Conformity Gradient Index Research Articles

Dosimetric feasibility of brain stereotactic radiosurgery with a 0.35 T MRI-guided linac and comparison vs a C-arm-mounted linac.

MRI is the gold-standard imaging modality for brain tumor diagnosis and delineation. The purpose of this work was to investigate the feasibility of performing brain stereotactic radiosurgery (SRS) with a 0.35T MRI-guided linear accelerator (MRL) equipped with a double-focused multileaf collimator (MLC). Dosimetric comparisons were made vs a conventional C-arm-mounted linac with a high-definition MLC. The quality of MRL single-isocenter brain SRS treatment plans was evaluated as a function of target size for a series of spherical targets with diameters from 0.6cm to 2.5cm in an anthropomorphic head phantom and six brain metastases (max linear dimension=0.7-1.9cm) previously treated at our clinic on a conventional linac. Each target was prescribed 20Gy to 99% of the target volume. Step-and-shoot IMRT plans were generated for the MRL using 11 static coplanar beams equally spaced over 360° about an isocenter placed at the center of the target. Couch and collimator angles are fixed for the MRL. Two MRL planning strategies (VR1 and VR2) were investigated. VR1 minimized the 12Gy isodose volume while constraining the maximum point dose to be within ±1Gy of 25Gy which corresponded to normalization to an 80% isodose volume. VR2 minimized the 12Gy isodose volume without the maximum dose constraint. For the conventional linac, the TB1 method followed the same strategy as VR1 while TB2 used five noncoplanar dynamic conformal arcs. Plan quality was evaluated in terms of conformity index (CI), conformity/gradient index (CGI), homogeneity index (HI), and volume of normal brain receiving ≥12Gy (V12Gy ). Quality assurance measurements were performed with Gafchromic EBT-XD film following an absolute dose calibration protocol. For the phantom study, the CI of MRL plans was not significantly different compared to a conventional linac (P>0.05). The use of dynamic conformal arcs and noncoplanar beams with a conventional linac spared significantly more normal brain (P=0.027) and maximized the CGI, as expected. The mean CGI was 95.9±4.5 for TB2 vs 86.6±3.7 (VR1), 88.2±4.8 (VR2), and 88.5±5.9 (TB1). Each method satisfied a normal brain V12Gy ≤10.0cm3 planning goal for targets with diameter ≤2.25cm. The mean V12Gy was 3.1cm3 for TB2 vs 5.5cm3 , 5.0cm3 and4.3cm3 , for VR1, VR2, and TB1, respectively. For a 2.5-cm diameter target, only TB2 met the V12Gy planning objective. The MRL clinical brain plans were deemed acceptable for patient treatment. The normal brain V12Gy was ≤6.0cm3 for all clinical targets (maximum target volume=3.51cm3 ). CI and CGI ranged from 1.12-1.65 and 81.2-88.3, respectively. Gamma analysis pass rates (3%/1mm criteria) exceeded 97.6% for six clinical targets planned and delivered on the MRL. The mean measured vs computed absolute dose difference was -0.1%. The MRL system can produce clinically acceptable brain SRS plans for spherical lesions with diameter ≤2.25cm. Large lesions (>2.25cm) should be treated with a linac capable of delivering noncoplanar beams.

Retrospective dosimetric analysis of brain lesions planned in Pinnacle 9.8 via a HDMLC linac

The University of Toledo Medical Center's Eleanor N. Dana Cancer Center located in northwest Ohio currently utilizes the Edge Radiosurgery System (Varian Medical Systems Inc., Palo Alto, CA) to deliver stereotactic radiosurgery for the treatment of brain lesions. The purpose of this study is to determine the quality of conformal arc radiotherapy in treating patients with brain lesions at The University of Toledo Medical Center and to provide more data for conformity and gradient indices (due to a lack of current data) to hopefully improve national standards by allowing centers to compare among each other. Treatment plans were assessed using the Pinnacle3 v9.8 Radiation Therapy Planning System (Philips Healthcare, Amsterdam, Netherlands). For patients (n = 41) presenting with small brain lesions (n = 82) and treated with conformal arc radiotherapy via the Edge Radiosurgery System, the RTOG conformity index, Paddick conformity index, conformity gradient index, gradient index, and dose gradient index were determined for each plan. This study additionally provides data to suggest the more accurate method of volume derivation provided by the Pinnacle3 v9.8 software. Using this method, average values for each of the following indices were calculated: RTOG conformity index = 1.36 ± 0.29; Paddick conformity index = 0.72 ± 0.12; conformity gradient index = 214.67 ± 12.35; gradient index = 3.64 ± 1.09; dose gradient index = −0.11 ± 0.16. Thus, The University of Toledo Medical Center provides favorable conformity of dose to intracranial target lesions.

Dosimetric comparison of different treatment modalities for stereotactic radiotherapy

BackgroundThe modalities for performing stereotactic radiotherapy (SRT) on the brain include the cone-based linear accelerator (linac), the flattening filter-free (FFF) volumetric modulated arc therapy (VMAT) linac, and tomotherapy. In this study, the cone-based linac, FFF-VMAT linac, and tomotherapy modalities were evaluated by measuring the differences in doses delivered during brain SRT and experimentally assessing the accuracy of the output radiation doses through clinical measurements.MethodsWe employed a homemade acrylic dosimetry phantom representing the head, within which a thermoluminescent dosimeter (TLD) and radiochromic EBT3 film were installed. Using the conformity/gradient index (CGI) and Paddick methods, the quality of the doses delivered by the various SRT modalities was evaluated. The quality indicators included the uniformity, conformity, and gradient indices. TLDs and EBT3 films were used to experimentally assess the accuracy of the SRT dose output.ResultsThe dose homogeneity indices of all the treatment modalities were lower than 1.25. The cone-based linac had the best conformity for all tumors, regardless of the tumor location and size, followed by the FFF-VMAT linac; tomography was the worst-performing treatment modality in this regard. The cone-based linac had the best gradient, regardless of the tumor location and size, whereas the FFF-VMAT linac had a better gradient than tomotherapy for a large tumor diameter (28 mm). The TLD and EBT3 measurements of the dose at the center of tumors indicated that the average difference between the measurements and the calculated dose was generally less than 4%. When the 3% 3-mm gamma passing rate metric was used, the average passing rates of all three treatment modalities exceeded 98%.ConclusionsRegarding the dose, the cone-based linac had the best conformity and steepest dose gradient for tumors of different sizes and distances from the brainstem. The results of this study suggest that SRT should be performed using the cone-based linac on tumors that require treatment plans with a steep dose gradient, even as the tumor is slightly irregular, we should also consider using a high dose gradient of the cone base to treat and protect the normal tissue. If normal tissues require special protection exist at positions that are superior or inferior to the tumor, we can consider using tomotherapy or Cone base with couch at 0° for treatment.

SU-F-T-605: Implementation of the Conformity/Gradient Index (CGI) to Intracranial, Linac-Based Stereotactic Plans to Evaluate Possible Improvements in Treatment Planning

Purpose: To retrospectively evaluate the planning quality of a sample of intracranial, linac-based, dynamic arc MLC and VMAT stereotactic plans by calculating the effective gradient, Conformity Score (CGIc) and Gradient Score Index (CGIg), and determine if the Conformity/Gradient Index (CGI) may be a useful metric to guide our current treatment planning process. Methods: We evaluated single lesion VMAT, single lesion dynamic arc, and single isocenter, 2-lesion linac-based, intracranial, VMAT treatment plans originally created in Eclipse™. The CGI index was calculated for each lesion using the Target Volume (TV), the Prescription Isodose Volume (PIV), and the 50% Isodose Volume. The effective gradient was obtained directly from the Eclipse “gradient measure.” Results: From the single lesion cases, target volumes of ≤ ∼2cc reliably achieved an effective gradient of ∼3mm and a CGI > 95. For lesions ≤ ∼20cc, the majority of cases have an effective gradient of 90-95. For our multilesion data, we determined that lesions separated by 80-85, and lesions separated by >3cm can have CGI over 90. Conclusion: The CGI metric is a simple tool that can be applied to MLC-based stereotactic planning, requires only three pieces of data, and quickly allows planners to evaluate and determine if a plan is suboptimal. Our data identified several cases that might be improved with additional effort and/or re-planning, but the majority of the treatment plans evaluated fell within a desirable CGI range. Cases deemed suboptimal are currently under investigation and will be re-planned to determine if CGI values can be improved.

Stereotactic Ablative Radiation Therapy Delivered by Helical Tomotherapy for Early-Stage Non-small Cell Lung Cancer: Dosimetric Evaluation and Toxicity

In stereotactic ablative radiation therapy (SABR), high doses per fraction are delivered so that the desired target receives the prescribed dose, with a minimal excess volume and a sharp dose fall-off outside the target volume. The purpose of this study is to evaluate the relationship between the conformity index (CI), the homogeneity index (HI), and the conformity gradient index (CGI) and the development of radiation pneumonitis (RP) in early stage non small cell lung cancer (NSCLC) patients treated with SABR delivered by Helical Tomotherapy (HT).

A Comparison of Helical Tomotherapy to Circular Collimator-Based Linear-Accelerator Radiosurgery for the Treatment of Brain Metastases

To compare stereotactic radiosurgery treatment plans for the treatment of patients with brain metastases generated using Tomotherapy and a circular collimator-based SRS approach. Twenty patients, previously treated with circular collimator-based radiosurgery, were replanned using Tomotherapy treatment planning software. Tomotherapy planning emphasized dose fall off peripheral to the target by allowing for inhomogeneous target coverage. Conformity and dose falloff were compared with the circular collimator-based plans using the following metrics: prescription isodose to tumor volume ratio, conformation number, and homogeneity index to assess effects on targets, whereas a combined conformity gradient index and the volume of the 12-Gy isodose volume were used to assess differences in dose to normal brain. Although a similar homogeneity index was achieved for both sets of plans, plan conformity was generally improved using the tomotherapy system whereas dose falloff at the target periphery was shallower. The 12-Gy isodose volume increased on average by 3.4 mL (range, -1.9 to +12.1 mL), for the 20 patients studied, but in spite of this, based on modeled predictions, the risk for symptomatic radiation necrosis associated with Tomotherapy SRS for each patient still falls within the clinically observed ranges for Gamma Knife SRS. Tomotherapy can be used to create treatment plans that meet the dosimetric and clinical requirements for stereotactic radiosurgery.

SU-E-T-585: A Dosimetric Analysis of Cranial Stereo-Tactic Radio Surgery Plans Treated by Gamma Knife

Purpose: : To quantify the impact of dose planning using different clinically important parameters such as volume conformity index (VCI), appropriateness of target volume(ATV), conformity gradient index(CGI), integral dose (ID), by categorizing size of tumor volume(TV) in small, medium and large for stereo‐tactic irradiation of cranial lesions treated by Gamma Knife. Method and Materials: Tumor volume(TV) were categorize as (a ) small tumor for less than 2 cc, (b) medium tumor for between 2– 5 cc and (c) large tumor for volume more than 5 cc for evaluating the dosimetric parameters. Randomly 100 patient's data of each category were selected for this study. A uniform treatment procedure for each patient is followed. Treatments steps are:1) Leksell frame fixation 2) MRI/CT/Angiography of the lesion 3) Treatment planning (GAMMAPLAN 5.4, Elekta, AB) and 4) Treatment delivery (Gamma Knife Model B, Elekta, AB). All input data for estimating the different dosimetric parameter were derived from the treatment planning system data. VCI is estimated by ratio of prescribed dose volume and tumor volume. ATV is determined by ratio of minimum target dose and prescribed dose. CGI were estimated using the Wagner formula. ID is in‐built function of treatment planning system. Results: For small TV, VCI is ranges from 1.2–1.4; ATV ranges from 5–10; CGI rages from 80–90 and ID ranges from 10–20. In case of medium TV, VCI is ranges from 1.2–1.4;ATV ranges from 2–3 ; CGI rages from 70– 80 and ID ranges from 40–100. For large TV, VCI is ranges from 1.3–1.6; ATV ranges from 0.3–1; CGI rages from 60–70 and ID ranges more than 100. Conclusions: Clinically important parameters VCI, ATV, CGI and ID were evaluated in small, medium and large TV cranial lesions treated by Gamma Knife. The value may be compared with other similar treatment modalities to compare the end result.

SU‐E‐T‐885: A Planning Comparison of Dynamic Conformal Arc (DCA), Static Non‐ Coplanar Intensity Modulated Radiotherapy (NCP‐IMRT), Volumetric Modulated Arc Therapy (RapidArc), Robotic Radiosurgery (Cyberknife), and Helical Tomotherapy (HI‐ART TomoTherapy) for SRS

Purpose: This work compares all linac‐based SRS treatment techniques currently available for single lesion cranial SRS. This study includes the planning comparison of dynamic conformal arc (DCA), static non‐coplanar intensity modulated radiotherapy (NCP‐IMRT), volumetric modulated arc therapy (RapidArc), robotic radiosurgery (Cyberknife), and helical tomotherapy (HI‐ART TomoTherapy) for cranial SRS. Methods: Thirteen target volumes with range 0.23 to 20.76 cc were retrospectively selected and transferred to a CT scan of a phantom designed for end‐to‐end SRS QA (Lucy phantom, Standard Imaging). Plans were developed using, iPlan TPS (v4.1, BrainLAB) for DCA (4 arcs) and NCP‐IMRT (16 beams), meanwhile the Eclipse TPS (v8.6, Varian Medical Systems) was used for the RapidArc (4 arcs) technique. Multiplan TPS (v3.5, Accuray) and TomoTherapy HI‐ART TPS (v3.1.4.23) was used for Cyberknife and TomoTherapy respectively. All plans were evaluated using four criteria, (1) Paddickˈs Conformity Index (CI), (2) Paddickˈs Gradient Index (GI), (3) Homogeneity Index and (4) Wagnerˈs Conformity/Gradient Index (CGI). Results: The average Paddick conformity index, CI was 0.64, 0.72, 0.76, 0.78, and 0.65 for the DCA, NCP‐IMRT, RapidArc, Cyberknife and Tomotherapy techniques respectively.The average Paddick gradient index, GI was 3.3, 3.6, 4.2, 4.4, and 4.9 for the DCA, NCP‐IMRT, RapidArc, Cyberknife and Tomotherapy techniques respectively. The average Wagnerˈs CGI was 71.6, 74.5, 72.2, 71.37, and 62.18 for the DCA, NCP‐IMRT, RapidArc, Cyberknife and Tomotherapy techniques respectively. IMRT‐based techniques and robotic radiosurgery showed better CI and CGI, whereas DCA showed the best dose fall off followed by NCP‐IMRT Conclusions: All methods were able to produce comparable plans for most of the targets tested. More importantly, it is suggested that for future planning studies the plan criteria must be explicit in their goals. In particular, the gradient index should be specified along with the desired dose prescription and conformality.

Tradeoffs for Assuming Rigid Target Motion in Mlc-Based Real Time Target Tracking Radiotherapy: A Dosimetric and Radiobiological Analysis

We report on our assessment of two types of real time target tracking modalities for lung cancer radiotherapy namely (1) single phase propagation (SPP) where motion compensation assumes a rigid target and (2) multi-phase propagation (MPP) where motion compensation considers a deformable target. In a retrospective study involving 4DCT volumes from six (n=6) previously treated lung cancer patients, four-dimensional treatment plans representative of the delivery scenarios were generated per modality and the corresponding dose distributions were derived. The modalities were then evaluated (a) Dosimetrically for target coverage adequacy and normal tissue sparing by computing the mean GTV dose, relative conformity gradient index (CGI), mean lung dose (MLD) and lung V(2)0; (b) Radiobiologically by calculating the biological effective uniform dose (D) for the target and organs at risk (OAR) and the complication free tumor control probability (P(+)). As a reference for the comparative study, we included a 4D Static modality, which was a conventional approach to account for organ motion and involved the use of individualized motion margins. With reference to the 4D Static modality, the average percent decrease in lung V(20) and MLD were respectively (13.1-/+6.9) % and (11.4-/+ 5.6)% for the MPP modality, whereas for the SPP modality they were (9.4-/+6.2) % and (7.2-/+4.7) %. On the other hand, the CGI was observed to improve by 15.3-/+13.2 and 9.6-/+10.0 points for the MPP and SPP modalities, respectively while the mean GTV dose agreed to better than 3% difference across all the modalities. A similar trend was observed in the radiobiological analysis where the P(+) improved on average by (6.7-/+4.9) % and (4.1-/+3.6) % for the MPP and SPP modalities, respectively while the D computed for the OAR decreased on average by (6.2-/+3.6) % and (3.8-/+3.5) % for the MPP and SPP tracking modalities, respectively. The D calculated for the GTV for all the modalities was in agreement to better than 2% difference. In general, respiratory motion induces target displacement and deformation and therefore the complex MPP real time target tracking modality is the preferred. On the other hand, the SPP approach affords simplicity in implementation at the expense of failing to account for target deformation. Radiobiological and dosimetric analyses enabled us to investigate the consequences of failing to compensate for deformation and assess the impact if any on the clinical outcome. While it is not possible to draw any general conclusions on a small patient cohort, our study suggests that the two tracking modalities can lead to comparable clinical outcomes and as expected are advantageous when compared with the static conventional modality.

SU-FF-T-573: Tracking Versus Gating for Lung Cancer Radiotherapy. A Retrospective Study Involving 5 Patient 4DCT Data Sets

Purpose: To compare tracking versus gating for lungcancerradiotherapy using actual patient 4DCT data. Method and Materials: This study involved 4DCT data from 5 patients treated for lungcancer with tumor extent of motion within the range [4mm, 10mm]. Ten image sets corresponding to various phases of the respiratory cycle were derived from each 4DCT data set and a single clinician contoured the target and organs at risk on each image set. Two treatment plans were developed per patient based on tracking and gated radiotherapy. The gating plan assumed a 30% duty cycle meaning that three out of the ten contiguous image sets were incorporated into the treatment plan. For these image sets, a composite target was derived which was the union of the delineated targets and a treatment plan was developed per image set based on the composite target. The tracking plan consisted of ten independent plans developed per image set. The 4D dose was derived by deforming each of the weighted dose distributions computed per image set to a chosen reference image set and summed. The two techniques were then evaluated for biological tumor coverage adequacy and normal tissue sparing by using the target generalized equivalent uniform dose (gEUD), lung V20, mean lungdose (MLD) and a relative conformity gradient index (CGI). The CGI ranks two plans in terms of quality and desirability. Results: On average, the gEUD discrepancy was within 4% among the plans. However, the lung V20 and MLD decreased by (12+/−6) % and (7+/−2) % respectively and the relative CGI increased by (12+/−8) points from gating to tracking radiotherapy plan. Conclusion: Adequate tumor coverage can be achieved using either of the delivery techniques. Tracking results in significantly improved normal tissue sparing, making it a more attractive delivery technique for lungcancerradiotherapy.

SU-FF-J-112: Evaluation of Tracking Versus Conventional Delivery for Lung Cancer Radiotherapy Via 4D Treatment Planning

Purpose: Apply 4D treatment planning techniques to evaluate tracking versus conventional delivery for lungradiotherapyMethod and Materials: This was a retrospective study involving 4DCT data from 6 patients previously treated for lungcancer with tumor extent of motion within the range [2mm, 10mm]. Ten image sets corresponding to various phases of the respiratory cycle were derived from each 4DCT data set and a single clinician contoured the target and organs at risk on all image sets. Two treatment plans were developed per patient based on tracking and conventional radiotherapy. In the conventional plan, bigger margins were used to account for organ motion. First, a composite target was derived based on the delineated targets across all image sets then a treatment plan was developed per image set based on the composite target. Tracking was characterized by reduced margins as the treatment plan consisted of ten independent plans developed per image set. The 4D dose was derived by deforming each of the dose distribution computed per image set to a chosen reference image set and summed. We then evaluated the two techniques for biological tumor coverage adequacy and normal tissue sparing by quantifying the target generalized equivalent uniform dose (gEUD), lung V20, mean lungdose (MLD) and a relative conformity gradient index (CGI). Results: On average, the gEUD discrepancy was within 5%, the lung V20 and MLD decreased by (17+/−4) % and (15+/−3) % respectively and the relative CGI increased by (22+/−12) points from conventional to tracking radiotherapy plan. This result excludes two patients with tumor motion less than 5mm as there was insignificant variation between the two techniques. Conclusion: Conventional and tracking techniques are comparable in terms of target coverage but tracking results in far more normal tissue sparing making it a more attractive delivery type for lungcancerradiotherapy.

A simple and reliable index for scoring rival stereotactic radiosurgery plans

Purpose A simple and robust index for ranking rival stereotactic radiosurgery plans is presented. Methods The radiosurgery plan score index, CGI (Conformity/Gradient Index), is an average of a conformity score and a gradient score. Computation of the CGI score is simple, requiring only three pieces of data: (1) the total volume irradiated to the prescription isodose level, (2) the volume of the target, and (3) the total volume irradiated at half of the prescription isodose level. The overall CGI Index is a simple function of these three pieces of data. Results When multiple sets of rival stereotactic radiosurgery plans were ranked with respect to this single score index, the resulting plan rankings closely matched the plan rankings according to biologic indices (calculated nontarget brain normal tissue complication probabilities). Conclusions The CGI is a simple and fast plan evaluation tool that can assist the radiosurgery planner in evaluating and optimizing multiple candidate radiosurgery plans.