Physics Contribution
Accelerated Partial-Breast Intensity-Modulated Radiotherapy Results in Improved Dose Distribution When Compared With Three-Dimensional Treatment-Planning Techniques

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Purpose

To compare dose distribution and normal tissue sparing in partial-breast treatment using three-dimensional conformal radiotherapy (3D-CRT) vs. intensity-modulated radiotherapy (IMRT).

Methods and Materials

Sixty-three patients with Tis-1N0M0 breast cancer were treated on a Phase II prospective accelerated partial-breast IMRT protocol at two facilities between April 2004 and January 2006. Fifty-six patients had data sets sufficient to adequately contour all structures. These cases were subsequently replanned with 3D-CRT techniques using the same contours, to compare the dose distribution patterns of 3D-CRT vs. IMRT.

Results

The average planning target volume (PTV) to ipsilateral breast (IB) ratio was 24% (range, 7–58%). The average volume of IB receiving 25%, 50%, 75%, and 100% of the prescribed dose was 4.0%, 5.0%, 5.5%, and 10.5% less with IMRT than with 3D (p < 0.01). The dose reduction to normal breast was further improved in the subset of patients whose PTV to IB ratio was >25%, and in patients with contoured breast volume <750 cm3. No difference was detected in delivery to the lumpectomy cavity or clinical target volume. The PTV volume receiving 95% of the dose was higher in the 3D conformal plans (p < 0.01), but no significant difference was observed in the PTV volume receiving 90% (p = 0.17). The irradiated heart and lung volumes were small with both techniques but also favored IMRT.

Conclusions

In T1N0 patients treated with external beam partial-breast radiotherapy, IMRT improves normal tissue sparing in the ipsilateral breast compared with 3DRT, without compromising dose delivery to the lumpectomy cavity and clinical target volume.

Introduction

The use of conservative surgery combined with whole-breast irradiation (WBI) has been established as a valid alternative to mastectomy for the local treatment of ductal carcinoma in situ and early invasive breast cancer. Recently updated 20-year results of breast-conservation therapy trials have shown comparable overall survival and disease-free survival results for limited surgery and WBI compared with mastectomy 1, 2, 3, 4.

Despite the efficacy of breast-conservation therapy in the treatment of early breast cancer, only 10–40% of patients who qualify for breast-conservation therapy are treated in this manner (5). In addition, a study querying the Surveillance, Epidemiology, and End Results database estimates that up to 25% of patients who are treated with breast-conservation surgery do not receive adjuvant radiotherapy (6). Breast-conservation therapy is time consuming, with WBI requiring 5–7 weeks to complete. Moreover, there has been a trend toward use of adjuvant chemotherapy after lumpectomy, delaying the start of radiotherapy (RT) (5). Although early delivery of chemotherapy is favored, the associated delay of conventional WBI negatively impacts local control (7).

Thus, accelerated partial-breast irradiation (APBI) offers potential advantages, allowing the completion of RT in 1 to 2 weeks, typically before the start of adjuvant chemotherapy. Several investigators have evaluated both low-dose-rate 8, 9, 10, 11, 12, 13, 14 and high-dose-rate brachytherapy 8, 14, 15, 16, 17, 18, 19, 20 and have demonstrated 5-year disease-free survival rates similar to those of historical controls. External beam RT (EBRT) has recently been established as an option in the delivery of APBI. The use of three-dimensional conformal radiotherapy (3D-CRT) has been tested in two Phase I/II clinical trials 21, 22, 23, 24, 25 and represents the most frequently used option for APBI in the Phase III National Surgical Adjuvant Breast and Bowel Project (NSABP) B39/Radiation Therapy Oncology Group (RTOG) 0413 randomized trial (26).

Intensity-modulated RT (IMRT) potentially offers further improvement in normal tissue sparing. Studies in prostate and head-and-neck cancer have shown that IMRT allows further dose reduction to normal tissues compared with 3D-CRT 27, 28. If the use of IMRT confers a significant reduction in uninvolved breast irradiation, then a reduction in the incidence of late toxicity and poor cosmetic outcomes may be achievable.

The current analysis compares target volume and normal tissue radiation doses using 3D-CRT vs. IMRT from an initial cohort of patients treated on a Phase II accelerated partial-breast IMRT protocol.

Section snippets

Patients

Sixty-three patients with T1N0 or ductal carcinoma in situ breast cancer were treated on a Phase II prospective accelerated partial-breast IMRT protocol at two facilities between April 2004 and January 2006. Eligibility requirements were a minimum age of 45 years, staged as a T1, N0, M0 Stage I American Joint Committee on Cancer classification (later amended to include patients with a minimum age of 40 years and to include Tis) and negative surgical margins (≥2 mm) after final surgery.

Treatment planning

Two

Ipsilateral breast

The maximum delivered dose was similar for IMRT and 3D-CRT. The mean Dmax as a percentage of the prescribed dose was 105.1% with IMRT and 105.8% with 3D-CRT. The present analysis showed a statistically significant reduction in the dose delivered to the IB favoring IMRT, particularly at the higher dose levels, as presented in Table 1. The mean volume of IB receiving 25% (V25), 50% (V50), 75% (V75), and 100% (V100) was reduced by 7%, 11%, 15%, and 53%, respectively, with IMRT compared with 3D

Discussion

The present analysis evaluated a cohort of 56 early-stage breast cancer patients planned for APBI with both 3D-CRT and IMRT planning techniques. Averaging all patients, the use of IMRT was associated with a reduction in dose and volume of normal breast irradiated.

The observation that IMRT has its greatest benefit at higher doses may be influential in decreasing toxicity. Skin tolerance to radiation as reported by Emani et al. is 30–40 Gy (30). In the setting of a hypofractionated accelerated

Acknowledgment

The authors thank Angel Sandoval, C.M.D., for assistance with treatment-planning calculations.

References (31)

Cited by (0)

Supported by Rocky Mountain Cancer Centers, Denver, CO.

Presented in poster form at the 48th Annual Meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO), November 5–9, 2006, Philadelphia, PA.

Conflict of interest: none.

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