Surface guidance compared with ultrasound-based monitoring and diaphragm position in cone-beam computed tomography during abdominal stereotactic radiotherapy in breath-hold.

Background and purpose: Spirometry induced deep-inspiration-breath-hold (DIBH) reduces intrafractional motion during upper abdominal stereotactic body radiotherapy (SBRT). The aim of this prospective study was to evaluate whether surface scanning (SGRT) is an adequate surrogate for monitoring residual internal motion during DIBH. Residual motion detected by SGRT was compared with experimental 4D-ultrasound (US) and an internal motion detection benchmark (diaphragm-dome-position in kV cone-beam computed tomography (CBCT) projections). Materials and methods: Intrafractional monitoring was performed with SGRT and US in 460 DIBHs of 12 patients. Residual motion detected by all modalities (SGRT (anterior-posterior (AP)), US (AP, craniocaudal (CC)) and CBCT (CC)) was analyzed. Agreement analysis included Wilcoxon signed rank test, Maloney and Rastogi's test, Pearson's correlation coefficient (PCC) and interclass correlation coefficient (ICC). Results: Interquartile range was 0.7 mm (US(AP)), 0.8 mm (US(CC)), 0.9 mm (SGRT) and 0.8 mm (CBCT). SGRT(AP) vs. CBCT(CC) and US(CC) vs. CBCT(CC) showed comparable agreement (PCCs 0.53 and 0.52, ICCs 0.51 and 0.49) with slightly higher precision of CBCT(CC). Most agreement was observed for SGRT(AP) vs. US(AP) with largest PCC (0.61) and ICC (0.60), least agreement for SGRT(AP) vs. US(CC) with smallest PCC (0.44) and ICC (0.42). Conclusions: Residual motion detected during spirometry induced DIBH is small. SGRT alone is no sufficient surrogate for residual internal motion in all patients as some high velocity motion could not be detected. Observed patient-specific residual errors may require individualized PTV-margins.

Transmission probability filter optimization for Agility MLC in Monaco treatment planning system.

In the Monte Carlo-based treatment planning system (TPS) Monaco, transmission probability filters (TPF) are utilized to describe the transmission through the multi leaf collimator (MLC). By having knowledge of the TPF parameters for various photon beam energies, adjusting the MLC transmission parameters becomes easier, enhancing the accuracy of the Monte Carlo algorithm in achieving a dose distribution that closely aligns with the irradiated dose at the Versa HD linear accelerator (linac). The objective of this study was to determine the TPF parameters for 6MV, 10MV, 6MV flattening filter free (FFF) and 10MV FFF for a Versa HD linac equipped with Agility MLC. The TPF parameters were adjusted using point dose measurements and vendor-provided fields specifically designed to fine-tune the MLC. After adjusting the TPF parameters, a gamma passing rate (GPR) analysis was conducted on 25 treatment plans to ensure that the Monte Carlo model, with the updated TPF parameters, accurately matched the actual linac delivery. The TPF values ranged from 0.0018 to 0.0032 for leaf transmission and 1.15 to 1.25 for Leaf Tip leakage across the different energies. The average GPR ranged from 97.8% for 10MV FFF to 98.5% for 6MV photon energies. Additionally, the TPF parameters for 6MV obtained in this study were consistent with previously published TPF values for 6MV photon energy. Hence, it was concluded that optimizing the TPF does not need to be performed for every individual Versa HD linac with Agility MLC. Instead, the published parameters can be applied to other Versa HD linacs to enhance clinical accuracy. In conclusion, this study determined the TPF parameters for 6MV and previously unpublished photon energies 10MV, 6MV FFF and 10MV FFF. These parameters can be easily transferred to other facilities, resulting in improved agreement between the dose distribution from the TPS and the linac.

Dosimetric benefits of daily treatment plan adaptation for prostate cancer stereotactic body radiotherapy.

BACKGROUND: Hypofractionation is increasingly being applied in radiotherapy for prostate cancer, requiring higher accuracy of daily treatment deliveries than in conventional image-guided radiotherapy (IGRT). Different adaptive radiotherapy (ART) strategies were evaluated with regard to dosimetric benefits. METHODS: Treatments plans for 32 patients were retrospectively generated and analyzed according to the PACE-C trial treatment scheme (40 Gy in 5 fractions). Using a previously trained cycle-generative adversarial network algorithm, synthetic CT (sCT) were generated out of five daily cone-beam CT. Dose calculation on sCT was performed for four different adaptation approaches: IGRT without adaptation, adaptation via segment aperture morphing (SAM) and segment weight optimization (ART1) or additional shape optimization (ART2) as well as a full re-optimization (ART3). Dose distributions were evaluated regarding dose-volume parameters and a penalty score. RESULTS: Compared to the IGRT approach, the ART1, ART2 and ART3 approaches substantially reduced the V37Gy(bladder) and V36Gy(rectum) from a mean of 7.4cm3 and 2.0cm3 to (5.9cm3, 6.1cm3, 5.2cm3) as well as to (1.4cm3, 1.4cm3, 1.0cm3), respectively. Plan adaptation required on average 2.6 min for the ART1 approach and yielded doses to the rectum being insignificantly different from the ART2 approach. Based on an accumulation over the total patient collective, a penalty score revealed dosimetric violations reduced by 79.2%, 75.7% and 93.2% through adaptation. CONCLUSION: Treatment plan adaptation was demonstrated to adequately restore relevant dose criteria on a daily basis. While for SAM adaptation approaches dosimetric benefits were realized through ensuring sufficient target coverage, a full re-optimization mainly improved OAR sparing which helps to guide the decision of when to apply which adaptation strategy.

Evaluation of a cycle-generative adversarial network-based cone-beam CT to synthetic CT conversion algorithm for adaptive radiation therapy.

PURPOSE: Image-guided radiation therapy could benefit from implementing adaptive radiation therapy (ART) techniques. A cycle-generative adversarial network (cycle-GAN)-based cone-beam computed tomography (CBCT)-to-synthetic CT (sCT) conversion algorithm was evaluated regarding image quality, image segmentation and dosimetric accuracy for head and neck (H&N), thoracic and pelvic body regions. METHODS: Using a cycle-GAN, three body site-specific models were priorly trained with independent paired CT and CBCT datasets of a kV imaging system (XVI, Elekta). sCT were generated based on first-fraction CBCT for 15 patients of each body region. Mean errors (ME) and mean absolute errors (MAE) were analyzed for the sCT. On the sCT, manually delineated structures were compared to deformed structures from the planning CT (pCT) and evaluated with standard segmentation metrics. Treatment plans were recalculated on sCT. A comparison of clinically relevant dose-volume parameters (D98, D50 and D2 of the target volume) and 3D-gamma (3%/3mm) analysis were performed. RESULTS: The mean ME and MAE were 1.4, 29.6, 5.4 Hounsfield units (HU) and 77.2, 94.2, 41.8 HU for H&N, thoracic and pelvic region, respectively. Dice similarity coefficients varied between 66.7 ± 8.3% (seminal vesicles) and 94.9 ± 2.0% (lungs). Maximum mean surface distances were 6.3 mm (heart), followed by 3.5 mm (brainstem). The mean dosimetric differences of the target volumes did not exceed 1.7%. Mean 3D gamma pass rates greater than 97.8% were achieved in all cases. CONCLUSIONS: The presented method generates sCT images with a quality close to pCT and yielded clinically acceptable dosimetric deviations. Thus, an important prerequisite towards clinical implementation of CBCT-based ART is fulfilled.

Characterization and clinical evaluation of a novel 2D detector array for conventional and flattening filter free (FFF) IMRT pre-treatment verification.

BACKGROUND AND PURPOSE: The novel MatriXXFFF (IBA Dosimetry, Germany) detector is a new 2D ionization chamber detector array designed for patient specific IMRT-plan verification including flattening-filter-free (FFF) beams. This study provides a detailed analysis of the characterization and clinical evaluation of the new detector array. MATERIAL AND METHODS: The verification of the MatriXXFFF was subdivided into (i) physical dosimetric tests including dose linearity, dose rate dependency and output factor measurements and (ii) patient specific IMRT pre-treatment plan verifications. The MatriXXFFF measurements were compared to the calculated dose distribution of a commissioned treatment planning system by gamma index and dose difference evaluations for 18 IMRT-sequences. All IMRT-sequences were measured with original gantry angles and with collapsing all beams to 0° gantry angle to exclude the influence of the detector's angle dependency. RESULTS: The MatriXXFFF was found to be linear and dose rate independent for all investigated modalities (deviations ≤0.6%). Furthermore, the output measurements of the MatriXXFFF were in very good agreement to reference measurements (deviations ≤1.8%). For the clinical evaluation an average pixel passing rate for γ(3%,3mm) of (98.5±1.5)% was achieved when applying a gantry angle correction. Also, with collapsing all beams to 0° gantry angle an excellent agreement to the calculated dose distribution was observed (γ(3%,3mm)=(99.1±1.1)%). CONCLUSIONS: The MatriXXFFF fulfills all physical requirements in terms of dosimetric accuracy. Furthermore, the evaluation of the IMRT-plan measurements showed that the detector particularly together with the gantry angle correction is a reliable device for IMRT-plan verification including FFF.