Bolus is commonly used in MV photon radiotherapy to increase superficial dose and improve dose uniformity for treating shallow lesions. However, irregular patient body contours can cause unwanted air gaps between a bolus and patient skin. The resulting dosimetric errors could be exacerbated in MR-Linac treatments, as secondary electrons generated by photons are affected by the magnetic field. This study aimed to quantify the dosimetric effect of unwanted gaps between bolus and skin surface in an MR-Linac. A parallel-plate ionization chamber and EBT3 films were utilized to evaluate the surface dose under bolus with various gantry angles, field sizes, and different air gaps. The results of surface dose measurements were then compared to Monaco 5.40 Treatment Planning System (TPS) calculations. The suitability of using a parallel-plate chamber in MR-Linac measurement was validated by benchmarking the percentage depth dose and output factors with the microDiamond detector and air-filled ionization chamber measurements in water. A non-symmetric response of the parallel-plate chamber to oblique beams in the magnetic field was characterized. Unwanted air gaps significantly reduced the skin dose. For a frontal beam, skin dose was halved when there was a 5 mm gap, a much larger difference than in a conventional linac. Skin dose manifested a non-symmetric pattern in terms of gantry angle and gap size. The TPS overestimated skin dose in general, but shared the same trend with measurement when there was no air gap, or the gap size was larger than 5 mm. However, the calculated and measured results had a large discrepancy when the bolus-skin gap was below 5 mm. When treating superficial lesions, unwanted air gaps under the bolus will compromise the dosimetric goals. Our results highlight the importance of avoiding air gaps between bolus and skin when treating superficial lesions using an MR-Linac system.

丸剂常用于 MV 光子放射治疗，以增加浅表剂量并改善治疗浅部病变的剂量均匀性。然而，不规则的患者身体轮廓会导致药丸和患者皮肤之间出现不希望的气隙。在 MR-Linac 治疗中，由此产生的剂量误差可能会加剧，因为光子产生的二次电子会受到磁场的影响。本研究旨在量化 MR-Linac 中丸剂和皮肤表面之间不需要的间隙的剂量学影响。平行板电离室和 EBT3 薄膜用于评估具有不同机架角度、场大小和不同气隙的丸剂下的表面剂量。然后将表面剂量测量的结果与 Monaco 5.40 治疗计划系统 (TPS) 计算进行比较。在 MR-Linac 测量中使用平行板室的适用性通过在水中使用 microDiamond 探测器和充气电离室测量对百分比深度剂量和输出因子进行基准测试来验证。表征了平行板室对磁场中的倾斜光束的非对称响应。不需要的气隙显着降低了皮肤剂量。对于正面光束，当有 5 毫米间隙时，皮肤剂量减半，这比传统直线加速器的差异要大得多。皮肤剂量在机架角度和间隙大小方面表现出非对称模式。TPS 总体上高估了皮肤剂量，但在没有气隙或气隙尺寸大于 5 mm 时与测量具有相同的趋势。然而，当丸-皮间隙小于5mm时，计算结果与实测结果有较大差异。在治疗浅表病变时，药丸下不需要的气隙会影响剂量测定目标。我们的结果强调了在使用 MR-Linac 系统治疗表面​​病变时避免丸剂和皮肤之间气隙的重要性。