Differences in superficial dose distribution in patients receiving intensity modulated radiotherapy (IMRT) after radical mastectomy

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Background: The accuracy of the dose on the chest wall in postoperative radiotherapy for breast cancer is critical for clinicians and patients. The objective of this study was to evaluate the superficial dose in patients undergoing radiotherapy after radical mastectomy using deep inspiration breath-hold (DIBH) and free breathing (FB) and to investigate the effects of respiratory amplitude in an anthropomorphic chest phantom. Methods: Thirty patients with left-breast radical mastectomy were prospectively enrolled before radiotherapy. During computed tomography (CT) simulation, half of the patients were treated with DIBH and half with FB. All patients were subjected to intensity modulated radiotherapy (IMRT) with a prescribed dose of 2 Gy. Eight quantum effect devices (QED) detectors were placed on the breast surface to measure the superficial dose. The measured average values from the detectors were compared with the dose calculated using the treatment planning system (TPS). The respiratory motion was simulated using a custom-made moving platform to drive the movement of an anthropomorphic chest phantom in the anterior-posterior direction at four different amplitudes-5, 10, 15, and 20 mm. Using the same detectors, the superficial dose was measured on the phantom during irradiation on the virtual left-side breast target with the same dose. Results: measurements were -2.63% (P=0.01) for FB and -0.107% (P=0.96) for DIBH, with an overall range of 5%. For the phantom, the relative dose differences were 5%, 6%, 8%, and 18% for movement amplitudes of 5, 10, 15, and 20 mm. Conclusions: During IMRT, slight respiratory movement does not cause variations in the superficial dose on the bolus-covered chest wall target. However, respiratory movement at high amplitudes may result in target dose variations of up to 10%.

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