Investigation of 4D dose in volumetric modulated arc therapy-based stereotactic body radiation therapy: does fractional dose or number of arcs matter?

A novel lung SBRT treatment planning: Inverse VMAT plan with leaf motion limitation to ensure the irradiation reproducibility of a moving target

This study exposed the implementation of a novel technique (VMATLSL) for the planning of moving targets in lung stereotactic body radiation therapy (SBRT). This new technique has been compared to static conformal radiotherapy (3D-CRT), volumetric-modulated arc therapy (VMAT), and dynamic conformal arc (DCA). The rationale of this study was to lower geometric complexity (54.9% lower than full VMAT) and hence ensure the reproducibility of the treatment delivery by reducing the risk for interplay errors induced by respiratory motion. Dosimetry metrics were studied with a cohort of 30 patients. Our results showed that leaf speed limitation provided conformal number (CN) close to the VMAT (median CN of VMATLSL is 0.78 vs 0.82 for full VMAT) and was a significant improvement on 3D-CRT and DCA with segment-weight optimized (respectively 0.55 and 0.57). This novel technique is an alternative to VMAT or DCA for lung SBRT treatments, combining independence from the patient's breathing pattern, from the size and amplitude of the lesion, free from interplay effect, and with dosimetry metrics close to the best that could be achieved with full VMAT.

Patient specific evaluation of breathing motion induced interplay effects

PURPOSE

In lung SABR, interplay between target motion and dynamically changing beam parameters can affect the target coverage. To identify the potential need for motion-management techniques, a comprehensive methodology for pre-treatment estimation of interplay effects has been implemented.

METHODS

In conjunction with an alpha-version of VeriSoft and OCTAVIUS 4D (PTW-Freiburg, Germany), a method is presented to calculate a virtual, motion-simulated 3D dose distribution based on measurement data acquired in a stationary phantom and a subsequent correction with time-dependent target-motion patterns. In-house software has been developed to create user-defined motion patterns based on either simplistic or real patient-breathing patterns including the definition of the exact beam starting phase. The approach was validated by programmed couch and phantom motion during beam delivery. Five different breathing traces with extremely altered beam-on phases (0 % and 50 % respiratory phase) and a superior-inferior motion altitude of 25 mm were used to probe the influence of interplay effects for 14 lung SABR plans. Gamma analysis (2 %/2mm) was used for quantification.

RESULTS

Validation measurements resulted in >98 % pass rates. Regarding the interplay effect evaluation, gamma pass rates of <92 % were observed for sinusoidal breathing patterns with <25 number of breaths per delivery time (NBs) and realistic patterns with <18 NBs.

CONCLUSION

The potential influence of interplay effects on the target coverage is highly dependent on the patient's breathing behaviour. The presented moving-platform-free approach can be used for verification of ITV-based treatment plans to identify whether the clinical goals are achievable without explicit use of a respiratory management technique.

Harmonization of dose prescription for lung stereotactic radiotherapy

•

Doses standardization achieved between dedicated linac and robotic-assisted unit.

•

Both methods delivered 3×18.3 Gy to the near minimum dose of the tumor volume.

•

Four-Dimensional deformable method allowed to estimate dose to a mobile tumor.

•

The reliability of a double-check software using a Monte-Carlo algorithm was validated.

•

Gross Tumor Volume-based prescription presented less dose heterogeneities to the tumor.

Background and purpose

Pulmonary stereotactic treatments can be performed using dedicated linear accelerators as well as robotic-assisted units, and different strategies can be used for dose prescription. This study aimed to compare the doses received by the tumor with a gross tumor volume (GTV)-based prescription on D_98%GTV using a robotic-assisted unit (method A) and planning target volume (PTV)-based prescription on D_95%PTV using a dedicated linac (method B).

Material & methods

Plans of 32 patients were collected for method A, and a dose of 3 × 18 Gy was prescribed using type A algorithm and recalculated using a Monte-Carlo (MC) algorithm. The plans were normalized to match D_98%GTV with the mean D98%GTV¯ of the cohort. The plans of 23 patients were collected for method B, and a dose of 3 × 18 Gy was prescribed to D_95%PTV using a MC algorithm. A 4D-sum method was developed to estimate doses for PTV and GTV. For validation, all plans were recalculated using an independent MC double-check software. A dose harmonization on D_{98% GTV} was determined for both methods.

Results

For method A, mean doses were D_2%GTV = 59.9 ± 2.1 Gy, D_50%GTV = 55.6 ± 1.2 Gy, D_98%GTV = 49.5 ± 0.0 Gy. For method B, the reported doses were D_2%GTV = 64.6 ± 2.1 Gy, D_50%GTV = 62.8 ± 1.7 Gy, and D_98%GTV = 60.0 ± 1.7 Gy. The dose trade-off of D_98%GTV = 55 Gy was obtained for both methods. For method A, it corresponded to a dose prescription of 3 × 20 Gy using type A algorithm, followed by rescaling to obtain D_98%GTV = 55 Gy. For method B, it corresponded to a dose prescription of D_95%PTV = 3 × 16.5 Gy using the MC algorithm.

Conclusions

This study determined similar near-minimum doses D_{98% GTV} of approximately 3 × 18.3 Gy (55 Gy) using a GTV-based prescription on a robotic-assisted unit (method A) and a PTV-based prescription on a dedicated linac (method B).

Multi-institutional feasibility study of intensity-modulated radiotherapy with chemotherapy for locally advanced non-small cell lung cancer

BACKGROUND

This multi-institutional clinical trial evaluated the feasibility of intensity-modulated radiotherapy (IMRT) for patients with locally advanced non-small cell lung cancer (NSCLC).

METHODS

The major inclusion criteria were clinical stage III NSCLC, age 20-74 years, and Eastern Cooperative Oncology Group performance status 0-1. Patients were treated with either cisplatin + S-1 (CS; four cycles every 4 weeks) or carboplatin + paclitaxel (CP; administered weekly with thoracic radiotherapy [RT], plus two consolidation cycles) concurrently with IMRT (60 Gy in 30 fractions). The primary endpoint was a treatment completion rate, defined as at least two cycles of CS or five cycles of CP during IMRT and completing 60 Gy IMRT within 56 days after the start of treatment, assumed its 90% confidence interval exceeds 60%. RT quality assurance was mandatory for all the patients.

RESULTS

Twenty-two patients were registered. One patient withdrew due to pulmonary infection before starting treatment. RT plans were reviewed and none was judged as a protocol violation. Grade 2 and 3 pneumonitis occurred in four (19%) and one (5%) patients, respectively. Seventeen patients met the primary endpoint, with a treatment completion rate of 77.3% (90% confidence interval [CI] 58.0%-90.6%). Four patients failed to complete chemotherapy due to chemotherapy-related adverse events, but 20 patients completed IMRT. There were no treatment-related deaths. The 2-year progression-free and overall survival rates were 31.8% (95% CI 17.3%-58.7%) and 77.3% (95% CI 61.6%-96.9%), respectively.

CONCLUSION

The treatment completion rate did not meet the primary endpoint, but 20 of 22 patients completed IMRT.

Incorporation of tumor motion directionality in margin recipe: The directional MidP strategy

PURPOSE

Planning target volume (PTV) definition based on Mid-Position (Mid-P) strategy typically integrates breathing motion from tumor positions variances along the conventional axes of the DICOM coordinate system. Tumor motion directionality is thus neglected even though it is one of its stable characteristics in time. We therefore propose the directional MidP approach (MidP dir), which allows motion directionality to be incorporated into PTV margins. A second objective consists in assessing the ability of the proposed method to better take care of respiratory motion uncertainty.

METHODS

11 lung tumors from 10 patients with supra-centimetric motion were included. PTV were generated according to the MidP and MidP dir strategies starting from planning 4D CT.

RESULTS

PTV_{MidP dir} volume didn't differ from the PTV_MidP volume: 31351 mm³ IC95% [17242-45459] vs. 31003 mm³ IC95% [ 17347-44659], p = 0.477 respectively. PTV_{MidP dir} morphology was different and appeared more oblong along the main motion axis. The relative difference between 3D and 4D doses was on average 1.09%, p = 0.011 and 0.74%, p = 0.032 improved with directional MidP for D_99% and D_95%. D_2% was not significantly different between both approaches. The improvement in dosimetric coverage fluctuated substantially from one lesion to another and was all the more important as motion showed a large amplitude, some obliquity with respect to conventional axes and small hysteresis.

CONCLUSIONS

Directional MidP method allows tumor motion to be taken into account more tightly as a geometrical uncertainty without increasing the irradiation volume.