PURPOSE Treatment planning for proton therapy requires the relative proton stopping power ratio (RSP) information of the patient for accurate dose calculations. RSP are conventionally obtained after mapping of the Hounsfield units (HU) from a calibrated patient computed tomography (CT). One or multiple CT are needed for a given treatment which represents additional, undesired dose to the patient. For prostate cancer, magnetic resonance imaging (MRI) scans are the gold standard for segmentation while offering dose-less imaging. We here quantify the clinical applicability of converted MR images as a substitute for intensity modulated proton therapy (IMPT) treatment of the prostate. METHODS MRCAT (Magnetic Resonance for Calculating ATtenuation) is a Philips-developed technology which produces a synthetic CT image consisting of five HU from a specific set of MRI acquisitions. MRCAT and original planning CT data sets were obtained for ten patients. An IMPT plan was generated on the MRCAT for each patient. Plans were produced such that they fulfill the prostate protocol in use at Massachusetts General Hospital (MGH). The plans were then recomputed onto the nominal planning CT for each patient. Robustness analyses (±5 mm setup shifts and ±3.5 % range uncertainties) were also performed. RESULTS Comparison of MRCAT plans and their recomputation onto the planning CT plan showed excellent agreement. Likewise, dose perturbations due to setup shifts and range uncertainties were well within clinical acceptance demonstrating the clinical viability of the approach. CONCLUSIONS This work demonstrate the clinical acceptability of substituting MR converted RSP images instead of CT for IMPT planning of prostate cancer. This further translates into higher contouring accuracy along with lesser imaging dose.

中文翻译：

---

基于 MRI 的前列腺癌 IMPT 计划

目的 质子治疗的治疗计划需要患者的相对质子停止能力比 (RSP) 信息，以便准确计算剂量。RSP 通常是在从校准的患者计算机断层扫描 (CT) 映射 Hounsfield 单位 (HU) 之后获得的。给定的治疗需要一个或多个 CT，这代表了对患者的额外的、不希望的剂量。对于前列腺癌，磁共振成像 (MRI) 扫描是分割的黄金标准，同时提供无剂量成像。我们在这里量化转换后的 MR 图像作为前列腺调强质子治疗 (IMPT) 治疗的替代品的临床适用性。方法 MRCAT（用于计算衰减的磁共振）是飞利浦开发的一项技术，可生成由一组特定 MRI 采集的五个 HU 组成的合成 CT 图像。获得了 10 名患者的 MRCAT 和原始计划 CT 数据集。在 MRCAT 上为每位患者生成了 IMPT 计划。制定的计划符合马萨诸塞州综合医院 (MGH) 使用的前列腺协议。然后将计划重新计算到每个患者的标称计划 CT 上。还进行了稳健性分析（±5 毫米设置偏移和±3.5 % 范围不确定性）。结果 MRCAT 计划的比较及其在计划 CT 计划上的重新计算显示出极好的一致性。同样地，由于设置变化和范围不确定性引起的剂量扰动在临床可接受范围内，证明了该方法的临床可行性。结论 这项工作证明了用 MR 转换的 RSP 图像代替 CT 代替前列腺癌的 IMPT 计划的临床可接受性。这进一步转化为更高的轮廓精度以及更少的成像剂量。