Complexity metrics for IMRT and VMAT plans: a review of current literature and applications

Purpose: To assess the feasibility of automated stereotactic volumetric modulated arc therapy (SBRT-VMAT) planning using a simultaneous integrated boost (SIB) approach as a dose escalation strategy for SBRT in pancreatic cancer. Methods: Twelve patients with pancreatic cancer were retrospectively replanned. Dose prescription was 30 Gy to the planning target volume (PTV) and was escalated up to 50 Gy to the boost target volume (BTV) using a SIB technique in 5 fractions. All plans were generated by Pinnacle 3 Autoplanning using 6MV dual-arc VMAT technique for flattened (FF) and flattening filter-free beams (FFF). An overlap volume (OLV) between the PRV duodenum and the PTV was defined to correlate with the ability to boost the BTV. Dosimetric metrics for BTV and PTV coverage, maximal doses for serial OARs, integral dose, conformation numbers, and dose contrast indexes were used to analyze the dosimetric results. Dose accuracy was validated using the PTW Octavius-4D phantom together with the 1500 2D-array. Differences between FF and FFF plans were quantified using the Wilcoxon matched-pair signed rank. Results: Full prescription doses to the 95% of PTV and BTV can be delivered to patients with no OLV. BTV mean dose was >90% of the prescribed doses for all patients at all dose levels. Compared to FF plans, FFF plans showed significant reduced integral doses, larger number of MUs, and reduced beam-on-times up to 51% for the highest dose level. Despite plan complexity, pre-treatment verification reported a gamma pass-rate greater than the acceptance threshold of 95% for all FF and FFF plans for 3%-2 mm criteria. Conclusions: The SIB-SBRT strategy with Autoplanning was dosimetrically feasible. Ablative doses up to 50 Gy in 5 fractions can be delivered to the BTV for almost all patients respecting all the normal tissue constraints. A prospective clinical trial based on SBRT strategy using SIB-VMAT technique with FFF beams seems to be justified.

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Automated treatment planning as a dose escalation strategy for stereotactic radiation therapy in pancreatic cancer

Cilla

Ianiro

Romano

et al. 2020

“…Possible correlations between measured dose deviations and the modulation complexity score applied to VMAT (MCSv), MLC leaf travel (LT), the average change in dose rate per control point (ΔDR/cp), monitor units pr. Gy (MU/Gy), and homogeneity index for the PTV (HI) were investigated . A lower MCSv indicates a plan with more modulation, while a MCSv of 1 indicates no modulation, that is, a static, open field.…”

Section: Methodsmentioning

confidence: 99%

Dose deviations induced by respiratory motion for radiotherapy of lung tumors: Impact of CT reconstruction, plan complexity, and fraction size

Sande

Roa

Hellebust

2020

A thorax phantom was used to assess radiotherapy dose deviations induced by respiratory motion of the target volume. Both intensity modulated and static, non-modulated treatment plans were planned on CT scans of the phantom. The plans were optimized using various CT reconstructions, to investigate whether they had an impact on robustness to target motion during delivery. During irradiation, the target was programmed to simulate respiration-induced motion of a lung tumor, using both patient-specific and sinusoidal motion patterns in three dimensions. Dose was measured in the center of the target using an ion chamber. Differences between reference measurements with a stationary target and dynamic measurements were assessed. Possible correlations between plan complexity metrics and measured dose deviations were investigated. The maximum observed motion-induced dose differences were 7.8% and 4.5% for single 2 Gy and 15 Gy fractions, respectively. The measurements performed with the largest target motion amplitude in the superiorinferior direction yielded the largest dosimetric deviations. For 2 Gy fractionation schemes, the summed dose deviation after 33 fractions is likely to be less than 2%.Measured motion-induced dose deviations were significantly larger for one CT reconstruction compared to all the others. Static, non-modulated plans showed superior robustness to target motion during delivery. Moderate correlations between the modulation complexity score applied to VMAT (MCSv) and measured dose deviations were found for 15 Gy SBRT treatment plans. Correlations between other plan complexity metrics and measured dose deviations were not found. K E Y W O R D S complexity, interplay, lung, radiotherapy, SBRT, VMAT

“…While measurement‐based PSQA is regarded as the most accurate method of assessing delivery accuracy, 1,3 extensive literature exists in developing pre‐treatment quality assurance (PTQA) tools for this purpose by quantifying plan complexity as indications of dose agreement. These complexity metrics can be used to describe the degree of dose modulation in a treatment.…”

Section: Introductionmentioning

confidence: 99%

“…An increased modulation often leads to increased uncertainty in dose because of the limitations of accurately modeling linac components such as MLC tongue and groove which affects the interleaf leakage, and leaf offset between the calculated rounded leaf tip value and the measured one. Generally, higher degrees of modulation suggest more complex treatments, and consequently increased uncertainty in delivery 3–12 …”

Section: Introductionmentioning

confidence: 99%

Quantified VMAT plan complexity in relation to measurement‐based quality assurance results

Nguyen

Chan

2020

Volumetric‐modulated arc therapy (VMAT) treatment plans that are highly modulated or complex may result in disagreements between the planned dose distribution and the measured dose distribution. This study investigated established VMAT complexity metrics as a means of predicting phantom‐based measurement results for 93 treatments delivered on a TrueBeam linac, and 91 treatments delivered on two TrueBeam STx linacs. Complexity metrics investigated showed weak correlations to gamma passing rate, with the exception of the Modulation Complexity Score for VMAT, yielding moderate correlations. The Spearman’s rho values for this metric were 0.502 (P < 0.001) and 0.528 (P < 0.001) for the TrueBeam and TrueBeam STx, respectively. Receiver operating characteristic analysis was also performed. The aperture irregularity on the TrueBeam achieved a 53% true positive rate and a 9% false‐positive rate to correctly identify complex plans. Similarly, the average field width on the TrueBeam STx achieved a 60% true‐positive rate and an 8% false‐positive rate. If incorporated into clinical workflow, these thresholds can identify highly modulated plans and reduce the number of dose verification measurements required.