WE-E-213CD-01: Best in Physics (Joint Imaging-Therapy) - Evaluation of Deformation Algorithm Accuracy with a Two-Dimensional Anatomical Pelvic Phantom

PURPOSE

To objectively evaluate the accuracy of 11 different deformable registration techniques for bladder filling.

METHODS

The phantom represents an axial plane of the pelvic anatomy. Urethane plastic serves as the bony anatomy and urethane rubber with three levels of Hounsfield units (HU) is used to represent fat and organs, including the prostate. A plastic insert is placed into the phantom to simulate bladder filling. Nonradiopaque markers reside on the phantom surface. Optical camera images of these markers are used to measure the positions and determine the deformation from the bladder insert. Eleven different deformable registration techniques are applied to the full- and empty-bladder computed tomography images of the phantom to calculate the deformation. The applied algorithms include those from MIMVista Software and Velocity Medical Solutions and 9 different implementations from the Deformable Image Registration and Adaptive Radiotherapy Toolbox for Matlab. The distance to agreement between the measured and calculated deformations is used to evaluate algorithm error. Deformable registration warps one image to make it similar to another. The root-mean-square (RMS) difference between the HUs at the marker locations on the empty-bladder phantom and those at the calculated marker locations on the full-bladder phantom is used as a metric for image similarity.

RESULTS

The percentage of the markers with an error larger than 3 mm ranges from 3.1% to 28.2% with the different registration techniques. This range is 1.1% to 3.7% for a 7 mm error. The least accurate algorithm at 3 mm is also the most accurate at 7 mm. Also, the least accurate algorithm at 7 mm produces the lowest RMS difference.

CONCLUSIONS

Different deformation algorithms generate very different results and the outcome of any one algorithm can be misleading. Thus, these algorithms require quality assurance. The two-dimensional phantom is an objective tool for this purpose.

SU-E-J-96: Investigation into the Accuracy of Deformable Registration Algorithms for Use in Dose Summation: A Case Study in Combined Modalities

PURPOSE

To better understand the accuracy and limitation of deformable registration algorithms for dose summation by studying the results produced through two different deformable registration techniques for a clinical case of combining a Cyberknife radiosurgery plan and an electron boost to the scar tissues.

METHODS

A patient was treated with a Cyberknife plan with supine CT and an electron boost plan with prone CT concurrently. The CT, RT structures and dose grids were exported from the Cyberknife MultiPlan system and the Pinnacle system and imported into two commercial DICOM viewing systems equipped with deformable registration algorithms. One of the systems uses intensity-based free-form deformable registration while the other uses B-spline free-form deformable registration. The electron boost plan was first registered to the CK plan using rigid registration, then secondly using deformable registration in each system. The region of interest used for registration was chosen to encompass the whole CT volumes due to the supine vs. prone positions.

RESULTS

The summation doses for the Cyberknife CTV and critical structures do not differ between rigid registration and deformable registration for both systems. The electron boost volume does show higher mean dose received for the deformable registration compared with rigid registration for both systems (12.90 Gy vs. 11.71 Gy and 12.39 Gy vs. 11.53 Gy). There are slight variations between the doses produced by the two systems for all the structures, with an averaged difference of approximately 0.02% to 2.63%.

CONCLUSIONS

These results show that for cases like this one, where the two treatment volumes do not overlap, there will not be significant differences between rigid and deformable registration, and that the only significant difference in summation dose between the different deformable registration algorithms is where the volume is deformed the most, in this case, the electron boost volume.

SU-E-T-123: Understanding the Meaning of IMRT QA Passing Rates with a 2D Diode Array

PURPOSE

A lower than ideal tolerance limit is used in intensity-modulated radiation therapy quality assurance (IMRT QA) with a 2D diode array due to passing rate fluctuations. The objective is to identify patterns in the passing rates to predict sources of uncertainty that can affect treatment delivery, for example, the need to re-calibrate the multileaf collimator when the passing rates start to decrease.

METHODS

Five complex clinical prostate IMRT plans were evaluated with a 2D diode array. The QA for each plan was repeated five times during one and a half month period. One of the plans was randomly selected and repeated the same day five consecutive times. The planar doses calculated by the treatment planning system were compared to the measurements of the 2D diode array. The individual passing rates per beam per plan were compared.

RESULTS

The average passing rate for each plan ranged from 94% to 97%. While the average percent difference of this ranged between -7.67% to 17.61%. Additionally, the minimum and maximum standard deviation among all beams was 0.13% and 9.63% respectively. We also compared the standard deviation of a plan QA repeated during different days versus a plan QA repeated during the same day. For the former the highest standard deviation was 6.05 % while for the later 0.21%. We noticed that the largest discrepancy between the passing rates was for angles at around 155° and 205°.

CONCLUSION

These results show some inconsistency in the IMRT QA passing rates from one day to the next. Moreover, lower passing rates for a specific angle like the ones shown here can represent possible mechanical or tuning problems with the linear accelerator at these specific locations. Early identification of these sources of uncertainty can greatly improve the precision of the treatment delivery.

SU-D-211-04: Sector Intensity Modulated (SIM) Gamma Knife Stereotactic Radiosurgery

PURPOSE

The latest Gamma Knife (GK) system, Perfexion, consists of 192 Co-60 sources divided into eight sectors. Treatment delivery includes multiple shots placed at different positions. For every shot, each sector can be either blocked or open with four different aperture sizes. However, the beam-on time is designed to be fixed. We proposed an innovative concept, Sector Intensity Modulated (SIM) Gamma Knife by dynamically varying the beam-on time for each individual sector to improve stereotactic radiosurgery planning quality.

METHODS

The anatomic structures and dose matrices from each sector for every shot were obtained from the GK workstation. The beam-on time for each sector was decomposed with various discrete levels and brute-force algorithm was used to get the optimal solution. The resulting SIM plan was then re-entered into the GK workstation. Six indices were used to benchmark the plan quality: Coverage, Conformality, Gradient, Maximum Dose(s) to critical structure(s), Volume receiving over 8 and 12 Gy. All the SIM plans in comparison with the original plans were further reviewed by an experienced oncologist.

RESULTS

The simulations were tested on various pituitary adenoma cases. Results consistently showed that SIM yielded better plans with all quantitative indices improved compared to original plan. It provides better conformality, quicker drop off of the isodose line outside the tumor, lower doses to the critical structures as optical- nerve/chiasm while maintaining at least 99% coverage of the tumor. Results were more favorable according to oncologist's view. In particular, up to 20% or 0.6 cc volume decrease in healthy tissue receiving 8 Gy was observed. This may translate into clinically observable reduction in acute/late toxicities.

CONCLUSIONS

Our preliminary results show that Sector Intensity Modulated Gamma Knife offers superior treatment plans compared to the originally delivered plans. Further works as adding dynamic shot location and dynamic shot shaping will be discussed.