Department of Radiation Oncology, Washington University School of Medicine, St Louis, Missouri.
Division of Biomedical Statistics and Informatics, Mayo Clinic, Scottsdale, Arizona.
Int J Radiat Oncol Biol Phys. 2018 Nov 15;102(4):987-995. doi: 10.1016/j.ijrobp.2018.06.022. Epub 2018 Jun 25.
Hypofractionated (>5 fraction) stereotactic radiation therapy (HSRT) may allow for ablative biologically equivalent dose to tumors with a lower risk of organ-at-risk (OAR) toxicity in central thoracic tumors. Adaptive planning may further improve OAR sparing while maintaining planning target volume (PTV) coverage. We hypothesized that midtreatment adaptive replanning would offer dosimetric advantages during HSRT for central thorax malignancies using magnetic resonance imaging (MRI)-guided radiation therapy.
Twelve patients with central thorax tumors received HSRT using MRI-guided radiation therapy. Clinically delivered regimens were 60 Gy in 12 fractions or 62.5 Gy in 10 fractions, with low-field magnetic resonance (0.35 T) volumetric setup imaging acquired at each fraction. Daily gross tumor volume (GTV) and OARs were retrospectively redefined on fraction 1, 6, and 10 MRIs, and GTV response was recorded. Simulated initial plans prescribed a dose of 60 Gy in 12 fractions based on fraction 1 MRI. Midtreatment adaptive plans were created based on fraction 6 anatomy-of-the-day. All plans were created using an isotoxicity approach with a goal of 95% PTV coverage, subject to hard OAR constraints, to represent clinically ideal OAR sparing. Plans were then compared for projected OAR sparing and PTV coverage.
Patients demonstrated significant on-treatment MRI-defined GTV reduction (median 41.8%; range 16.7%-65.7%). At fraction 6, median reduction was 26.7%. All initial plans met OAR constraints. Initial plan application to fraction 6 and fraction 10 anatomy resulted in 8 OAR violations (5 of 13 patients) and 10 OAR violations (6 of 13 patients). All fraction 6 violations persisted at fraction 10. Midpoint adaptive planning reversed 100% of midpoint OAR violations and tended to reduce the magnitude of OAR violations incurred at fraction 10. In 40% of fractions (2 of 5) in which OAR violation resulted from initial plan application to fraction 6 anatomy, PTV coverage was increased concomitant with violation reversal.
Midtreatment adaptive planning based on tumor response may be dosimetrically advantageous for sparing of surrounding critical structures in HSRT for central thorax malignancies and could be applied using either an online or offline paradigm.
大分割（＞5 次分割）立体定向放疗（HSRT）可能允许对中央胸部肿瘤进行肿瘤消融性生物等效剂量治疗，同时降低危及器官（OAR）毒性的风险。自适应计划可能进一步提高 OAR 保护，同时保持计划靶区（PTV）覆盖。我们假设在使用磁共振成像（MRI）引导放疗的中央胸部恶性肿瘤的 HSRT 中，中期自适应再计划将提供剂量学优势。
12 例中央胸部肿瘤患者接受 MRI 引导放疗的 HSRT。临床给予的方案为 60 Gy/12 次分割或 62.5 Gy/10 次分割，每次分割时采集低场磁共振（0.35 T）容积设置成像。在第 1、6 和 10 次 MRI 上回顾性重新定义 GTV 和 OAR，并记录 GTV 反应。基于第 1 次 MRI 模拟初始计划，将 60 Gy/12 次分割剂量设定为处方剂量。根据第 6 次分割的解剖结构，制定中期自适应计划。所有计划均采用等毒性方法进行制定，目标是 95%PTV 覆盖，同时受 OAR 硬限制的约束，以代表临床理想的 OAR 保护。然后比较计划以评估预测的 OAR 保护和 PTV 覆盖。
患者在治疗过程中表现出明显的 MRI 定义的 GTV 缩小（中位数 41.8%；范围 16.7%-65.7%）。在第 6 次分割时，中位数缩小率为 26.7%。所有初始计划均符合 OAR 限制。初始计划在第 6 次和第 10 次分割的解剖结构上的应用导致 8 次 OAR 侵犯（13 例患者中的 5 例）和 10 次 OAR 侵犯（13 例患者中的 6 例）。所有第 6 次分割的侵犯均持续到第 10 次分割。中期自适应计划完全逆转了 100%的中期 OAR 侵犯，并倾向于减少第 10 次分割时发生的 OAR 侵犯程度。在导致初始计划应用于第 6 次分割解剖结构的 OAR 侵犯的 5 次分割中的 40%（2/5）中，PTV 覆盖同时增加，同时逆转了侵犯。
基于肿瘤反应的中期自适应计划对于中央胸部恶性肿瘤的 HSRT 中保护周围关键结构具有剂量学优势，并且可以使用在线或离线模式应用。
