Paddick Gradient Index Research Articles

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.

Dose Gradient Analyses in Linac-based Intracranial Stereotactic Radiosurgery Using Paddick's Gradient Index: Consideration of the Optimal Method for Plan Evaluation

The objective of our study was to describe the dose gradient characteristics of Linac-based stereotactic radiosurgery using Paddick's gradient index (GI) and to elucidate the factors influencing the GI value. Seventy-three plans for brain metastases using the dynamic conformal arcs were reviewed. The GI values were calculated at the 80% and 90% isodose surfaces (IDSs) and at the different target coverage IDSs (D99, D95, D90, and D85). The GI values significantly decreased as the target coverage of the reference IDS increased (the percentage of the IDS decreased). There was a significant inverse correlation between the GI values and target volume. The plans generated with the addition of a 1-mm leaf margin had worse GI values both at the D99 and D95 relative to those without leaf margin. The number and arrangement of arcs also affected the GI value. The GI values are highly sensitive to (1) the IDS selection variability for dose prescription or evaluation, (2) the target volume, and (3) the planning method. To objectively compare the quality of dose gradient between rival plans, it would be preferable to employ the GI defined at the reference IDS indicating the specific target coverage (e.g., D95), irrespective of the intended marginal dose. The modified GI (mGI), defined in this study, substituting the denominator of the original GI with the target volume, would be useful to compensate for the false superior GI value in cases of target over-coverage with the reference IDS and to objectively evaluate the dose gradient outside the target boundary.

Leksell GammaPlan version 10.0 preview: performance of the new inverse treatment planning algorithm applied to Gamma Knife surgery for pituitary adenoma

Treatment planning for Gamma Knife surgery has traditionally been a forward planning (FP)-only approach with results that depend significantly on the experience of the user. Leksell GammaPlan version 10.0, currently in beta testing, introduces a new inverse planning (IP) engine that may allow more reproducible results across dosimetrists and individual institutions. In this study the authors compared the FP and IP approaches to Gamma Knife surgery. Forty-three patients with pituitary adenomas were evaluated after dose planning was performed using FP and IP treatment approaches. Treatment plans were compared for target coverage, target selectivity, Paddick gradient index, number of isocenters, optic pathways dose, and treatment time. Differences between the forward and inverse treatment plans were evaluated in a statistical fashion. The IP software generated a dose plan within approximately 10 minutes. The FP approach delivered the prescribed isodose to a larger treatment volume than the IP system (p < 0.001). The mean (± SD) FP and IP coverage indices were 0.85 ± 0.23 and 0.85 ± 0.13, respectively (no significant difference). The mean FP and IP gradient indices were 2.78 ± 0.20 and 3.08 ± 0.37, respectively (p < 0.001). The number of isocenters did not appreciably differ between approaches. The maximum doses directed to the optic apparatus for the FP and IP methods were 8.67 ± 1.97 Gy and 12.33 ± 5.86 Gy, respectively (p < 0.001). The Leksell GammaPlan IP system was easy to operate and provided a reasonable, first approximation dose plan. Particularly in cases in which there are eloquent structures at risk, experience and user-based optimization will be required to achieve an acceptable Gamma Knife dose plan.

Comparison Study of Intensity Modulated Arc Therapy (IMAT) using Single or Multiple Arcs to Intensity Modulated Radiation Therapy (IMRT) for High-risk Prostate Cancer

IMAT is a complex form of IMRT that allows dose delivery in a single or multiple arcs. The treatment of nodal stations in high-risk prostate cancer is controversial, however, there is emerging data to support enhanced biochemical disease free survival with nodal irradiation. We compared the plan quality and plan efficiency of IMRT versus one arc (1Arc) versus three arcs (3Arc) for treatment of high-risk prostate cancer. Fifteen patients with high-risk prostate cancer as defined by NCCN, were studied. Prostate (PTVp), right pelvic lymph nodes (PTVrln), left pelvic lymph nodes (PTVlln), and organs at risk were contoured. The total planned dose was 79.20Gy/1.8Gy/fraction. PTVp, PTVrln, PTVlln received 50.40Gy/1.8Gy/fraction followed by a boost to PTVb of 28.80Gy. Three plans were generated for each patient: 7 beams IMRT, 1Arc (1full rotation, 1 isocenter), 3Arc (1 full, 2 partial rotations for the lymph nodes, 3 isocenters) using Rapid Arc technology. All 3 plans were calculated to maintain Dmax ≤110% with 95% coverage to PTVt (PTVp+PTVrln+PTVlln). All plans were optimized using the same dose constraints. Nine patients were treated using 3Arc technique, 4 with IMRT and 3 using 1Arc. All plans had excellent coverage, i.e., 95% of dose to 95% of PTVt. The average RTOG conformity index (prescription volume/target volume), was 1.17, 1.20, 1.15 for IMRT, 1Arc, 3Arc, respectively, with 3Arc exhibiting the highest conformity. The 3 plans' homogeneity index was within 1% of each other. Similarly, Dmean of bladder was within 2% of each other. The lowest mean dose for the rectum was achieved with the IMRT plan, with 10% lower dose than the arc plans. The Paddick gradient index (50% isodose volume/prescription volume) of the 3Arc was superior to IMRT by 27.4% (p = 0.006). The average monitor unit (MU) was highest in the IMRT plan (1685) versus 1Arc (575) versus 3Arc (1079), (p =0.0001 for 1arc). The average beam on time was 6 minutes for IMRT compared to IMAT, p = 0.001. The beam on time of the arc therapies was 1.3 and 2.9 minutes for 1Arc and 3Arc, respectively. Intensity Modulated Arc Therapy delivers acceptable coverage compared to IMRT for high-risk prostate cancer, it also has the potential to achieve a more favorable dose gradient profile, conformity, lower MU and beam on time compared to IMRT. The 3Arc plan proved to have a superior gradient and conformity indices in contrast to the 1Arc plan. In regards to dose to organs at risk, there was no significant difference except that a lower rectal dose was seen in the IMRT plans.

Feasibility of Single-Isocenter Volumetric Modulated Arc Radiosurgery for Treatment of Multiple Brain Metastases

To evaluate the relative plan quality of single-isocenter vs. multi-isocenter volumetric modulated arc therapy (VMAT) for radiosurgical treatment of multiple central nervous system metastases. VMAT plans were created using RapidArc technology for treatment of simulated patients with three brain metastases. The plans consisted of single-arc/single-isocenter, triple-arc (noncoplanar)/single-isocenter, and triple-arc (coplanar)/triple-isocenter configurations. All VMAT plans were normalized to deliver 100% of the 20-Gy prescription dose to all lesions. The plans were evaluated by calculation of Paddick and Radiation Therapy Oncology Group conformity index scores, Paddick gradient index scores, and 12-Gy isodose volumes. All plans were judged clinically acceptable, but differences were observed in the dosimetric parameters, with the use of multiple noncoplanar arcs showing small improvements in the conformity indexes compared with the single-arc/single-isocenter and triple-arc (coplanar)/triple-isocenter plans. Multiple arc plans (triple-arc [noncoplanar]/single-isocenter and triple-arc [coplanar]/triple-isocenter) showed smaller 12-Gy isodose volumes in scenarios involving three metastases spaced closely together, with only small differences noted among all plans involving lesions spaced further apart. Our initial results suggest that single-isocenter VMAT plans can be used to deliver conformity equivalent to that of multiple isocenter VMAT techniques. For targets that are closely spaced, multiple noncoplanar single-isocenter arcs might be required. VMAT radiosurgery for multiple targets using a single isocenter can be efficiently delivered, requiring less than one-half the beam time required for multiple isocenter set ups. VMAT radiosurgery will likely replace multi-isocenter techniques for linear accelerator-based treatment of multiple targets.