New conformity indices based on the calculation of distances between the target volume and the volume of reference isodose

Dose conformity and falloff in single-lesion intracranial SRS with DCA and VMAT methods

BACKGROUND

Intracranial stereotactic radiosurgery (SRS) aims at achieving highly conformal dose distribution and, at the same time, attaining rapid dose falloff outside the treatment target. SRS is performed using different techniques including dynamic conformal arcs (DCA) and volumetric modulated arc therapy (VMAT).

PURPOSE

In this study, we compare dose conformity and falloff in DCA and VMAT plans for SRS with a single target.

METHODS

To compare dose conformity in SRS plans, we employ a novel conformity indexC I d e x p $C{I}_{{d}_{exp}}$ , RTOG conformity index (C I R T O G $C{I}_{RTOG}$ ), and Riet-Paddick conformity index (C I R P $C{I}_{RP}$ ). In addition, we use indicesR 50 % $R50\% $ ,V 10 G y ${V}_{10Gy}$ , andV 12 G y ${V}_{12Gy}$ to evaluate dose falloff. For each of the considered 118 cases of SRS, two plans were created using DCA and VMAT. A two-tailed Student's t-test was used to evaluate the difference between the employed indices for the DCA and VMAT plans.

RESULTS

The studied VMAT plans were characterized by higher dose conformity than the DCA plans. The differences between the conformity indices for the DCA plans and VMAT plans were statistically significant. The DCA plans had a smaller number of monitor units (MUs) and smaller indices R50%, V10 Gy, and V12 Gy than the VMAT plans. However, the differences between R50%, V10 Gy, and V12 Gy for the DCA and VMAT plans were not statistically significant.

CONCLUSIONS

Although the studied VMAT plans had higher dose conformity, they also had larger MUs than the DCA plans. In terms of dose falloff characterized by parameters R50%, V10 Gy, and V12 Gy, DCA serves as a reasonable alternative to VMAT in the case of a single brain metastasis.

Toward an improved assessment of dose conformity in radiotherapy

BACKGROUND

Evaluation of dose conformity is important to ensure minimum dose to normal tissue and sufficient dose coverage of the planning target volume (PTV). The existing conformity indices depend on the PTV volume and do not differentiate between two different scenarios: overdosing normal tissue and underdosing PTV.

PURPOSE

In this study, we introduce a novel index to assess conformity of dose distributions in radiotherapy.

METHODS

The suggested conformity indexC I d e x p $C{I_{{d_{exp}}}}$ is defined by the ratio of the volume representing actual "non-conformity" of the planned dose and the volume representing acceptable "non-conformity." The latter volume is produced by expanding the PTV. If both the average distance (d ¯ $\overline d $ ) between the reference isodose surface and planning target volume and arbitrarily selected PTV expansion margin (d e x p ${d_{exp}}$ ) are much smaller than the size of the PTV,C I d e x p $C{I_{{d_{exp}}}}$ approximately equals the ratiod ¯ d e x p $\dfrac{{\bar d}}{{{d_{exp}}}}$ . In this work,C I d e x p $C{I_{{d_{exp}}}}$ was utilized to analyze 90 cases of brain metastases treated with stereotactic radiation therapy (SRS) and 102 cases of lung cancer treated with stereotactic body radiation therapy (SBRT).

RESULTS

Ford e x p ${d_{exp}}$  = 0.1 cm, all considered SRS treatment plans were characterized byC I d e x p < 1 $C{I_{{d_{exp}}}} < 1$ while 2 out of 102 SBRT plans hadC I d e x p > 1 $C{I_{{d_{exp}}}} > 1$ . The average values ofC I d e x p $C{I_{{d_{exp}}}}$ for SRS and SBRT plans were 0.31 and 0.43, respectively. Ford e x p ${d_{exp}}$  = 0.2 cm, all studied treatment plans hadC I d e x p < 1 $C{I_{{d_{exp}}}} < 1$ , and the average values ofC I d e x p $C{I_{{d_{exp}}}}$ for SRS and SBRT plans were 0.15 and 0.25, respectively.

CONCLUSIONS

The suggested conformity indexC I d e x p $C{I_{{d_{exp}}}}$ varies less with PTV volume than the RTOG and Riet-Paddick indices frequently used for evaluation of dose conformity. In addition,C I d e x p $C{I_{{d_{exp}}}}$ can be expressed as a sum of two terms which describe "over-coverage" and "under-coverage" of the treatment target. The results confirm thatC I d e x p $C{I_{{d_{exp}}}}$ can be used for evaluation of dose conformity in SRS and SBRT.

Proposal of a New Conformal Factor and Normal Tissue Penalty Factor for Radiosurgery Treatments

The quality of a treatment plan is evaluated by the conformality of the prescribed isodose around the target and the homogeneity of dose distribution inside the target. Presently, to check the target volume conformality, a number of published conformity indices are in use. Most of these indices are based on the target volume coverage by prescribed isodose, with respect to the total volume of the target. Some take into account the normal tissue covered by the prescribed isodose and suitably weigh the target coverage to evaluate conformity. In this study, for the irradiation of normal tissue by the prescription isodose, a normal tissue penalty factor is proposed and for the target conformality, a new conformal factor is proposed by applying this normal tissue penalty factor to the target coverage. The proposed conformal factor is evaluated for a few sample analytical cases and the results are compared with those obtained using the published conformity indices.

Progressive resolution optimizer (PRO) predominates over photon optimizer (PO) in sparing of spinal cord for spine SABR VMAT plans

BACKGROUND

we assessed the performance of the optimization algorithms by comparing volumetric modulated arc therapy generated by a progressive resolution optimized (VMATPRO) and photon optimizer (VMATPO) in terms of plan quality, MU reduction, sparing of the spinal cord (or cauda equina), and plan complexity.

METHODS

Fifty-seven patients who received spine stereotactic ablative radiotherapy (SABR) with tumors located in the cervical, thoracic, and lumbar spine were retrospectively selected. For each patient, VMATPRO and VMATPO with two full arcs were generated with using the PRO and PO algorithms. For dosimetric evaluation, the dose-volumetric (DV) parameters of the planning target volume (PTV), organs at risk (OARs), the corresponding planning organs at risk (PRV), and 1.5-cm ring structure surrounding the PTV (Ring1.5 cm) were calculated for all VMAT plans. The total number of monitor units (MUs) and the modulation complexity score for the VMAT (MCSv) were compared. To investigate the correlations of OAR sparing to plan complexity, Pearson's and Spearman's correlation tests were conducted between the two algorithms (PO - PRO, denoted as Δ) in the DV parameters for normal tissues, total MUs, and MCSv.

RESULTS

For the PTVs, Target conformity and dose homogeneity in the PTVs of VMATPRO were better than those of VMATPO with statistical significance. For the spinal cords (or cauda equine) and the corresponding PRVs, all of the DV parameters for VMATPRO were markedly lower than those for VMATPO, with statistical significance (all p < 0.0001). Among them, the difference in the maximum dose to the spinal cord between VMATPRO and VMATPO was remarkable (9.04 Gy vs. 11.08 Gy with p < 0.0001). For Ring1.5 cm, no significant difference in V115% for VMATPRO and VMATPO was observed.

CONCLUSIONS

The use of VMATPRO resulted in improved coverage and uniformity of dose to the PTV, as well as OARs sparing, compared with that of VMATPO for cervical, thoracic, and lumbar spine SABR. Better dosimetric plan quality generated by the PRO algorithm was observed to result in higher total MUs and plan complexity. Therefore, careful evaluation of its deliverability should be performed with caution during the routine use of the PRO algorithm.

STANDARD-DEVIATION BASED CONFORMITY INDEX FOR EVALUATING TREATMENT PLAN OF INTENSITY MODULATED RADIOTHERAPY IN LUNG CANCER

This paper attempts to find a new conformity index (CI) calculation method with slice and angle information for evaluating lung cancer radiation treatment plan. A total of 20 lung cancer patients in 2016-2019 were selected. Treatment plans were made for each patient. Parameters used in the process of making treatment plans were set the same. The CI and the standard-deviation based CI (SDCI) that contains angle and slice information were calculated. Comparison of results calculated with SDCI and CI were made. The results of the two methods for the patients showed the same trend. Different shapes of simulated dose distribution line shows SDCI can provide more detail information about the target area. Special shapes of simulated dose distribution line for SDCI showed inaccuracy in angle information. The parameter SDCI has more advantage towards the traditional CI for it can provide angle and slice information. However, more angles need to be calculated.

Novel approach for the evaluation of dose conformity in radiotherapy

PURPOSE

We describe a new approach to evaluate conformity of dose distributions in radiotherapy.

METHODS

The suggested conformity factor λ is defined by using existing conformity indices and expansion of the planning target volume (PTV). If the average distance ( d ¯ $\bar d$ ) between the PTV and reference isodose surface and an arbitrarily selected PTV expansion margin ( d e x p ${d_{exp}}$ ) are both much smaller than the size of the PTV, then λ approximately equals the ratio d ¯ d e x p $\frac{{\bar d}}{{{d_{exp}}}}$ . We use λ to analyze several cases of stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT).

RESULTS

In the case of SRS with a single target or multiple targets, treatment plans produced with the help of volumetric modulated arc therapy (VMAT) have smaller λ than plans produced by using dynamic conformal arcs (DCA). Likewise, it is demonstrated that in the case of SBRT, λ is reduced by employing VMAT instead of DCA. It is also shown that if the distance between the reference isodose surface and surface of the PTV is fixed, λ varies less with variations in PTV volume compared to frequently used conformity indices.

CONCLUSIONS

The described conformity factor λ can be applied clinically to compare and rank treatment plans for lesions of different sizes. It is suggested that conditions λ < 1 $\lambda &lt; 1$ and λ > 1 can be employed as "pass" and "fail" criteria, respectively, for dose conformity assessment with appropriate choice of d e x p ${d_{exp}}$ .

A systematically compiled set of quantitative metrics to describe spatial characteristics of radiotherapy dose distributions and aid in treatment planning

PURPOSE

Many quantitative metrics have been proposed in literature for characterization of spatial dose properties. The aim of this study is to work towards much-needed consensus in the radiotherapy community on which of these metrics to use. We do this by comparing characteristics of the metrics and providing a systematically selected set of metrics to comprehensively quantify properties of the spatial dose distribution.

METHODS

We searched the literature for metrics to quantitatively evaluate dose conformity, homogeneity, gradient (overall and directional), and distribution and location of over- and under-dosed sub-volumes. For each spatial dose property, we compared the responses of its corresponding metrics to simulated dose variations in a virtual water phantom. Selection criteria were a metric's ability to describe simulated scenarios robustly and to be visualized in an intuitive way.

RESULTS

We saw substantial differences in the responses of metrics to the simulated dose variations. Some conformity and homogeneity metrics were unable to quantify certain types of changes (e.g. target under-coverage). Others showed a large dependency on the shape and volume of targets and isodoses. Metric values differed between calculations in a static plan and in simulated full treatment courses including setup errors, especially for metrics quantifying distribution and location of hot and cold spots. We provide an Eclipse plugin script to calculate and visualize selected metrics.

CONCLUSION

The selected set of metrics provides complementary and comprehensive quantitative information about the spatial dose distribution. This work serves as a step towards broader consensus on the use of spatial dose metrics.

Breast clinical target volume: HU-based glandular CTVs and ESTRO CTVs in modern and historical radiotherapy treatment planning

PURPOSE

The current study aimed to compare contouring of glandular tissue only (gCTV) with the clinical target volume (CTV) as defined according to European Society for Radiotherapy and Oncology (ESTRO) guidelines (eCTV) and historically treated volumes (marked by wire and determined by palpation and anatomic landmarks) in breast cancer radiotherapy.

METHODS

A total of 56 consecutive breast cancer patients underwent treatment planning based solely on anatomic landmarks/wire markings ("wire based"). From these treatment plans, the 50% and 95% isodoses were transferred as structures and compared to the following CT-based volumes: eCTV; a Hounsfield unit (HU)-based automatic contouring of the gCTV; and standardized planning target volumes (PTVs) generated with 1‑cm safety margins (resulting in the ePTVs and gPTVs, respectively).

RESULTS

The 95% isodose volume of the wire-based plan was larger than the eCTV by 352.39 ± 176.06 cm³ but smaller than the ePTV by 157.58 ± 189.32 cm³. The 95% isodose was larger than the gCTV by 921.20 ± 419.78 cm³ and larger than the gPTV by 190.91 ± 233.49 cm³. Patients with larger breasts had significantly less glandular tissue than those with small breasts. There was a trend toward a lower percentage of glandular tissue in older patients.

CONCLUSION

Historical wire and anatomic landmarks-based treatment planning sufficiently covers the glandular tissue and the theoretical gPTV generated for the glandular tissue. Modern CT-based CTV and PTV definition according to ESTRO results in a larger treated volume than the historical wire-based techniques. HU-standardized glandular tissue contouring results in a significantly smaller CTV and might be an option for reducing the treatment volume and improving reproducibility of contouring between institutions.

Robust Treatment Planning in Intrafraction Motion Using TomoDirect™ Intensity-modulated Radiotherapy for Breast Cancer

BACKGROUND/AIM

To evaluate the robustness of radiotherapy treatment planning optimization for respiratory-moving breast cancer using fixed-angle beams planning TomoDirect™ intensity-modulated radiotherapy (IMRT).

MATERIALS AND METHODS

A minimax optimisation algorithm was applied to 10 breast cancer patients. Two sets of treatment plans with or without robust techniques were prepared considering anterior-posterior and head-tail movements due to respiration. Parameters were compared between treatment plans: 95% planned target volume (PTV) dose, conformal index and homogeneity index (HI), and organs at risk (OAR) parameters including the lung volume receiving 20 Gy or more (V20) and 5 Gy (V5).

RESULTS

Robust planning significantly improved parameters of 95% PTV dose and HI, without deteriorating V20 or V5 in the anterior-posterior movement, while it slightly improved 95% PTV and slightly deteriorated V20 in the head-tail movement.

CONCLUSION

Robust treatment planning improves coverage of targets moving because of respiration in the treatment of breast cancer using TomoDirect; however, normal lung doses should be cautiously evaluated on a case-by-case basis.

Delivery of online adaptive magnetic resonance guided radiotherapy based on isodose boundaries

Magnetic Resonance-guided Radiotherapy (MRgRT) allows direct monitoring of treated volumes. The aim of this study was to investigate the feasibility of a new gating strategy consisting in using an isodose as boundary. Forty-four patients treated for thoracic and abdominal lesions using MRgRT were enrolled. The accuracy of the new strategy was compared to the conventional one in terms of area improvement available for gating without compromising target coverage. A mean increase of 24% for lung, 15% for liver and 11% for pancreas was observed, demonstrating how the new method can be useful in challenging situations with low dose conformality.

A new index for evaluating the fit of dose distribution to target volume: Dose distribution fix index

To develop a new dose evaluation index, fit index (FI), to help evaluate the fit between isodose surfaces at different percentages of the prescription dose and the target volume. Two types of FI, differential and cumulative, were defined. The differential fit index (dFI) was defined as the ratio of the integral dose of volume occupied by an isodose surface to the integral dose of the planning target volume. The cumulative fit index (cFI) was defined as the integral of dFI from the minimum dose of clinical significance to the 100% prescription dose. Performance of the cFI was evaluated with virtual dose distributions. In addition, non-coplanar and coplanar VMAT plans of 20 brain metastasis cases were evaluated using the FI, and the results were compared with results from the dose gradient index (GI) and conformity index (CI). Correlations between cFI and GI, and between cFI and CI were studied and Pearson's correlation coefficients were calculated. dFI and cFI provided comprehensive and objective results in evaluating the dose fit between isodose surfaces at different percentages of the prescription dose and the target volume. Analysis showed a positive correlation between cFI and GI with a Pearson correlation coefficient of 0.928 (p < 0.01) and a negative correlation between cFI and CI with a Pearson correlation coefficient of -0.831 (p < 0.01). dFI and cFI were shown to be effective and convenient tools for evaluating the dose fit of a radiotherapy plan.

Plan evaluation indices: A journey of evolution

AIM

A systemic review and analysis of evolution journey of indices, such as conformity index (CI), homogeneity index (HI) and gradient index (GI), described in the literature.

BACKGROUND

Modern radiotherapy techniques like VMAT, SRS and SBRT produce highly conformal plans and provide better critical structure and normal tissue sparing. These treatment techniques can generate a number of competitive plans for the same patients with different dose distributions. Therefore, indices like CI, HI and GI serve as complementary tools in addition to visual slice by slice isodose verification while plan evaluation. Reliability and accuracy of these indices have been tested in the past and found shortcomings and benefits when compared to one another.

MATERIAL AND METHODS

Potentially relevant studies published after 1993 were identified through a pubmed and web of science search using words "conformity index", "Homogeneity index", "Gradient index"," Stereotactic radiosurgery"," stereotactic Body radiotherapy" "complexity metrics" and "plan evaluation index". Combinations of words "plan evaluation index conformity index" were also searched as were bibliographies of downloaded papers.

RESULTS AND CONCLUSIONS

Mathematical definitions of plan evaluation indices modified with time. CI definitions presented by various authors tested at their own and could not be generalized. Those mathematical definitions of CI which take into account OAR sparing grant more confidence in plan evaluation. Gradient index emerged as a significant plan evaluation index in addition to CI whereas homogeneity index losing its credibility. Biological index base plan evaluation is becoming popular and may replace or alter the role of dosimetrical indices.

The superficially averaged dose gradient at the target volume's boundary: A two-dimensional formulation and solution of anisotropic dose gradient problems

BACKGROUND AND PURPOSE

Dose conformity and steepness of dose fall-off at a target volume's boundary are important quality criteria in treatment planning to predict complication rates in normal tissue and critical structures. Several dose gradient measures are in use; ICRU Report 91 recommends one of two gradient metrics for reporting. All of the common gradient indices are one-dimensional, although dose gradient problems are at least two-dimensional and anisotropic. Four of ten investigated gradient indices show false characteristics on the mean value of the physical dose gradients. Anisotropic dose gradient measures can be the basis of clinical therapeutic decisions if tumours are surrounded by organs at risk with various tolerance dose values. To close this information gap, the author presents a mathematical description and solution of anisotropic dose gradient problems.

MATERIALS AND METHODS

The new two-dimensional dose gradient measure is called the superficially averaged dose gradient (SADG). The particular informative content of the ten common dose gradient indices was assessed by classification and analysing their properties. The correlations between all of the dose gradient measures were investigated for linac-based stereotactic radiosurgery of 13 brain metastases.

RESULTS

From all of the one-dimensional dose gradient indices, the approximated SADG* showed the best correlation on the SADG. Here, Pearson's correlation coefficient was 1.000 and the relative errors were in a range of -0.2 to 2.9%. Distributions of anisotropic dose gradients were graphically represented by dose gradient-solid angle histograms.

CONCLUSIONS

Two-dimensional dose gradient measures such as the SADG are urgently required for lesions that are located in non-homogeneous normal tissue. The quality of each present and future dose gradient measure concerning the description of anisotropic dose gradient problems is now verifiable by use of the SADG. Through the SADG, the influences of collimation types on the dose fall-off at the target volume's boundary can be investigated. The algorithm for determining the SADG should be implemented in treatment planning systems to utilise the formalism for all users. The dose gradient indices recommended in ICRU Report 91 overestimate and underestimate physical dose gradients, respectively.

Development and dosimetric assessment of a patient-specific elastic skin applicator for high-dose-rate brachytherapy

PURPOSE

The purpose of this study was to develop a patient-specific elastic skin applicator and to evaluate its dosimetric characteristics for high-dose-rate (HDR) brachytherapy.

METHODS AND MATERIALS

We simulated the treatment of a nonmelanoma skin cancer on the nose. An elastic skin applicator was manufactured by pouring the Dragon Skin (Smooth-On Inc., Easton, PA) with a shore hardness of 10A into an applicator mold. The rigid skin applicator was printed using high-impact polystyrene with a shore hardness of 73D. HDR plans were generated using a Freiburg Flap (FF) applicator and patient-specific rigid and elastic applicators. For dosimetric assessment, dose-volumetric parameters for target volume and normal organs were evaluated. Global gamma evaluations were performed, comparing film measurements and treatment planning system calculations with various gamma criteria. The 10% low-dose threshold was applied.

RESULTS

The V_120% values of the target volume were 56.9%, 70.3%, and 70.2% for HDR plans using FF, rigid, and elastic applicators, respectively. The maximum doses of the right eyeball were 21.7 Gy, 20.5 Gy, and 20.5 Gy for the HDR plans using FF, rigid, and elastic applicators, respectively. The average gamma passing rates were 82.5% ± 1.5%, 91.6% ± 0.8%, and 94.8% ± 0.2% for FF, rigid, and elastic applicators, respectively, with 3%/3 mm criterion.

CONCLUSIONS

Patient-specific elastic skin applicator showed better adhesion to irregular or curved body surfaces, resulting in better agreement between planned and delivered dose distributions. The applicator suggested in this study can be effectively implemented clinically.

A novel index for assessing treatment plan quality in stereotactic radiosurgery

OBJECTIVE

Stereotactic radiosurgery (SRS) is characterized by high levels of conformity and steep dose gradients from the periphery of the target to surrounding tissue. Clinical studies have backed up the importance of these factors through evidence of symptomatic complications. Available data suggest that there are threshold doses above which the risk of symptomatic radionecrosis increases with the volume irradiated. Therefore, radiosurgical treatment plans should be optimized by minimizing dose to the surrounding tissue while maximizing dose to the target volume. Several metrics have been proposed to quantify radiosurgical plan quality, but all present certain weaknesses. To overcome limitations of the currently used metrics, a novel metric is proposed, the efficiency index (η50%), which is based on the principle of calculating integral doses: η50% = integral doseTV/integral dosePIV50%.

METHODS

The value of η50% can be easily calculated by dividing the integral dose (mean dose × volume) to the target volume (TV) by the integral dose to the volume of 50% of the prescription isodose (PIV50%). Alternatively, differential dose-volume histograms (DVHs) of the TV and PIV50% can be used. The resulting η50% value is effectively the proportion of energy within the PIV50% that falls into the target. This value has theoretical limits of 0 and 1, with 1 being perfect. The index combines conformity, gradient, and mean dose to the target into a single value. The value of η50% was retrospectively calculated for 100 clinical SRS plans.

RESULTS

The value of η50% for the 100 clinical SRS plans ranged from 37.7% to 58.0% with a mean value of 49.0%. This study also showed that the same principles used for the calculation of η50% can be adapted to produce an index suitable for multiple-target plans (Gη12Gy). Furthermore, the authors present another adaptation of the index that may play a role in plan optimization by calculating and minimizing the proportion of energy delivered to surrounding organs at risk (OARη50%).

CONCLUSIONS

The proposed efficiency index is a novel approach in quantifying plan quality by combining conformity, gradient, and mean dose into a single value. It quantifies the ratio of the dose “doing good” versus the dose “doing harm,” and its adaptations can be used for multiple-target plan optimization and OAR sparing.

Optimal collimator rotation based on the outline of multiple brain targets in VMAT

Background

The aim of this study was to investigate the dosimetric quality in volumetric modulated arc therapy (VMAT) plans with optimal collimator angles that can represent the outline of multiple brain targets.

Methods

Twenty patients with multiple target volumes in the brain cases were selected retrospectively. To better represent the outline of the multiple brain targets, four conformal arc plans were generated for each patient using one full arc with four collimator settings. The optimal collimator angles calculated from the integrated multi-leaf collimator (MLC) aperture that had the smallest aperture size for certain collimator settings of the conformal arc plan were selected. VMAT plans with the optimal collimator angles with angular sections of 40° and 60° (Colli-VMAT (40°), Colli-VMAT (60°)) were generated, followed by evaluation of field sizes, dose-volumetric parameters and total monitor units (MUs).

Results

Patient-averaged values of field sizes for Colli-VMAT (40°) (111.5 cm²) were lowest and 1.3 times smaller than those for Std-VMAT (143.6 cm²). Colli-VMAT plans improved sparing of most normal organs but for brain stem and left parotid gland. For the total MUs, the averaged values obtained with the Colli-VMAT (40°) (390 ± 148 MU) were smaller than those obtained with the Std-VMAT (472 ± 235 MU).

Conclusions

The Colli-VMAT plans with smaller angular sections could be suitable in the clinic for multiple brain targets as well as for irregularly shaped targets. Determination of the optimal collimator rotation generally showed good normal tissue sparing and MU reduction for multiple brain targets.

Dose gradient curve: A new tool for evaluating dose gradient

Purpose

Stereotactic radiotherapy, which delivers an ablative high radiation dose to a target volume for maximum local tumor control, requires a rapid dose fall-off outside the target volume to prevent extensive damage to nearby normal tissue. Currently, there is no tool to comprehensively evaluate the dose gradient near the target volume. We propose the dose gradient curve (DGC) as a new tool to evaluate the quality of a treatment plan with respect to the dose fall-off characteristics.

Methods

The average distance between two isodose surfaces was represented by the dose gradient index (DGI) estimated by a simple equation using the volume and surface area of isodose levels. The surface area was calculated by mesh generation and surface triangulation. The DGC was defined as a plot of the DGI of each dose interval as a function of the dose. Two types of DGCs, differential and cumulative, were generated. The performance of the DGC was evaluated using stereotactic radiosurgery plans for virtual targets.

Results

Over the range of dose distributions, the dose gradient of each dose interval was well-characterized by the DGC in an easily understandable graph format. Significant changes in the DGC were observed reflecting the differences in planning situations and various prescription doses.

Conclusions

The DGC is a rational method for visualizing the dose gradient as the average distance between two isodose surfaces; the shorter the distance, the steeper the dose gradient. By combining the DGC with the dose-volume histogram (DVH) in a single plot, the DGC can be utilized to evaluate not only the dose gradient but also the target coverage in routine clinical practice.

A comparison of treatment plan quality between Tri-Co-60 intensity modulated radiation therapy and volumetric modulated arc therapy for cervical cancer

PURPOSE

To investigate the plan quality of tri-Co-60 intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) for cervical cancer.

METHODS

A total of 20 patients who received postoperative radiotherapy for cervical cancer were selected. For each patient, a tri-Co-60 IMRT plan for which the target volume was the planning target volume (PTV) generated by adding 1mm isotropic margins from the clinical target volume (CTV) and a VMAT plan for which the target volume was the PTV generated by adding 7mm and 10mm margins from the CTV were generated. The tri-Co-60 IMRT plans were generated with the ViewRay™ system while the VMAT plans were generated with 15-MV photon beams from a linear accelerator (prescription dose=50.4Gy in 28 fractions).

RESULTS

The average volumes of the PTVs and CTVs were 704.9cc±87.8cc and 271.6cc±51.6cc, respectively. No noticeable differences in the dose-volumetric parameters for the target volumes were observed between the tri-Co-60 IMRT and VMAT plans. The values of V_40Gy for the small bowel and rectal wall, V_45Gy of the bladder, and V_35Gy of the femoral heads for the VMAT plans were 14.6%±7.8%, 54.4%±4.2%, 30.0%±4.7%, and 8.9%±3.3%, respectively. Those of the tri-Co-60 IMRT plans were 2.8%±2.1%, 23.0%±8.9%, 17.1%±6.1%, and 0.3%±0.4%, respectively.

CONCLUSIONS

Owing to the target margin reduction capability, the tri-Co-60 IMRT plans were more favorable than the VMAT plans for cervical cancer.

Dosimetric effects of sectional adjustments of collimator angles on volumetric modulated arc therapy for irregularly-shaped targets

PURPOSE

To calculate an optimal collimator angle at each of sectional arcs in a full-arc volumetric modulated arc therapy (VMAT) plan and evaluate dosimetric quality of these VMAT plans comparing full-arc VMAT plans with a fixed collimator angle.

METHODS

Seventeen patients who had irregularly-shaped target in abdominal, head and neck, and chest cases were selected retrospectively. To calculate an optimal collimator angle at each of sectional arcs in VMAT, integrated MLC apertures which could cover all shapes of target determined by beam's-eye view (BEV) within angular sections were obtained for each VMAT plan. The angular sections were 40°, 60°, 90° and 120°. When the collimator settings were rotated at intervals of 2°, we obtained the optimal collimator angle to minimize area size difference between the integrated MLC aperture and collimator settings with 5 mm-margins to the integrated MLC aperture. The VMAT plans with the optimal collimator angles (Colli-VMAT) were generated in the EclipseTM. For comparison purposes, one full-arc VMAT plans with a fixed collimator angles (Std-VMAT) were generated. The dose-volumetric parameters and total MUs were evaluated.

RESULTS

The mean dose-volumetric parameters for target volume of Colli-VMAT were comparable to Std-VMAT. Colli-VMAT improved sparing of most normal organs but for brain stem, compared to Std-VMAT for all cases. There were decreasing tendencies in mean total MUs with decreasing angular section. The mean total MUs for Colli-VMAT with the angular section of 40° (434 ± 95 MU, 317 ± 81 MU, and 371 ± 43 MU for abdominal, head and neck, and chest cases, respectively) were lower than those for Std-VMAT (654 ± 182 MU, 517 ± 116 MU, and 533 ± 25 MU, respectively).

CONCLUSIONS

For an irregularly-shaped target, Colli-VMAT with the angular section of 40° reduced total MUs and improved sparing of normal organs, compared to Std-VMAT.

The effect of extremely narrow MLC leaf width on the plan quality of VMAT for prostate cancer

BACKGROUND

To investigate the effect of multi-leaf collimators (MLCs) with leaf width of 1.25 mm on the plan quality of volumetric modulated arc therapy (VMAT) for prostate cancer.

METHODS

A total of 20 patients with prostate cancer were retrospectively selected. Using a high definition MLC (HD MLC), primary and boost VMAT plans with two full arcs were generated for each patient (original plan). After that, by shifting the isocenter position of the 2nd arc by 1.25 mm in the cranio-caudal direction, we simulated fluences made with MLCs with leaf width of 1.25 mm. After shifting, primary and boost plans were generated for each patient (shifted plan). A sum plan was generated by summation of the primary and boost plan for each patient. Dose-volumetric parameters were calculated and compared.

RESULTS

Both the homogeneity index (HI) and conformity index (CI) of the shifted plans were better than those of the original plans in primary plans (HI = 0.065 vs. 0.059 with p < 0.001 and CI = 1.056 vs. 1.044 with p = 0.006). Similarly, the shifted plans for the boost target volume showed better homogeneity and conformity than did the original plans (HI = 0.060 vs. 0.053 with p < 0.001 and CI = 1.015 vs. 1.009 with p < 0.001). The target mean dose of the original plans was closer to the prescription dose than that of the shifted plans in the case of sum plans (81.45 Gy vs. 81.12 Gy with p = 0.001).

CONCLUSIONS

Use of extremely narrow MLCs could increase dose homogeneity and conformity of the target volume for prostate VMAT.

Modulation index for VMAT considering both mechanical and dose calculation uncertainties

The aim of this study is to present a modulation index considering both mechanical and dose calculation uncertainties for volumetric modulated arc therapy (VMAT). As a modulation index considering only mechanical uncertainty of VMAT, MIt has been previously suggested. In this study, we developed a weighting factor which represents dose calculation uncertainty based on the aperture shapes of fluence maps at every control point of VMAT plans. In order to calculate the weighting factor, the thinning algorithm of image processing techniques was applied to measure field aperture irregularity. By combining this weighting factor with the previously suggested modulation index, MIt, comprehensive modulation index (MIc) was designed. To evaluate the performance of MIc, gamma passing rates, differences in mechanical parameters between plans and log files and differences in dose-volume parameters between plans and the plans reconstructed from log files were acquired with a total of 52 VMAT plans. Spearman's correlation coefficients (rs) between the values of MIc and measures of VMAT delivery accuracy were calculated. The rs values of MIc (f = 0.5) to global gamma passing rates with 2%/2 mm, 1%/2 mm and 2%/1 mm were  -0.728,-0.847 and  -0.617, respectively (p  <  0.001). Those to local gamma passing rates were  -0.765,-0.767 and  -0.748, respectively (p  <  0.001). The rs values of MIc (f = 0.5) to multi-leaf collimator and gantry angle errors were 0.800 and  -0.712, respectively (p  <  0.001). The MIc (f = 0.5) showed a total of 20 rs values (p  <  0.05) to the differences in dose-volumetric parameters from a total of 35 tested cases. The MIc (f = 0.5) demonstrated considerable power to predict VMAT delivery accuracy showing strong correlations to various measures of VMAT delivery accuracy.