Challenges of Treating Lung Cancer Patients at MR-Linac Using MR-Based Synthetic CT Calculation in the Adaptive Workflow

PURPOSE/OBJECTIVE(S)

Magnetic Resonance guided adaptive radiotherapy (MRgART) allows plan adaptation according to the new patient anatomy; the contours of the structures are adjusted based on the patient's daily MRI, and in the adapt to shape (ATS) workflow, the adapted plan is recalculated on the MRI-based synthetic CT (sCT) generated by bulk density assignment. For sites where there is a high electronic density (ED) gradient between the target and surrounding tissues, such as in lung cancer treatments, the assignment of an average ED may not be able to reproduce an accurate dose calculation. This study evaluates the accuracy of the sCT adapted plan calculation for lung cancer patients and assesses whether the assignment of an optimized ED can reduce dosimetric differences should they arise MATERIALS/METHODS: Nine lung cancer patients treated at Unity 1.5 MR-Linac were selected for this retrospective study. The patient's target and organs at risk (OARs) were contoured, and a CT reference plan containing the ED bulk assignment information i.e., the contours to use in the ATS workflow, and their corresponding average ED was generated. To assess the accuracy of the dosimetry of the adapted plan calculated on the sCT, the plan was recalculated on an ideal sCT (sCTref) obtained from the reference CT by forcing the drawn contours to the average ED as defined on the CT reference plan. Targets and OARs dose-volume histogram (DVH) of the CT and sCTref plans and the dose distributions using gamma (γ) analysis with 2%-2mm criteria were compared. In the case of a discrepancy between the DVHs, the average Eds used for the recalculation on the sCTref were adjusted by several attempts to obtain a sCT optimized (sCTopt) for which a superposition of DVHs on CT and sCTopt was achieved.

RESULTS

For 7 of the 9 patients CT and sCTref target DVHs were not comparable, with a mean dosimetric difference of 5.55% (range 2.35%-7.46%) in the target volume receiving the prescription dose (VDpre), while OARs DVH dose differences remained below 1% for the nine patients. The adjustment of the ED of the homolateral lung in the sCTopt, reduced the mean target VDpre dosimetric difference between CT and sCTopt to 0.66% (range 0.17%-1.64%). In addition, the results of the gamma analysis increased from values ranging between 39.5%-70.3% to 88.5%-93.2%, as shown in the Table.

CONCLUSION

Dosimetric errors in the use of the sCT calculation for targets in high ED gradient areas may arise; the use of optimized ED for sCT calculation may be a promising strand to investigate in order to proceed with MR-based sCT plan adaptation for lung cancer treatment.

Effectiveness of Bladder Filling Control during Online MR-Guided Adaptive Radiotherapy for Rectum Cancer

PURPOSE/OBJECTIVE(S)

MR-guided adaptive radiotherapy (MRgART) treatment sessions at MR-Linac are time-consuming and changes in bladder filling during the session can impact the treatment dosimetry. In this work, we present the procedure implemented in the clinical workflow to stabilize bladder filling during the MR based adaptive radiotherapy sessions and evaluate its effectiveness and the resulting dosimetric impact on the adaptive plan.

MATERIALS/METHODS

Twenty-five rectum cancer patients treated at 1.5T MR-Linac with a short course radiotherapy (25 Gy in 5 fractions of 5 Gy each) were included in this retrospective study. Patients were treated with the adapt-to-shape workflow consisting of a plan adaptation based on the MRI acquired in each session and optimized on the corresponding MR-based synthetic CT. Considering the significant interval time between the acquisition of the first daily MRI used for plan adaptation, and the beam delivery, a bladder catheter was used to stabilize the bladder filling; the procedure consists of emptying the bladder and refilling it with a well-known amount of physiological solution before each MRI acquisition. Two MRIs were acquired at each session: the first was used for plan adaptation and the second was acquired while approving the adapted plan, to be rigidly registered with the first to ensure the appropriateness of the isodoses on the ongoing delivery treatment. A total of 125 sessions and 250 MRI images and bladder contours were analyzed; for each fraction, the time interval between the first and second MRI and the corresponding bladder volumes were recorded; the consistency of bladder volumes and shapes along each online session was assessed with the dice similarity index (DSC) and Hausdorff distance (HD); the impact on plan dosimetry was evaluated by comparing target and bladder DVH cut off points of the plan on the two different MRI datasets.

RESULTS

The time interval between the first and second MRI, averaged over the 125 sessions is 39.0 min, range (18.6-75.8) min. The changes in bladder volumes, DSC index, HD, and the differences between the bladder and target DVH cut-off points are shown in the table below. The DSC and HD are comparable to inter-observer variability in manual contour segmentation, with an average DSC of 0.91 and average HD of 2.13 mm; the average differences in bladder and target dosimetry remain under 0.63% and 0.10%, respectively.

CONCLUSION

The use of a procedure in the clinical workflow of MRgART to stabilize the bladder filling throughout the online session may be helpful to guarantee the accuracy of the ongoing delivered treatment.

Study on the Clinical Use of a Respiratory Navigator Combined with Breath-Hold for MRI- Guided Liver SBRT

PURPOSE/OBJECTIVE(S)

Respiratory movement strongly affects the accuracy of stereotactic body radiation therapy (SBRT) of liver malignancies treated without the use of a respiratory gating system. This study investigates the feasibility and advantages of using a respiratory navigator-guided combined with patient breath-hold for liver SBRT in an adaptive magnetic-resonance guided workflow.

MATERIALS/METHODS

Clinical datasets of 10 liver cancer patients treated with 1.5T MR-Linac with respiratory navigator-guided SBRT combined with patient breath-hold were retrospectively analyzed. All patients underwent simulation CT with and without contrast, and 4D-CT and 3D-T2w MRI without contrast. Patients received a prescription dose ranging from 36 to 50 Gy in 5 to 8 fractions and followed the adapt to shape (ATS) workflow including contours adjustment and a subsequent MR-based synthetic CT (sCT) calculation on the online MRI acquired. The reference treatment plan was optimized on the expiratory phase of the 4D-CT, and during the online session the contours and the adapted plans were performed using the 3D-T2w navigator MRI of the patient's end-expiratory signal; 2D-T2w real-time monitoring MRI was also used as support for the contour's definition. The radiation therapist instructed the patients to hold their breath at the end of the breathing cycle for the time of the beam on. A total of 59 fractions were analyzed. For each fraction the dosimetric parameters of the target and normal liver of the adaptive and reference plans were compared; particularly the volume, the conformity index (CI) and gradient index (GI) for the target, and V5, V10 and Dmean for the normal liver. T-student statistical analysis was performed; a p-value less than 0.05 was considered statistically significant.

RESULTS

In the free breathing state, the 3D-T2w navigator MRI images enable a clear visualization of the tumor and its boundaries. The average target CI of the adaptive and reference plans is not significantly different (p = 0.448), while the GI is significantly higher (p = 0.043). Normal liver V10 and Dmean are lower and V5 is slightly increased, but without statistical differences. The mean values and standard deviation of the dosimetric parameters of the reference and adapted plans are shown in the Table below.

CONCLUSION

The use of a respiratory navigator combined with the breath-hold for MRI- guided liver SBRT allows clear visualization of the tumor, ensures the accuracy of the delivered dose and may be considered an alternative when the respiratory gating system is not available during MRgART sessions.

Study on Lattice Radiotherapy Treatments (LRT) for Head and Neck Bulky Tumors

PURPOSE/OBJECTIVE(S)

Lattice radiotherapy (LRT) exploits various effects of radiation, such as the bystander effect and the abscopal effect, and consists on the administration of high dose fraction in small areas with large tumor masses, helping to solve the problem of treating bulky disease, especially if it is located in a critical anatomical area. The optimization of LRT treatment plans is challenging due to the difficulty to generate spots of high dose within the tumor with consequent high gradient. This study compares the plan dosimetry and delivery time of two delivery techniques VMAT and CyberKnife for LRT treatments of bulky head and neck lesions.

MATERIALS/METHODS

Six patients with giant head and neck tumors who received LRT at our institution were included in this study. Target and OARs were contoured following international guidelines; to allow easy identification of the desired high gradient zones, an artificial geometrical lattice structure with spherical vertices was arranged inside the target volume (GTV), and the vertices of the lattice representing the high dose boost volumes (GTVboost) were delineated. The GTVboost and GTV were prescribed to receive 12 Gy and 3 Gy, respectively in a single fraction. Separate VMAT and CyberKnife LRT plans were optimized for each patient with lattice vertex of 0.5 diameter and center-to-center distances of 1.5 cm (LRT1.5) and 3 cm (LRT3). The dose heterogeneity was measured as the peak-to-valley dose ratio (PVDR), with the traditional definition being replaced by the D10/D90 ratio, where D10 and D90 represent the doses covering 10% and 90% of the GTV, respectively. For each plan generated, the treatment delivery time, the monitor units (MU), and the PVDR were assessed. Pre-treatment plan verifications were performed with ArcCheck array and Gafchromics film for VMAT and CyberKnife, respectively, using gamma analysis criteria of 3%-3mm.

RESULTS

The mean PVDR obtained for VMAT LRT plans were 2.0 and 2.6 for LRT1.5 and LRT3, respectively, and 3.2 and 4.7, respectively for CyberKnife LRT plans. For each pre-treatment plan dose verification, the gamma passing rate (GPR) was higher than 95.0 %; CyberKnife delivery time and MU were more than 10 times higher than that of VMAT, nevertheless, VMAT had a lower PVDR. The detailed results are shown in the table below.

CONCLUSION

CyberKnife LRT has a strong ability to place the peak dose within the target, generating a higher peak-to-valley dose ratio, however its use is partially invalidated by the long beam delivery times and the resulting high MU number; the use of the VMAT LRT technique allows clinically adequate dosimetry with acceptable delivery times.

Evaluation of Flattening-Filter-Free and Flattening Filter Dosimetric and Radiobiological Criteria for Lung SBRT: A Volume-Based Analysis

PURPOSE/OBJECTIVE(S)

The use of volumetric modulated arc therapy (VMAT) with flattening-filter-free (FFF) beams is becoming more prevalent in lung cancer stereotactic body radiotherapy (SBRT). The aim in this study was to assess the impact of dosimetric and radiobiological differences between FFF and flattening filter (FF) beams for lung SBRT based on the target volume.

MATERIALS/METHODS

A total of 198 lung stereotactic body radiation therapy treatment plans with FFF beams and FF beams were retrospectively selected for this study. For all plans, the prescribed dose was 50 Gy/5 fractions, and the dose volume histogram (DVH) for the target and organs at risk (OAR) and the normal tissue complication probability (NTCP) of the lung were recorded and compared. Moreover, monitor units (MUs), the beam on-time and the treatment time were evaluated. The study was performed following the Radiation Therapy Oncology Group (RTOG) 0813 and 0915 protocols.

RESULTS

No significant differences in D90, coverage rate (CR) or conformity index (CI) of the target were observed between FFF beams and FF beams (p>0.05). The D2, R50% and gradient index (GI) for the target improved with FFF beams compared with FF beams (p<0.05). FFF beams also significantly reduced the dose for the lung, heart, spinal cord, esophagus and NTCP of the lung (p<0.05), compared with FF beams. However, there was no significant difference in sparing of the trachea (p>0.05). The mean MUs, beam on-time and treatment time were 1871±278 MUs, 3.2 ±0.2 min and 3.9 ±0.3 min for FFF beams, and 1890±260 MUs, 4.2±0.3 min and 4.8 ±0.4 min for FF beams, respectively.

CONCLUSION

The FFF beam technique for lung SBRT with VMAT results in a better dose fall-off, better dose-sparing of OAR, lower NTCP of the lung and a shorter beam on-time compared with the FF beam technique. Additionally, the improvement in target and OAR-sparing for FFF beams was increased with increasing target volume.

Improving dose delivery accuracy with EPID in vivo dosimetry: results from a multicenter study

PURPOSE

To investigate critical aspects and effectiveness of in vivo dosimetry (IVD) tests obtained by an electronic portal imaging device (EPID) in a multicenter and multisystem context.

MATERIALS AND METHODS

Eight centers with three commercial systems-SoftDiso (SD, Best Medical Italy, Chianciano, Italy), Dosimetry Check (DC, Math Resolution, LCC), and PerFRACTION (PF, Sun Nuclear Corporation, SNC, Melbourne, FL)-collected IVD results for a total of 2002 patients and 32,276 tests. Data are summarized for IVD software, radiotherapy technique, and anatomical site. Every center reported the number of patients and tests analyzed, and the percentage of tests outside of the tolerance level (OTL%). OTL% was categorized as being due to incorrect patient setup, incorrect use of immobilization devices, incorrect dose computation, anatomical variations, and unknown causes.

RESULTS

The three systems use different approaches and customized alert indices, based on local protocols. For Volumetric Modulated Arc Therapy (VMAT) treatments OTL% mean values were up to 8.9% for SD, 18.0% for DC, and 16.0% for PF. Errors due to "anatomical variations" for head and neck were up to 9.0% for SD and DC and 8.0% for PF systems, while for abdomen and pelvis/prostate treatments were up to 9%, 17.0%, and 9.0% for SD, DC, and PF, respectively. The comparison among techniques gave 3% for Stereotactic Body Radiation Therapy, 7.0% (range 4.7-8.9%) for VMAT, 10.4% (range 7.0-12.2%) for Intensity Modulated Radiation Therapy, and 13.2% (range 8.8-21.0%) for 3D Conformal Radiation Therapy.

CONCLUSION

The results obtained with different IVD software and among centers were consistent and showed an acceptable homogeneity. EPID IVD was effective in intercepting important errors.

Improvement in clinical evaluation of PET/CT images with high resolution algorithms

AIM

Positron emission tomography/computed tomography (PET/CT) diagnosis relies on quality of the reconstructed images which strongly depends on the algorithms used. The aim of this work was to assess if the introduction of high resolution algorithms allows a better identification of reduced diameter lesions, leading to improved diagnosis in clinical setting.

METHODS

The performances of a Siemens Biograph6 True Point PET/CT used for this work were checked for both standard and high resolution algorithms. Clinical studies of thirteen patients referred for PET/CT were selected and grouped according to the metabolic tumor volume and their position in the FOV and then reconstructed with both algorithms; clinical studies were estimated in terms of geometric characteristics and uptake values (SUVmax, SUVmean) of the lesions.

RESULTS

FWHM, spatial resolution, contrast ratio and image quality of the PET/CT scanner used for this work are in agreement with the performances declared by the manufacturers. For the clinical studies, the results obtained using TrueX algorithm showed an increase in SUVmax and SUVmean of 20% and 10% respectively for lesions with volume higher than 2 cm3 and of about 26% and 15% for smaller lesions. The enhancement of SUVmean was around 10% for in axis lesions and of about 12% for off axis lesions. For SUVmax the increase was 23% for both the positions. For small lesions TrueX algorithm led to a metabolic volume higher than with the iterative one while no significant differences were found for big lesions.

CONCLUSION

The advances in 3D PET reconstruction algorithms lead to images with improved quantitative accuracy and image quality performance.