Department of Radiation Oncology, University Hospital Tübingen, Tübingen, Germany.
Clinic for Radiation Oncology, Oberschwaben Hospital Group, Ravensburg, Germany.
Acta Oncol. 2020 Aug;59(8):911-917. doi: 10.1080/0284186X.2020.1760349. Epub 2020 May 21.
Cure- and toxicity rates of prostate IGRT can both be affected by ill-chosen planning target volume (PTV) margins. For dose-escalated prostate radiotherapy, we studied the potential for organ at risk (OAR) sparing and compensation of prostate motion with robust plan optimization using the coverage probability (CovP) concept compared to conventional PTV-based IMRT. We evaluated plan quality of CovP-plans for 27 intermediate risk prostate cancer patients treated in a prospective study (78 Gy/39 fractions). Clinical target volume (CTV) and OARs were contoured on three separate CTs to capture movement and deformation. To define the internal target volume (ITV), the union of CTV1-3 was encompassed by an isotropic margin of 7 mm for the planning process. CovP-dose distribution is optimized considering weight factors for IMRT constraints derived from probabilities of systematic organ displacement in the three CTs. CovP-dose volume histograms (DVHs) were compared with additionally calculated conventional PTV-based IMRT plans. PTV-based IMRT was planned on one-single CT with an isotropically expanded CTV to generate the PTV (i.e., CTV1 + 7mm) and was evaluated on the two other CTs. The CovP-concept showed higher robustness in target volume coverage. Target miss was frequently observed with PTV-based IMRT, resulting in cold spots until 70 Gy with the CovP-concept. The target dose at 74 Gy was comparable, while further the dose-escalation (75-78 Gy) was improved with PTV-based IMRT. However, dose-escalation with PTV-based IMRT was associated with increased OAR-doses, especially in high-dose areas. Probabilistic dose-escalated IMRT was feasible in this prospective study. Comparison of the CovP-concept with PTV-based IMRT revealed superiority with regard to target-coverage and sparing of OARs. The CovP-concept implements a robust plan optimization strategy for organ deformation and motions and could, therefore, serve as a less demanding compromise on the way to adaptive IGRT avoiding daily time-consuming re-planning. SUMMARYWe evaluated the robustness of coverage probability (CovP)-based IMRT plans within a prospective study for prostate cancer radiotherapy. The treatment plans were compared with newly calculated conventional PTV-based IMRT plans. We were able to show that CovP led to a clearly more robust target coverage by avoiding hot spots at OARs compared to conventional PTV-based IMRT. In addition, negative consequences of an inflated PTV can be ameliorated by a more relaxed CovP-based dose prescription.
前列腺 IGRT 的治愈率和毒性率都可能受到计划靶区（PTV）边界选择不当的影响。对于剂量递增的前列腺放疗，我们使用覆盖概率（CovP）概念研究了使用稳健的计划优化来保护危及器官（OAR）和补偿前列腺运动的潜力，与传统的基于 PTV 的调强放疗（IMRT）相比。我们评估了 27 例中危前列腺癌前瞻性研究患者（78Gy/39 次）的 CovP 计划的质量。临床靶区（CTV）和 OAR 分别在 3 个独立的 CT 上勾画，以捕捉运动和变形。为了定义内部靶区（ITV），CTV1-3 的联合通过 7mm 的各向同性边界来确定计划过程。CovP 剂量分布是根据在三个 CT 中系统器官移位概率得出的 IMRT 约束的权重因素进行优化的。将 CovP 剂量体积直方图（DVH）与另外计算的基于传统 PTV 的 IMRT 计划进行比较。基于 PTV 的 IMRT 是在一个单独的 CT 上进行的，CTV1 均匀扩大生成 PTV（即 CTV1+7mm），并在另外两个 CT 上进行评估。CovP 概念在靶区覆盖方面表现出更高的稳健性。基于 PTV 的 IMRT 经常观察到靶区漏诊，导致冷点，直到 CovP 概念达到 70Gy。在 74Gy 时，靶区剂量相当，而基于 PTV 的 IMRT 则提高了剂量递增（75-78Gy）。然而，基于 PTV 的 IMRT 的剂量递增与 OAR 剂量增加有关，尤其是在高剂量区域。这项前瞻性研究中，概率性剂量递增 IMRT 是可行的。CovP 概念与基于 PTV 的 IMRT 的比较显示，在靶区覆盖和 OAR 保护方面具有优势。CovP 概念为器官变形和运动实施了稳健的计划优化策略，因此可以作为避免日常耗时重新计划的适应性 IGRT 的一种不那么苛刻的妥协。总结我们在前列腺癌放疗的前瞻性研究中评估了基于覆盖概率（CovP）的 IMRT 计划的稳健性。将治疗计划与新计算的传统 PTV 为基础的 IMRT 计划进行比较。我们能够表明，与传统的基于 PTV 的 IMRT 相比，CovP 通过避免 OAR 中的热点，明显提高了靶区的覆盖能力。此外，通过更宽松的 CovP 剂量处方，可以改善 PTV 膨胀的负面影响。
