Improving Health Through Medical Physics


Collaboration with VCU (M. Ostyn, M. Riblett, M. Wagar and S. Wang), Henry Ford Cancer Institute (J. Cunningham, A. Doemer, J. Dolan, X. Liu, S. Rusu and Q. Wu), and William Beaumont (C. Knill)

AAPM Newsletter — Volume 43 No. 6 — November | December 2018

1. To propose adding index of achievement (IOA) to IMRT QA process
Kim et al. performed a systematic analysis of a newly proposed dosimetric performance metric, the index of achievement, in relation to spatial and dosimetric uncertainties. Known spatial and dosimetric perturbations were added to model 1D geometric cases and clinical 2D cases, which were then analyzed through conventional gamma analysis. The IOA, IOH (index of hotness), and IOC (index of coldness) were then calculated and compared to the gamma passing rate. The IOA metrics all showed greater granularity in results than gamma analysis with generous criteria (such as 3-mm distance to agreement or 3% dose tolerance), while also showing weak-to-moderate correlation between index and gamma passing rate. The authors argue that because the IOA indices are purely dosimetric measures, they are a more realistic measure of quality of deliverability of a given plan compared to gamma analysis, and that the indices could be easily added to existing patient-specific QA workflows. An IOA value of 1.025 was proposed as a reference threshold for considering a plan of sufficient quality.
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2. Dosimetric Evaluation of Incorporating Patient Geometric Variations Into Adaptive Plan Optimization Through Probabilistic Treatment Planning in Head and Neck Cancers
Liu et al. performed a retrospective dosimetric study (N=18) of head and neck cancer patients using four different adaptive radiotherapy strategies, comparing non-adaptive radiotherapy to two different conventional adaptive radiotherapy techniques and a probabilistically adaptive radiotherapy method (4D ART). The conventional adaptive plans redrew contours for the 13th–22nd and 25th–35th fractions with 0-mm and 3-mm PTV margins based on diagnostic scans acquired at the 10th and 22nd fractions, respectively. In the 4D ART scheme, adaptations to the plan were based on patient-specific interfraction geometric variations measured directly by CT or CBCT. Mean and maximum dose to all typical head and neck OARs were found to be reduced with 4D ART compared to 0-mm margin ART, and lower in 0-mm margin ART than compared to 3-mm ART. Target coverage was better in 3-mm ART compared to either 0-mm ART or 4D ART, but significantly better in 4D ART compared to 0-mm ART, in terms of percentage of volume of the target CTV receiving the prescription dose.
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3. Monte Carlo dose verification of VMAT treatment using Elekta Agility 160-leaf MLC
Onizuka et al. attempts to do for the Elekta Agility MLC on the Synergy systems what had previously been done of Varian Millennium and HD 120-MLCs on Varian linear accelerators. In this analysis, the group performs an in-depth Monte Carlo dose calculation method as a verification tool for VMAT treatment plans. The ability of the Monte Carlo method to property model the dosimetric properties of the Agility MLC were evaluated my comparing to a combination of film and chamber measurements. VMAT plans were created and verified using the Monte Carlo method and evaluated using 3D Gamma and DVH analysis. Results of this study show good agreement between Monte Carlo and the measured dosimentric properties as well as between VMAT plans from the TPS and the Monte Carlo Verification. This study shows there is utility in verifying VMAT plans using Monte Carlo dose calculation.
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4. Differences in lung injury after IMRT or proton therapy assessed by 18FDG PET imaging
Shusharina et al. explores post-treatment lung injury for IMRT and Proton therapy treatments, in hopes of finding predictors of symptomatic radiation pneumonitis. This study used 18FDG uptake to visualize the inflammatory process for each case on PET-CT imaging. While the dose distributions differed expectedly, with IMRT plan result in a larger low dose bath of V5 to the lung and the proton plans having greater mid to high dose to the lung, there was minimal difference in the perceived dose response between the two different modalities. While it was confirmed that higher MLD and slope of SUV are indicators or development of radiation pneumonitis, this occurred in both cohorts without any notable correlation to modality.
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5. Evaluation of machine log files/MC-based treatment planning and delivery QA as compared to ArcCHECK QA
Stanhope et al. evaluated the use of linear accelerator log files and Monte Carlo dose calculation as an alternative to phantom-based QA for planned IMRT and VMAT treatment. An Elekta Infinity was used to deliver eleven clinical treatment plans onto a Sun Nuclear ArcCheck phantom. Two of the eleven treatment plans were simple brain cases; nine were head and neck treatments with higher complexity and low gamma-pass rates. All tested cases were delivered at 6MV with a flattening filter. Planned doses were optimized initially using Pinnacle's convolution-superposition (CS) algorithm and subsequently recalculated using ScientificRT's SciMoCa Monte Carlo (MC) algorithm. For each delivered plan, machine log files (LF) were recorded and used to reconstruct a delivered dose using one of the two dose algorithms. The agreements between the three delivered, reconstructed doses (LF-CS, LF-MC, and ArcCheck) and the two planned doses (Plan-CS and Plan-MC) were compared using statistical (mean percent dose difference) and gamma-pass analysis methods in order to characterized dosimetric uncertainties. These comparisons demonstrated that highlighted greater agreement between both Plan-MC and LF-MC doses and the ArcCheck measurement than between the analogous CS doses. Both Plan-MC and LF-MC were found to generally agree with ArcCheck measurements with 3% across evaluated treatment plans. The researchers concluded that LF-based QA which incorporates MC-based dose reconstruction is a viable method of performing patient-specific QA without additional physical measurement.
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6. Second Cancer Risk After Radiation Therapy Of Ependymoma Using The Flattening Filter Free Irradiation Mode Of A Linear Accelerator
Moret et al. investigated the possibility of reducing secondary malignancies arising from the treatment of ependymoma by using flattening filter free (FFF) mode in 3D conformal radiotherapy (3DCRT), IMRT and VMAT treatments. Five treatment plans were developed for each of 11 pediatric subjects consisting of IMRT with/without FFF, VMAT with/without FFF, and conventional 3DCRT. The quality of these treatment plans, developed on Oncentra External Beam for an Elekta Synergy accelerator operating at 6MV, was evaluated through the comparison of corresponding DVHs. Treatment plans were shot on a 2D ionization chamber array for dose verification. Excess absolute risk (EAR) of secondary brain malignancies was computed for each treatment plan. It was determined that IMRT and VMAT plans produced better coverage of the PTV and greater sparing of organs-at-risk (OARs) than 3DCRT. EAR was not found to be reduced for the brain using FFF plans, however lower EAR was assessed for peripheral OARs (e.g. gonads, bladder, bowel). Additionally, FFF plans resulted in a reduction in overall treatment time compared to their flattening filtered counterparts. The researchers suggest that the combination of low treatment delivery time and reduced peripheral EAR to OARs resulting from VMAT treated in FFF mode make it an advantageous option for the treatment of pediatric ependymoma.
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7. Deep-inspiration breath-hold intensity modulated radiation therapy to the mediastinum for lymphoma patients: setup uncertainties and margins
Aristophanous et al. investigated the setup uncertainties and margins in IMRT for mediastinal lymphoma patients using DIBH. The setup errors were retrospectively measured, using autoregistration software, for the total PTV and 6 anatomic subregions in 3 directions. Large differences were found among various subregions and directions, most noticeably in the lower heart, neck, and axilla regions and in the superoinferior direction. In addition, 3 IGRT daily setup strategies were also examined: no IGRT, CBCT, and CT on rails (CTOR). Despite the measurable improvement in margin reduction from using IGRT, there was no clear conclusion whether CTOR offers an advantage over CBCT.
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8. Intensity modulated radiation therapy and second cancer risk in adults (Commentary)
Filippi et al. commented on the second cancer (SC) risk of IMRT in comparison with that of 3D-CRT. Citing recent radiobiological modeling studies, preclinical data, and preliminary clinical data, the authors suggest that there is at least an equivalence in SC induction risk between 3D-CRT and IMRT, with possible reduced risks of certain solid tumors for IMRT. The article also addresses the need for more clinical data in order to gain further insights.
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9. Multi-Center validation of automated MCO VMAT prostate cases.
Heijmen et al. reported on a multi-center study to investigate the potential for apriorMCO automated planning to improve on manually planned prostate and seminal vesicles VMAT cases. The apriorMCO optimization created a single Paerto-optimal plan per patient. The automated plans showed dosimetric improvements in bladder and rectum metrics and were preferred by physicians over the manual plans. The authors noted the improvements were very center and patient specific, suggesting the automated planning may have been overcoming inconsistencies in manual planning.
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10. Spinal Metastases post-radiotherapy pain response: single-fraction SBRT versus 3DCRT
Sprave et al. performed a randomized phase II study (N=55) to evaluate the 3-month and 6-month post-radiotherapy pain response of palliative spinal metastases patients treated with single-fraction SBRT (24Gy) versus 3DCRT (30Gy in 10 fractions). SBRT was treated using IMRT/VMAT on an Elekta Versa HD or TomoTherapy. Pain relief was primarily quantified as a >2 point change on the visual analog scale. Pain response was similar after 3-months (p=0.13), however SBRT showed a faster initial pain reduction (p=0.01). SBRT showed improved pain reduction after 6-months (p= 0.002). No ≥3 grade toxicities were observed in the SBRT arm (24 month follow up).
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11. Optimized radiotherapy to improve clinical outcomes for locally advanced lung cancer
Jaksic et al. optimized IMRT radiotherapy treatment schedule to improve outcome for locally advanced lung cancer. Seventy-three patients were consecutively treated with IMRT. A total dose of 66 Gy was delivered using two different schedules of radiotherapy: simultaneous modulated accelerated radiotherapy (30 × 2.2 Gy, across 6 weeks) with or without chemotherapy, or moderate hypofractionated radiotherapy (24 × 2.75 Gy, across 4 weeks) in patients unfit to receive concomitant chemotherapy. They found the 1-year and 2-year local-regional control (LRC) were 76% [95 confidence interval (CI) %: 66–87%] and 62% [95 CI%: 49–77%] respectively. The 1 and 2-year overall survival rates were 72% [95% CI: 63–83%] and 54% [95 CI%: 43–68%] respectively. No significant differences were observed in the toxicity rates associated with each of the RT schedules. Authors concluded that Accelerated IMRT for locally advanced lung cancer is associated with low toxicities and high LRC. Moderate hypo fractionated radiotherapy, by decreasing the total treatment time, may be promising in improving clinical outcomes.
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12. The clinical target distribution: a probabilistic alternative to the clinical target volume
Shusharina et al. incorporate a clinical target distribution (CTD) to address the challenge of decreasing the significant uncertainty stemming from the delineation of the traditional CTV (on the order of 1 cm in contrast with millimeter uncertainties in delivery). Their paper discusses the incorporation of the CTD into treatment plan optimization algorithms, the flexibility it provides in optimizing the therapeutic ratio, and the reduction observed in inter-user variability of the CTV delineation. Increased conformality in radiation treatments via IMRT has increased the significance of the binary decisions made when contouring the CTV. The CTD concept proposes a continuous probabilistic portrayal of the CTV where each voxel is given a probability of containing tumor cells. The authors suggest a few approaches to the development of the CTD and focus on its inclusion in plan optimization and plan quality. Shells of defined tumor probabilities were placed around the GTV and weighted according to those probabilities in the commercial TPS. Results are based on synthetic geometries and two clinical cases. The authors demonstrate dosimetric improvements from the CTD approach. Additionally, a sensitivity analysis of the dose distributions is performed where CTD parameter adjustment is compared to variations in CTV delineation, proving more robust distributions for the CTD approach.
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13. Agility MLC transmission optimization in the Monaco treatment planning system
This study performed by Roche et al concentrates on the optimization of the transmission probability filters (TPF) using measurements from the linear accelerator with an Agility multi-leaf collimator (MLC). The TPFs in the TPS were characterized by both geometry and probability of particle transmission, specifically, the secondary collimator TPFs are editable which can help differentiate the variation in transmission through the distinct MLC regions (through the MLC body, adjacent leafs and MLC tips). The minimum leaf gap width, secondary collimator transmission, leaf offset, dosimetric leaf gap, tongue and groove effect were used to adjust the corresponding TPF parameters through the use of vendor provided fields and a set of additional fields to perform optimization. The validation of the optimization was then performed using point dose measurements and 2D dose matrices for a number of clinical IMRT and VMAT plans and compared to those from the TPS. All plans calculated with the optimized beam model had a gamma pass rate of > 95% using 2%/2mm criterion. On the other hand, plans calculated with the default beam model had pass rates as low as 88.4%. For measured point doses, the most noticeable difference was achieved in low dose regions. Although it is possible to achieve good clinical results by randomly selecting TPF parameter values, it is recommended that the optimization process outlined in this study is followed so that the transmission through the TPF is characterized appropriately.
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14. Comparison of two different EPID-based solutions performing pretreatment quality assurance: 2D portal dosimetry versus 3D forward projection method
Bresciani et al. studied the ability of 2D portal dosimetry and the 3D projection technique, for pre-treatment VMAT quality assurance, to catch main critical delivery errors. Using a linear accelerator equipped with EPID aSi1000, Portal Dose Image Prediction (PDIP) software for the 2D forward method, and PerFRACTION software for the 3D projection technique, measurements were acquired. Perturbations of the reference plan were applied through systematic variations in dose values and micromultileaf collimator position. To assess the performance of these software, PDIP was evaluated calculating gamma passing rate between no-error and error-simulated measurements and PerFRACTION was analyzed by calculating the difference between reference and perturbed DVH. Subsequently pre-treatment verification with PerFRACTION was done for 27 patients of different pathologies. The sensitivity of PerFRACTION was slightly higher than sensitivity of PDIP and the specificity of PerFRACTION was also higher than PDIP. The analysis of patients' DVHs indicated that the mean percent dose difference was (1.2 ± 1.9)% for D2%, (0.6 ± 1.7)% for D95% and (−0.0 ± 1.2)% for Dmean of PTV. For the OARs, important discrepancies on DVH were observed but the higher dose variations were in low dose area (< 10 Gy). These findings support the use of the 3D forward projection method for pretreatment QA and highlight the potential major advantages of visualizing the delivered dose distribution on patient anatomy versus traditional portal dosimetry QA systems.
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