Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114, USA.
Med Phys. 2011 Aug;38(8):4623-33. doi: 10.1118/1.3604152.
To quantify interfractional anatomical variations and their dosimetric impact during the course of fractionated proton therapy (PT) of prostate cancer and to assess the robustness of the current treatment planning techniques.
Simulation and daily in-room CT scans from ten prostate carcinoma patients were analyzed. PT treatment plans (78 Gy in 39 fractions of 2 Gy) were created on the simulation CT, delivering 25 fractions to PTV1 (expanded from prostate and seminal vesicles), followed by 14 boost fractions to PTV2 (expanded from prostate). Plans were subsequently applied to daily CT, with beams aligned to the prostate center in the sagittal plane. For five patients having a sufficiently large daily imaging volume, structure contours were manually drawn, and plans were evaluated for all CT sets. For the other five patients, the plans were evaluated for six selected fractions. The daily CT was matched to the simulation CT through deformable registration. The registration accuracy was validated for each fraction, and the three patients with a large number of accurately registered fractions were used for dose accumulation.
In individual fractions, the coverage of the prostate, seminal vesicles, and PTV1 was generally maintained at the corresponding prescription dose. For PTV2, the volume covered by the fractional prescription dose of 2 Gy (i.e., V2) was, on average, reduced by less than 3% compared to the simulation plan. Among the 225 (39 x 5 + 6 x 5) fractions examined, 15 showed a V2 reduction larger than 5%, of which ten were caused by a large variation in rectal gas, and five were due to a prostate shift in the craniocaudal direction. The fractional dose to the anterior rectal wall was found to increase for one patient who had large rectal gas volume in 25 of the 39 fractions, and another who experienced significant prostate volume reduction during the treatment. The fractional bladder dose generally increased with decreasing fullness. In the total accumulated dose for the three patients after excluding a few fractions with inaccurate registration due to a large amount of rectal gas (a condition inconsistent with RTOG protocol), 98.5%, 96.6%, and 98.2% of the PTV2 received the prescription dose of 78 Gy. The V75 and V70 of the anterior rectal wall and bladder both remained within tolerance.
The results confirm that the PT planning techniques and dose constraints used at our institution ensure that target coverage to the prescription dose is maintained in the presence of interfractional anatomical variations. Dose coverage in individual fractions can be compromised, and normal tissue dose increased, due to deviations in the bladder and rectal volume compared to the simulation plans or progressive changes in the prostate volume during the treatment. Deviations from the plan can be reduced with efforts aimed at maintaining consistent daily patient anatomy.
量化前列腺癌分次质子治疗过程中的分次间解剖学变化及其剂量学影响，并评估当前治疗计划技术的稳健性。
对 10 例前列腺癌患者的模拟和每日机房 CT 扫描进行分析。在模拟 CT 上创建 78 Gy/39 次/2 Gy 的 PT 治疗计划，先对 PTV1（前列腺和精囊的扩大范围）进行 25 次照射，然后对 PTV2（前列腺的扩大范围）进行 14 次加量照射。随后将计划应用于每日 CT，在矢状面中将光束与前列腺中心对准。对于 5 名具有足够大的每日成像体积的患者，手动绘制结构轮廓，并对所有 CT 集进行评估。对于另外 5 名患者，仅对 6 个选定的分次进行评估。通过变形配准将每日 CT 与模拟 CT 匹配。验证了每个分次的配准精度，并使用 3 名配准精度高的患者进行剂量累加。
在各个分次中，前列腺、精囊和 PTV1 的覆盖范围通常保持在相应的处方剂量。对于 PTV2，与模拟计划相比，单次处方剂量 2 Gy 覆盖的体积（即 V2）平均减少不到 3%。在检查的 225 个分次（39×5+6×5）中，有 15 个显示 V2 减少超过 5%，其中 10 个是由于直肠气体大量变化引起的，5 个是由于前列腺在头尾方向上的移位引起的。在 39 个分次中有 25 个分次中，一名患者直肠气体体积较大，另一名患者在治疗过程中前列腺体积明显减少，导致前直肠壁的分次剂量增加。膀胱的分次剂量通常随充盈度的降低而增加。在排除由于直肠气体大量存在导致配准不准确的少数几个分次（一种与 RTOG 方案不一致的情况）后，对 3 名患者的累积总剂量进行分析，98.5%、96.6%和 98.2%的 PTV2 接受了 78 Gy 的处方剂量。前直肠壁和膀胱的 V75 和 V70 均保持在耐受范围内。
结果证实，本机构使用的 PT 计划技术和剂量限制确保了在存在分次间解剖学变化的情况下，目标覆盖范围保持在处方剂量。由于与模拟计划相比膀胱和直肠体积的偏差或治疗过程中前列腺体积的逐渐变化，个别分次的剂量覆盖可能会受到影响，并且正常组织的剂量可能会增加。通过努力维持患者每天的解剖结构一致，可以减少与计划的偏差。
