Abstract
BackgroundThe assessment of organ motion is a crucial feature for prostate stereotactic body radiotherapy (SBRT). Rectal spacer may represent a helpful device in order to outdistance rectal wall from clinical target, but its impact on organ motion is still a matter of debate. MRI-Linac is a new frontier in radiation oncology as it allows a superior visualization of the real-time anatomy of the patient and the current highest level of adaptive radiotherapy.MethodsWe present data regarding a total of 100 fractions in 20 patients who underwent MRI-guided prostate SBRT for low-to-intermediate risk prostate cancer with or without spacer. Translational and rotational shifts were computed on the pre- and post-treatment MRI acquisitions referring to the delivery position for antero-posterior, latero-lateral and cranio-caudal direction, and assessed using the Mann-Whitney U-Test.ResultsAll patients were treated with a five sessions schedule (35 Gy/5fx) using MRI-Linac for a median fraction treatment time of 50 min (range, 46–65). In the entire study sample, median rotational displacement was 0.1° in cranio-caudal, − 0.002° in latero-lateral and 0.01° in antero-posterior direction; median translational shift was 0.11 mm in cranio-caudal, − 0.24 mm in latero-lateral and − 0.22 mm in antero-posterior. A significant difference between spacer and no-spacer patients in terms of rotational shifts in the antero-posterior direction (p = 0.033) was observed; also for translational shifts a positive trend was detected in antero-posterior direction (p = 0.07), although with no statistical significance. We observed statistically significant differences in the pre-treatment planning phase in favor of the spacer cohort for several rectum dose constraints: rectum V32Gy < 5% (p = 0.001), V28 Gy < 10% (p = 0.001) and V18Gy < 35% (p = 0.039). Also for bladder V35 Gy < 1 cc, the use of spacer provided a dosimetric advantage compared to the no-spacer subpopulation (p = 0.04). Furthermore, PTV V33.2Gy > 95% was higher in the spacer cohort compared to the no-spacer one (p = 0.036).ConclusionIn our experience, the application of rectal hydrogel spacer for prostate SBRT resulted in a significant impact on rotational antero-posterior shifts contributing to limit prostate intra-fraction motion. Further studies with larger sample size and longer follow-up are required to confirm this ideally favorable effect and to assess any potential impact on clinical outcomes.
Highlights
The use of hypofractionated radiotherapy for prostate cancer has globally widespread, being endorsed by international guidelines [1, 2]
In our experience, the application of rectal hydrogel spacer for prostate stereotactic body radiotherapy (SBRT) resulted in a significant impact on rotational antero-posterior shifts contributing to limit prostate intra-fraction motion
The pre- and post-treatment Magnetic resonance imaging (MRI) data regarding a total of 100 fractions in 20 consecutive patients who underwent MRI-guided prostate SBRT are presented
Summary
The use of hypofractionated radiotherapy for prostate cancer has globally widespread, being endorsed by international guidelines [1, 2]. The radiobiological rationale for using higher doses per fraction in prostate cancer lies on the known low alpha-beta ratio of the tumor, estimated in 1.5 Gy, which reflects a superior sensitivity to > 2 Gy/fractions in terms of biological effect [3]. Technological progress has allowed clinicians to deliver very high doses per fraction with higher levels of accuracy and confidence [4]. The use of SBRT in prostate cancer has been reported by several experiences in the literature, with long-term data that report this technique as a safe and effective treatment for localized prostate cancer [9]. The assessment of organ motion is a crucial feature for prostate stereotactic body radiotherapy (SBRT). MRI-Linac is a new frontier in radiation oncology as it allows a superior visualization of the real-time anatomy of the patient and the current highest level of adaptive radiotherapy
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