Dosimetric Impact of Intrafraction Prostate Motion and Interfraction Anatomical Changes in Dose-Escalated Linac-Based SBRT

Abstract

The dosimetric impact of intrafraction prostate motion and interfraction anatomical changes and the effect of beam gating and motion correction were investigated in dose-escalated linac-based SBRT. Fifty-six gated fractions were delivered using a novel electromagnetic tracking device with a 2 mm threshold. Real-time prostate motion data were incorporated into the patient’s original plan with an isocenter shift method. Delivered dose distributions were obtained by recalculating these motion-encoded plans on deformed CTs reflecting the patient’s CBCT daily anatomy. Non-gated treatments were simulated using the prostate motion data assuming that no treatment interruptions have occurred. The mean relative dose differences between delivered and planned treatments were −3.0% [−18.5–2.8] for CTV D99% and −2.6% [−17.8–1.0] for PTV D95%. The median cumulative CTV coverage with 93% of the prescribed dose was satisfactory. Urethra sparing was slightly degraded, with the maximum dose increased by only 1.0% on average, and a mean reduction in the rectum and bladder doses was seen in almost all dose metrics. Intrafraction prostate motion marginally contributed in gated treatments, while in non-gated treatments, further deteriorations in the minimum target coverage and bladder dose metrics would have occurred on average. The implemented motion management strategy and the strict patient preparation regimen, along with other treatment optimization strategies, ensured no significant degradations of dose metrics in delivered treatments.

Stereotactic body radiation therapy (SBRT) for prostate cancer: Improving treatment delivery efficiency and accuracy

“…Faccenda et al [13] compared the dosimetric impact of intrafraction prostate motion in linac-based gated and non-gated SBRT treatments using the same device, dose prescriptions and constraints described above [11] , [12] . As expected, the authors found larger dose deviations in non-gated treatments due to intrafraction prostate motion.…”

“…Nowadays, prostate volumetric modulated arc therapy (VMAT)-SBRT treatments are commonly planned with multiple flattening filter free (FFF) arcs, generally two [8] , [9] , [10] , [11] , [12] , [13] , [14] but even up to four [15] , thus requiring intrafraction motion monitoring.…”

Treatment Optimization in Linac-Based SBRT for Localized Prostate Cancer: A Single-Arc versus Dual-Arc Plan Comparison

“…The benefit of reducing treatment time in prostate SBRT to potentially mitigate the effects of intrafraction motion has been demonstrated by several groups [ 14 , 15 , 16 , 17 , 18 , 19 ]. In previous studies [ 19 , 26 ], it has been demonstrated that the probabilities of prostate motion >3 mm and >5 mm within 5 min were 1.8% and 0.0%, respectively, and that intrafraction prostate motion had a noticeable impact on some individual fractions using 2 mm PTV margins but tended to average out when considering the cumulative effect over 4–5 treatment fractions. The use of SA VMAT plans as planning strategy to shorten the delivery time has been widely recommended as well [ 34 , 37 , 38 , 42 ].…”

“…Nonetheless, due to the higher doses per fraction and fewer fractions compared to conventional treatment, SBRT requires more precise dose gradients with tighter margins. Since several studies [ 14 , 15 , 16 , 17 , 18 , 19 ] have identified substantial and unpredictable motion of the prostate during the beams’ delivery, significant intrafractional displacement can have a relevant impact on treatment, potentially leading to either excessive exposure of healthy tissue or insufficient target coverage [ 20 , 21 , 22 , 23 , 24 , 25 , 26 ].…”

Planning CT Identifies Patients at Risk of High Prostate Intrafraction Motion

“…The intrafraction motion of the prostate can significantly impact delivery to the prostate gland. The prostate gland is a mobile organ, and larger movements during treatment could result in irradiation of healthy tissue or underdosing of the tumor target volume, adversely affecting tumor control [ 4 , 5 ]. Conversely, patients with higher intrafraction motion might benefit from continuous tracking and intrabeam adjustments through smaller required safety margins [ 6 , 7 ].…”