Improving Health Through Medical Physics

WGIMRT Article Watch

Collaboration with the Karmanos Cancer Center Medical Physics Residency Program (Geoff Baran, Claire Coughlin, Rebecca Culcasi, Yair Hillman, Justin Kamp, Marisa Leney, and Brian Loughery) and the University of California San Diego Medical Physics Program (Xenia Fave and Everardo Flores-Martinez)

AAPM Newsletter — Volume 43 No. 4 — July | August 2018

1. A Multi-institutional Comparison of SBRT and IMRT for Definitive Reirradiation of Recurrent or Second Primary Head and Neck Cancer
Vargo et al. retrospectively compared survival and toxicity differences between the use of IMRT (n = 217 patients) and SBRT (n = 197 patients) for the reirradiation of recurrent or second primary squamous cell carcinoma of the head and neck (rSCCHN). When accounting for baseline differences between IMRT and SBRT patient populations, the majority of the statistical tests suggest there are no significant differences in overall survival or cumulative incidence of locoregional failure between IMRT and SBRT. Outside of an increase in incidence of grade ≥ 4 acute toxicity for IMRT, IMRT and SBRT show no significant differences in acute or late toxicity in the treatment of rSCCHN with both regimens performing better than historically expected.

Int J Radiat Oncol Biol Phys. 2018 March 1; 100(3):595-605

2. Correlation between gamma passing rate and complexity of IMRT plan due to MLC position errors.
Wang et al. investigated a correlation between the complexity of an IMRT plan and the gamma passing rate of delivery quality assurance (DQA). Varying systematic MLC leaf bank positioning errors, ranging from 0.3 mm to 1 mm, were introduced into several types of plans in order to see the effect on the gamma passing rate. The plans varied in delivery technique and treatment site. The complexity of each type of plan was assessed using modulation complexity score (MCS), in order to determine if the sensitivity of the gamma passing rate to the MLC errors was dependent on the complexity of the plan. The results show that the susceptibility of DQA results to the MLC leaf bank errors correlates with the IMRT plan complexity, and suggests that clinically, with all other factors equal, a plan with a lower complexity should be chosen for treatment in order to avoid the effects of potential MLC leaf positioning errors.

Physica Medica, Volume 47, Pages 112-120.

3. Linac-based VMAT radiosurgery for multiple brain lesions: comparison between a conventional multi-isocenter approach and a new dedicated mono-isocenter technique.
Ruggieri et al. compared two VMAT radiosurgery approaches for treating multiple brain lesions. Typical plans involved one isocenter per lesion, translating to multiple time-consuming IGRT setups for each isocenter. As an alternative, HyperArc’s mono-isocenter approach with multiple non-coplanar arcs is being utilized for patients with ≤10 brain metastases having only a single IGRT setup. This study included 20 patients with 2-10 brain metastases that were treated with HyperArc (HA) plans, then generated RapidArc (RA) plans using the same optimizer, dose calculation algorithm, and dose grid resolution. HA and RA plans were analyzed by dosimetric plan quality (V12Gy and Dmean to brain-minus-PTV, CI and GI for target dose coverage) and overall treatment time (OTT). HA had significantly improved CI and GI and reduced OTT compared to RA. This early clinical implementation of HyperArc resulted in better plan quality and treatment delivery was safely completed within 20 minutes. Analysis of patient response to treatment is ongoing. Radiation Oncology. 2018 March; 13(38).

Radiation Oncology, Volume 13(38).

4. Retrospective dosimetry study of intensity-modulated radiation therapy for nasopharyngeal carcinoma: measurement-guided dose reconstruction and analysis.
Sun et al. evaluated measurement-guided 3D dose reconstruction (3DVH) as a method for determining dose distribution accuracy, compared to conventional phantom-based planar dosimetry limited by inadequate spatial information and ability to determine clinically relevant dose errors in IMRT. They analyzed 30 nasopharyngeal patients treated with 9-field static IMRT plans. Pre-treatment QA included gamma analysis of 2D diode detector array measurements. Retrospectively, pre-treatment 2D dose distributions were utilized to reconstruct the 3D dose distribution in the patient’s CT images using a planned dose perturbation algorithm. Organ-specific and global 3D gamma pass rates and DVH parameters were calculated for the reconstructed dose distribution. A strong correlation was found between organ-specific gamma pass rates and DVH deviation using the 3DVH method, providing a more effective IMRT QA technique.

Radiation Oncology, Volume 13(42).

5. Minimizing dose variation from the interplay effect in stereotactic radiation therapy using volumetric modulated arc therapy for lung cancer
Kubo et al. investigated the risk of developing hot or cold spots from interplay effects in VMAT-SBRT lung cases. Since VMAT-SBRT provides fast treatments with few fractions, there is an increased risk of clinically-relevant interplay effects. To assess this issue, Kubo collected respiratory waveforms of 30 patients using Varian’s RPM system and applied these waveforms to a Quasar motion phantom. The patients’ VMAT-SBRT plans were delivered to the Quasar phantom and dynamic measurements were taken with radiochromic film and a pinpoint ion chamber. When comparing the expected and measured doses, the authors considered 4 factors (degree of modulation, amplitude of tumor motion, irradiation time, and number of breaths during treatment) to assess their role in interplay effects. The authors concluded that a combination of these 4 factors can significantly predict the likelihood of having large dose variations due to interplay effects. They also suggested that a plan, which can be completed in fewer than 40 of the patient’s breaths, may not be suitable for VMAT-SBRT.

J Applied Clin Med Phys, Volume 19, Issue 2, Pages 121-127.

6. Improving Quality and Consistency in NRG Oncology Radiation Therapy Oncology Group 0631 for Spine Radiosurgery via Knowledge-Based Planning
Younge et. al. designed a knowledge-based planning (KBP) method to create high-quality treatment plans for the NRG Oncology 0631 protocol on spine radiosurgery. Twenty two cases were replanned using KBP and compared to the NRG-provided plan. Using the protocol objectives with a prescription dose of 16 Gy, all KBP plans were protocol compliant. The high-dose spillage (evaluated with ‘NonPTV1600’ volume) V16.8 average decreased from 2.1 cm3 for the submitted plan to 1.8 cm3 for the KBP plan. The PTV V16 improved with the KBP plan from 93.3%±3.2% to 98.3%±1.4%. The average conformity and gradient indices for the submitted plans were 0.8±0.11 and 4.91±2.0 respectively, compared with 0.84±0.07 and 4.65±1.0 for the KBP plans. Plans that were non-compliant due to high-dose spillage were compliant when using KBP. Some plans prioritized spinal cord sparing over PTV coverage causing a wide range of acceptable plans; however, with KBP, the plans were significantly more consistent. The investigators created a KBP method that can provide high-quality plans on the first optimization which saves time and provides consistency critical to clinical trials.

Int J Radiat Oncol Biol Phys, 100(4), Pages 1067-1074

7. Accuracy of radiotherapy dose calculations based on cone-beam CT: comparison of deformable registration and image correction based methods
Marchant et al investigated CBCT-based dose calculation accuracy for 44 patients receiving IMRT or VMAT treatments to prostate, head-and-neck, or lung. Three CBCT images were used per patient and adjusted each to the planning CT: one using a histogram-matching technique to the patient CT, and two using different deformable image registration techniques. Dose accuracy was evaluated by using a density override technique from previous work (Marchant et al, 2017) and comparing to doses from the density-overridden CBCT. The mean error was less than 1% for all sites, but use of DIR leads to a larger spread of errors in lung sites, possibly due to more complex anatomical changes. Manual adjustment of the registration was required in some cases in which registration failures resulted in obvious non-physical distortions of the deformed CT image.

Phys. Med. Bio., 63 065003

8. Variations in dosimetric distribution and plan complexity with collimator angles in hypofractionated volumetric arc radiotherapy for treating prostate cancer.
Li et al. investigated the impact of collimator angle on dual-arc VMAT prostate plan quality. Volumetric modulated arc therapy (VMAT) is the standard radiation option for these patients, however the specific parameters for the plans (number of arcs, collimator angle, etc) vary between institutions. They recalculated 10 prostate plans using different collimator angles between 0°-90°. The conformity index (CI), homogeneity index (HI), gradient index (GI), normalized dose contrast (NDC), MUs, and modulation complexity score (MCSv) were used to compare the plans. A collimator angle of 45 resulted in the optimal values for CI, HI, and MCSv while an angle of 0 had the best results for GI and NDC. The authors hope that their results will serve to guide VMAT plan design for hypofractionated prostates.

JACMP. March 2018 Volume 19, Issue 2, Pages 93-102.

9.Technical Note: A planning technique to lower normal tissue toxicity in lung SBRT plans based on two likely dependent RTOG metrics.
Narayanasamy et al. retrospectively evaluated the independence of two RTOG metrics (R50% and D2cm) from each other and determined which planning methods were correlated to high normal tissue dose. Both intermediate and low-dose fall off are correlated to normal tissue toxicity for lung SBRT patients. By examining 105 lung tumor SBRT plans authors observed that R50% and D2cm were not independent. Additionally, while all the plans they examined met objectives for tumor coverage, conformity index, homogeneity index, and critical organ dose tolerance objectives, those with coplanar beam arrangements were more likely to have deviations in both of the RTOG metrics. Thus the authors recommended the use of noncoplanar beams to improve normal tissue sparing in these patients.

Medical Physics. Volume 45, Issue 5, Pages 2325-2328.

10. Electromagnetic-guided MLC tracking radiation therapy for prostate cancer patients: prospective clinical trial results.
Keall et al. analyzed the results of a prospective clinical trial evaluating the feasibility of electromagnetic-guided multileaf collimator (MLC) tracking therapy. With this technology the leaves track the tumor in real-time as it moves and reshape to potentially increase treatment accuracy. For the trial, 28 prostate patients were treated using VMAT with doses per fraction between 2-13.75 Gy. The primary outcome was feasibility of the technique and all 858 planned fractions were successfully delivered. Furthermore the secondary outcomes of improving beam-target geometric alignment, improving dosimetric coverage of the prostate and critical structure avoidance, and observing no acute grade ≥3 genitourinary or gastrointestinal toxicity were all also met.

Int. Journal of Rad Onc Bio Phy. June 2018, Volume 101, Issue 2, Pages 387-395.

11. 4π plan optimization for cortical-sparing brain radiotherapy.
Murzin et al. replanned 13 IMRT plans for brain tumors using 4π to determine if better normal brain sparing could be achieved. Normal brain tissue and irradiation is correlated to cognitive decline and cortical atrophy. Plans were evaluated by comparing the homogeneity index (HI), gradient measure, doses to cortex, white matter, brainstem, optics, and hippocampus, and probability for cortical atrophy. Each measure of plan quality improved when 4π optimization was used. Additionally PTV dose homogeneity improved with the non-coplanar technique. As dose reductions in these tissues is directly linked to reduced probabilities for complications, non-coplanar beams may significantly improve outcomes for patients with brain tumors.

Radiotherapy & Oncology. April 2018, Volume 127, Issue 1, Pages 128-135.

12. Comparison of four techniques for spine stereotactic body radiotherapy: Dosimetric and efficiency analysis
Aljabab et al. replanned 10 spine stereotactic body radiotherapy (SBRT) patient plans using four techniques: CyberKnife (CK), volumetric modulated arc therapy (VMAT), and helical tomotherapy with dynamic jaws (HT-D) and fixed jaws (HT-F). This study was motivated by the fact that SBRT utilization for spine metastases is increasing however evidence for selecting the best delivery technique is not available. The resulting plans for each technique were compared based on their target volume coverage, conformity index (CI), gradient index (GI), homogeneity index (HI), treatment time (TT) per fraction, and monitor units (MU) per fraction. CK and HT plans had the most conformal target coverage without surpassing the cord tolerance value of 17 Gy. VMAT had the shortest treatment times and lowest values for the MU. Improvements in treatment planning algorithms or technology may result in minimizing the differences between techniques.

JACMP. March 2018, Volume 19, Issue 2, Pages 160-167.

13. Tolerance limits and methodologies for IMRT measurement‐based verification QA: Recommendations of AAPM Task Group No. 218
Miften et al. investigated methodologies, metrics and tolerances for patient-specific IMRT QA. They analyzed the performance and limitations of the dose difference, distance to agreement and gamma distributions providing recommendations with respect to the normalization and interpretation of the results. A discussion of the advantages and disadvantages of the true composite (TC), perpendicular field by field (PFF) and perpendicular composite (PC) methods is provided. After analysis, the authors recommend the TC delivery method or the PFF if the measurement device is not suitable for TC. The PC method is not recommended as is prone to masking errors. A number of commercial IMRT QA tools were compared as well as gamma analysis software, investigated by providing two tests to the vendors for the evaluation with their software. Results showed differences showing that the vendors are not using a standardized approach for dose comparison. Recommendations for the gamma analysis using global normalization in absolute dose are provided: a gamma passing rate ≥ 95%, with 3%/2 mm with a 10% dose threshold for universal tolerance limits and ≥ 90%, with 3%/2 mm at 10% dose threshold for universal action limits. Tighter tolerances should be considered for SRS and SBRT. A list of actions to check and evaluate when IMRT QA results fail is provided

Med. Phys. 45 (4), April 2018

14. Correcting TG 119 confidence limits
Kearney et al. reviewed the confidence limits (CLs) adopted by TG-119. In TG-119 the CLs were established assuming a dataset Gaussian distribution in which symmetry about the mean was assumed. The authors investigated the moment estimator of the Gamma distribution family and compared the results with the Gaussian families. More than 300 plans were investigated using the 3%/3mm error local criteria. It was found that the Gamma distribution underestimated the 95% CL by 0.09% while the Gaussian distribution overestimated the 95% CL by 4.12%. The authors concluded that the gamma distribution is superior over the Gaussian formalism to model IMRT failing rates. The proposed mathematical formalism can be applied to different treatment planning and delivery systems.

Med. Phys. 45 (3), March 2018

15. Motion induced interplay effects for VMAT radiotherapy
Edvarsson et al. investigated the interplay effect due to respiratory motion for patients treated with VMAT Rapidarc. Simulations were performed by creating plans on a Delta4 phantom and breathing patterns were simulated for each treatment plan by dividing the plans into smaller sub-arcs with a shifted isocenter using an in-house developed software. Four different diameters for CTVs were investigated ranging from 1 to 8 cm and added an ITV to avoid the dose blurring effects at the edge of the target. Results showed that interplay effects were larger for FFF compared to FF and increased for higher breathing amplitudes, longer period times, lower dose levels and more complex treatment plans. Also, the interplay effects varied considerably with the initial breathing phase and larger variations were observed for smaller CTV sizes. The findings were verified through measurements using a motion platform.

Physics in Medicine & Biology 63 085012

16. Institutional experience with SRS VMAT planning for multiple cranial metastases
Ballangrud et al. report the experience at their institution when treating multiple cranial metastases with VMAT. 40 plans were analyzed with a median number of lesions of 5 and a maximum number of 8. Treatment delivery was performed using a Trubeam STx having an MLC with 2.5 and 5.0 mm leaves. The authors report limitations in the TPS for treating multiple lesions as only one DLG is used for both leave widths. For each lesion, the homogeneity index ranged from 1.2 to 1.5, the conformity index ranged from 1.0 to 2.9 and the gradient index ranged from 2.5 to 8.4.The authors conclude that a significant improvement would come from better dose modelling in the TPS, allowing a customized DLG for an SRS model allowing planning with one isocenter.

J Appl Clin Med Phys 2018; 19:2:176–183

17. Volumetric modulated arc therapy for total body irradiation: A feasibility study using Pinnacle3 treatment planning system and Elekta Agility™ linac
Symons et al. investigated the feasibility of using VMAT for total body irradiation. Five patient plans were created using a 6 MV beam and a dose prescription of 12 Gy in 6 fractions. The PTV was subdivided into four subsections: head, chest, abdomen and pelvis. The final optimization was performed simultaneously for all beams. Setup errors were simulated by shifting the isocenter in every axis, recalculating the plans and evaluating the dosimetric effect. All VMAT arcs were delivered and measured using the ArcCheck diode array. The five plans achieved the prescribed dose to 90% of the PTV and a dose reduction to organs at risk of up to 40%. The authors conclude that total body irradiation is feasible using VMAT.

J Appl Clin Med Phys. 2018 Mar;19(2):103-110

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