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

PURPOSE/OBJECTIVE(S)

The aim of this study was to investigate the impact of intrafraction prostate motion and interfraction anatomical changes on dose metrics and the effect of beam gating and motion correction in dose-escalated Linac-based SBRT.

MATERIALS/METHODS

Thirteen patients (56 fractions) with organ-confined prostate cancer underwent dose-escalated FFF-VMAT SBRT. Accurate patient setup was ensured by CBCT, and real-time 3D prostate motion data were obtained using a novel electromagnetic tracking device. Treatment was interrupted when the signals exceeded a 2 mm threshold in any of the three spatial directions and couch position was corrected unless the offset was transient. Prostate trajectories with and without beam gating and motion correction events were reconstructed and analyzed. Rectum and bladder volumes on each daily CBCT were recorded and compared with volumes at simulation. The prostate motion observed for each fraction was incorporated into the patient original treatment plan with an isocenter shift method. Actually, delivered treatments were then obtained by recalculating the reconstructed motion-encoded plan on deformed CTs reflecting the patient CBCT-anatomy of the day. Non-gated treatments were also simulated using the prostate motion data assuming that no treatment interruptions have occurred.

RESULTS

Treatment interruptions because of target motion trespassing the predefined threshold occurred in 25 fractions (45%). Rectum and bladder volume changes were relevant in most patients. The mean relative dose differences between actually delivered and planned treatments were -3.0% [-18.5 - 2.8] for CTV D99% and -2.6% [-17.8 - 1.0] for PTV D95% over all 56 fractions. However, the median cumulative CTV coverage with 93% of the prescribed dose turned out to be satisfactory. Urethra sparing was slightly degraded and a mean reduction of rectum and bladder dose was seen in all but two dose metrics: the maximum dose to rectum mucosa and the bladder D40%. Nevertheless, only 2 major deviations in rectum mucosa D0.035cc were observed at the end of the treatment. The greatest contribution to target missing and OARs doses came from the interfraction anatomical variations, while intrafraction prostate motion marginally contributed in gated treatments. In non-gated treatments, further deteriorations by 2.4 - 2.8% in minimum target coverage metrics and by 3.1 - 11.6% in bladder dose parameters would have occurred on average.

CONCLUSION

The implemented motion management strategy and the strict patient preparation regimen, along with current PTV margins, robustness of original treatment plans, and fast FFF beam delivery, ensured no significant degradations of dose metrics due to both intrafraction motion and interfraction anatomical changes. Non-gated treatments would have more widely deteriorated the dosimetry of some individual fractions. Thus, continuous monitoring, beam gating and motion correction are recommended to safely deliver dose-escalated prostate SBRT.

Lexicographic Optimization-Based Planning for Single and Multiple Brain Metastasis Radiosurgery with Coplanar Arcs

PURPOSE/OBJECTIVE(S)

This study validated a not yet commercially available fully-automated lexicographic optimization planning system (LOps) for single and multiple lesions intracranial stereotactic radiosurgery (SRS).

MATERIALS/METHODS

Forty-four consecutive SRS treatment plans (21 Gy/1 fx) delivered between November 2019 and August 2022 were retrospectively selected and automatically re-planned by LOps: 25 of them has 1 lesion, 13 had 2 lesions, 4 had 3 lesions, and 2 had 4 lesions. An a-priori assigned priority list, Wish List (WL), was used to define the sequential LO: 4 patient sets (tuning set) were necessary to tune each WL, for single lesion-plans (SLp) and multiple lesion-plans (MLp). While in manual plans (MP), the arc setup is freely chosen, the WL was tuned to use 2 coplanar arcs of 140° and 360° for SLp and MLp, respectively. A 0° and 90° collimator rotation were set for counter-clockwise and clockwise arcs, respectively. The fluence optimization is followed by a Monte Carlo calculation (MCc) with 1 mm-dose grid and 0.5%-statistical uncertainty. A target coverage as high as possible was requested, with at least 80% of the prescription dose covering 99% of the PTV. The main criteria for SLp approval were a brain V12Gy <10 cm3. In MLp this criterion can be overcome to get the minimum target coverage. The remaining 36 SRS plans (21 SL and 15 MLp) were automatically re-planned (testing set). Testing plans were compared in terms of dose-volume constraints, conformality, and monitor units (MUs). Statistical significance was assessed by performing the Wilcoxon test and plan delivery accuracy was verified by pre-treatment QA.

RESULTS

WLs-tuning took 3 days. Overall manual and automatic MCc time can be estimated at 8 hours and 3 hours, respectively. Statistically significant increases in SLp and MLp target coverage (GTV_D98%: +3.0% SL, +5.7% ML, PTV_D98%: +4.4% SL, +13.4% ML) and conformity index were registered. Automatic plans showed acceptably higher median brain V12Gy (SL: MP 7.2 cm3, SLp 7.6 cm3; ML: MP 8.7 cm3, MLp 10.3 cm3). The SLp registered a lower median MU number (-4.2%) while MLp were obtained with a higher median number of MU (+9.8%). This not statistically significant difference did not affect gamma passing rates.

CONCLUSION

The novel LOps produced high-quality clinically acceptable SRS SL and ML plans with coplanar arcs according to institutional-specific planning protocols, significantly reducing the overall planning time from about one working day for one MP to about 3 hours for one automatic plan. Together with comparable OAR sparing, the target coverage was significantly increased and the plan deliverability preserved.

Dosimetric Impact of Setup Errors in Single-Isocenter VMAT Radiosurgery for Brain Metastases

PURPOSE/OBJECTIVE(S)

In stereotactic radiosurgery (SRS) and fractionated stereotactic radiosurgery (fSRS) of brain metastases (BM) using single-isocenter VMAT, intra-fraction positioning errors may affect target coverage. This study aims to investigate geometric and dosimetric accuracy in single and multiple BM treatments.

MATERIALS/METHODS

Seventy patients with single (n = 38) and multiple (n = 32) BM treated with 15-21 Gy in 1 (n = 59) or 27 Gy in 3 (n = 11) fractions using coplanar FFF-VMAT technique were analyzed. PTV was defined by a 2 mm isotropic GTV expansion. Pre-treatment setup errors were evaluated with CBCT and corrected with a robotic six degrees-of-freedom couch. For each fraction, intra-fractional errors were measured by post-treatment CBCT and applied to the planning CT. Plans involving translations and rotations (Fx-plan) were recalculated with Monte Carlo TPS. Original and Fx-plans were compared in terms of target and brain dose parameters, performing the Wilcoxon-Mann-Whitney test (alpha = 0.05). The relationships of the BM volume, maximum dimension, distance-to-isocenter (for multiple BM cases only) and barycenter shift with the difference in target coverage between the two plans were investigated.

RESULTS

The median post-treatment 3D error and maximum rotational error over all 129 BM were 0.5 mm [0.1-2.7] and 0.3° [0.0-1.3], respectively. The resulting median BM barycenter shift was 0.5 mm [0.1-2.7]. The percentage of fractions in which at least one BM barycenter shifted by more than 2 mm from the planned position was 4% and 1% for single and multiple BM cases, respectively. The median single GTV volume was 0.27 cc [0.01-10.48], while the PTV had a median volume of 1.05 cc [0.12-17.05]. The median BM maximum dimension was 10.7 mm [2.9-34.1] and for multiple BM the median distance-to-isocenter was 5.15 cm [0.89-7.52]. For single BM patients, the GTV D95% was never reduced by > 5%, while PTV D95% reductions > 1% occurred in only 11 (29%) PTV. For multiple BM patients, the target statistics were slightly worse, with dose deficits larger than 5% and 1% occurring respectively in 2 BM and 34 (37%) PTV. Anyway, the majority of single and multiple BM had a loss of coverage for both GTV and PTV below 1% in Fx-plans. Larger than 5% brain V12 Gy (SRS) and V20 Gy (fSRS) increases were observed for only one single BM patient. None of the two dosimetric comparisons resulted statistically significant (p>0.05). The differences in target coverage showed a moderate-to-strong correlation only with the BM barycenter shift in both cases (R2 = 0.70-0.73 for single and R2 = 0.44-0.50 for multiple BM).

CONCLUSION

Due to the optimal patient setup, as well as the full six degrees-of-freedom corrections, the safety PTV margin, and the fast beam delivery, the dosimetric effects of residual setup and patient motion errors for both single and multiple BM cases were negligible. These findings warrant a potential reduction in the PTV margin with this treatment technique.