A study on the application of range shifter and bolus in spot-scanning proton arc (SPArc) therapy after modified radical mastectomy for left-sided breast cancer

Background: This study aimed to compare the dosimetric differences between the use of range shifter and bolus in spot-scanning proton arc (SPArc) therapy for postoperative radiotherapy after left-sided breast cancer modified radical mastectomy, with a primary focus on target volume dose coverage and organs-at-risk sparing. By comparison with VMAT, this study further explored the potential of SPArc to reduce the normal tissue complication probability (NTCP) and the effective dose to immune cells (EDIC). Methods: This retrospective study utilized the imaging datasets of 18 patients who treated with VMAT. Three SPArc plans were designed for each patient using the RayStation v2025: 1. SPArc with bolus (SPArcB); 2. SPArc with range shifter (SPArcR); 3. SPArc with both bolus and range shifter (SPArcRB). The target volumes included the supraclavicular target volume and the chest wall target volume. The Lyman-Kutcher-Birman NTCP model was adopted to evaluate pulmonary and cardiac toxicities and the EDIC model was applied to assess the immune cell dose across different plans. Robustness optimization accounting for a ± 3.5% range uncertainty and a 5 mm setup error was incorporated into all SPArc treatment plans. Results: All SPArc plans met robustness criteria and significantly reduced doses to the heart and left lung, as well as EDIC, compared to VMAT. The SPArcR plan demonstrated superior cardiac protection, achieving the lowest mean heart dose and the most significant reduction in NTCP for both the heart and lungs. While SPArcB showed a slight advantage in reducing the mean left lung dose, SPArcR yielded the highest conformity index for target coverage. Conclusion: This study confirms that, in postoperative radiotherapy for left-sided breast cancer after modified radical mastectomy, the standalone application of a range shifter can optimally reduce cardiac dose and the associated risk of complications while ensuring adequate target volume coverage, and it circumvents the issues of setup variability and hygiene concerns associated with the clinical use of bolus

Bolus (digestion) · Breast cancer · Dosimetry · Modified Radical Mastectomy · Proton therapy · Radiation therapy · Advanced Radiotherapy Techniques · Breast Cancer Treatment Studies · Radiation Therapy and Dosimetry

• Global cancer statistics 2022

  Open Access•Francesca Bray, Mathieu Laversanne et al.•CA: A Cancer Journal for Clinicians•2024

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