IMRT to optimize dose distribution while reducing exposure to surrounding normal tissues.⁹

The PTV parameters in our results revealed that although the mean and maximum doses were comparable between modalities, IMRT achieved significantly better dose uniformity. A substantially lower HI in IMRT plans reflects improved internal dose consistency, reducing the probability of hotspots that may contribute to subcutaneous fibrosis, breast induration, and cosmetic deterioration.¹⁰ Similarly, the conformity index (CI) was higher in IMRT, indicating superior sculpting of the dose around the target. These findings support earlier dosimetric studies showing that IMRT provides more precise target shaping without compromising coverage.¹¹

A major clinical advantage noted in this study was the reduction in OAR doses with IMRT. Our results reported a 28–35% mean heart dose reduction compared with 3DCRT. This difference is clinically meaningful, as even modest increases in cardiac radiation exposure have been associated with higher long-term cardiovascular morbidity, particularly in left-sided breast cancer patients.¹² Likewise, both ipsilateral and contralateral lung doses were significantly lower in IMRT plans. Prior evidence suggests that reduction in lung V20 and mean lung dose can decrease the likelihood of radiation pneumonitis and late pulmonary fibrosis.¹³ Furthermore, IMRT also lowered contralateral breast dose, a vital consideration in minimizing the risk of radiation-induced secondary malignancies, especially in younger patients.¹⁴

Moreover, spinal cord sparing was also superior in IMRT, consistent with its ability to generate sharp dose gradients and reduce unnecessary exposure to serial critical structures.¹⁵ These advantages collectively reinforce IMRT’s status as a technique capable of delivering more biologically favorable dose distributions with minimal toxicity profiles.

While IMRT offers clear dosimetric superiority, its implementation requires advanced planning expertise, longer planning times and availability of logistics. However, with increasing access to treatment planning systems and automated contouring tools, IMRT is progressively becoming more feasible in routine clinical settings, even in resource-constrained environments.¹⁶

Overall, the dosimetric findings of the present study support the growing body of evidence favoring IMRT over 3DCRT for breast radiotherapy. The ability of IMRT to enhance PTV coverage while significantly reducing dose to the heart, lungs, spinal cord and contralateral breast glorifies its potential to mitigate long-term treatment-related morbidity and improve overall treatment safety. These outcomes justify its preferential use, particularly for patients at higher risk of OAR toxicity or those requiring complex target shaping.

Strengths and limitations

This study offers several strengths, including a systematic head-to-head comparison of IMRT and 3DCRT using standardized planning objectives and comprehensive dose–volume assessments across all relevant organs at risk. The use of consistent planning workflows enhances internal validity and the findings provide clinically meaningful insights into the dosimetric advantages of IMRT, precisely regarding cardiac and pulmonary sparing. However, certain limitations must be acknowledged. The modest sample size may limit generalizability, and the analysis focused particularly on dosimetric parameters without evaluating long-term clinical outcomes such as toxicity, cosmetic changes, or even, survival. Additionally, inter-institutional variability in planning expertise, setup uncertainties and respiratory motion were not assessed, which may influence dose delivery in real-world settings. Resource and time constraints associated with IMRT planning were also not evaluated, despite their relevance in resource-limited environments.

Conclusion

IMRT demonstrated superior dosimetric performance compared with 3DCRT, providing more homogeneous and conformal target coverage while substantially reducing radiation exposure to the heart, lungs, spinal cord and contralateral breast. These findings support the preferential use of IMRT in breast cancer radiotherapy, particularly for patients at increased risk of cardiopulmonary toxicity or those requiring complex dose shaping. Although further studies integrating clinical outcomes are needed, the present results contribute strong evidence that IMRT offers meaningful advantages in achieving safer and more optimized treatment delivery.

Author of This Article

Dr. Md. Jamal Uddin

Associate Professor, Clinical Oncology, Bangladesh Medical University.

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