Combining Step-and-Shoot Intensity Modulated Radiation Therapy Concomitant Boost to Tumor Bed and 3D Breast Field-in-Field Plans for Hypofractionated Breast Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> To investigate the efficacy and safety of step-and-shoot intensity modulated radiation therapy (IMRT) concomitant boost (CB) to tumor bed (TB) combined with 3D breast field-in-field (FIF) plans for hypofractionated breast radiotherapy. <h3>Materials/Methods</h3> We initiated prospective cohort study since January, 2020. The patients with early-stage breast cancer or breast ductal carcinoma in situ who received breast-conserving surgery underwent sixteen-fraction treatment. All received prescribed dose of 40 Gy 3D FIF plans to breast, CB with 8 Gy to TB by IMRT. We applied the identical contours and PTV coverage to compare the original plans with the re-plans using conventional fractions of breast FIF followed by en face electron beams to TB on the same patients. Plan quality including maximum dose, RTOG conformal index (CI) and homogeneity index (HI) of PTV_TB, mean dose and volumes irradiated to doses of 5 Gy (V₅) of heart and V₂₀ of ipsilateral lung, in addition to acute toxicity and any recurrence were evaluated. Lyman normal tissue complication probability (NTCP) model for moderate to severe breast fibrosis established by Cambridge and EORTC trials was also calculated. Paired sample t-test and Wilcoxon signed-rank test were used. <h3>Results</h3> Thirty patients with sixty treatment plans were reviewed. The majority (19/30) were pathological prognostic stage I breast cancer and half of the patients were left-sided diseases. No patient experienced more than grade 2 acute toxicity. No recurrence was observed during the follow-up. Mean HI (D₂-D₉₈/D_p × 100; where D₂ = minimum dose to the 2% of the target volume, D₉₈ = minimum dose to the 98% of the target volume, and D_p = prescribed dose) of PTV_TB is 9.4 vs. 18.5 (<i>P</i> < .05). Maximum dose divided by reference dose was also lower in the CB group by 2.9% (<i>P</i> < .05). Mean CI is 1.45 in the CB group compared to 1.63 in the conventional fraction plans (<i>P</i> = .07). In terms of EQD2, mean dose and V₅ of heart for left-sided breast cancer are similar (<i>P</i> = .12 and 0.10). Besides, mean dose and V₂₀ of ipsilateral lung are comparable (<i>P</i> = .26 and 0.43). NTCP for breast fibrosis were all lower in the CB group (Mean: 20.02% vs. 22.13%; <i>P</i> < .05). <h3>Conclusion</h3> Hypofractionated breast radiotherapy combing 3D FIF plans and IMRT CB to TB decreased breast moderate to severe fibrosis by Lyman NTCP model and improved homogenous dose distributions without excessive dose to normal tissue, when compared to conventional fractions using FIF and electron beams. It may reduce the toxicity, and is also labor-saving for patients, radiation oncologists and staff.