Cheng B. Saw

Synchrony – Cyberknife Respiratory Compensation Technology

Studies of organs in the thorax and abdomen have shown that these organs can move as much as 40 mm due to respiratory motion. Without compensation for this motion during the course of external beam radiation therapy, the dose coverage to target may be compromised. On the other hand, if compensation of this motion is by expansion of the margin around the target, a significant volume of normal tissue may be unnecessarily irradiated. In hypofractionated regimens, the issue of respiratory compensation becomes an important factor and is critical in single-fraction extracranial radiosurgery applications. CyberKnife is an image-guided radiosurgery system that consists of a 6-MV LINAC mounted to a robotic arm coupled through a control loop to a digital diagnostic x-ray imaging system. The robotic arm can point the beam anywhere in space with 6 degrees of freedom, without being constrained to a conventional isocenter. The CyberKnife has been recently upgraded with a real-time respiratory tracking and compensation system called Synchrony. Using external markers in conjunction with diagnostic x-ray images, Synchrony helps guide the robotic arm to move the radiation beam in real time such that the beam always remains aligned with the target. With the aid of Synchrony, the tumor motion can be tracked in three-dimensional space, and the motion-induced dosimetric change to target can be minimized with a limited margin. The working principles, advantages, limitations, and our clinical experience with this new technology will be discussed.

Concept of dose nonuniformity in interstitial brachytherapy

PURPOSE Evaluation of the 3-dimensional dose distributions of interstitial implants using the dose uniformity ratio. METHODS AND MATERIALS Single source, two sources, three and four sources arranged both linearly and in the form of a triangle or a square, ribbons with different seed spacings, a single-plane and double-plane implants were evaluated. The evaluations involved the use of differential dose volume histograms and the dose nonuniformity ratio defined as the ratio of the high dose volume to the reference volume. RESULTS For a single source, the dose nonuniformity is the same regardless which dose rate is selected as the treatment dose rate. For any multi-source implant, the dose nonuniformity is altered depending on the selection of the reference dose rate. In addition, the dose nonuniformity curve exhibited three characteristics zones. CONCLUSION The dose nonuniformity ratio can be a useful tool in assessing and optimizing interstitial implants.

The effect of respiratory cycle and radiation beam-on timing on the dose distribution of free-breathing breast treatment using dynamic IMRT.

In breast cancer treatment, intensity-modulated radiation therapy (IMRT) can be utilized to deliver more homogeneous dose to target tissues to minimize the cosmetic impact. We have investigated the effect of the respiratory cycle and radiation beam-on timing on the dose distribution in free-breathing dynamic breast IMRT treatment. Six patients with early stage cancer of the left breast were included in this study. A helical computed tomography (CT) scan was acquired for treatment planning. A four-dimensional computed tomography (4D CT) scan was obtained right after the helical CT scan with little or no setup uncertainty to simulate patient respiratory motion. After optimizing based on the helical CT scan, the sliding-window dynamic multileaf collimator (DMLC) leaf sequence was segmented into multiple sections that corresponded to various respiratory phases per respiratory cycle and radiation beam-on timing. The segmented DMLC leaf sections were grouped according to respiratory phases and superimposed over the radiation fields of corresponding 4D CT image set. Dose calculation was then performed for each phase of the 4D CT scan. The total dose distribution was computed by accumulating the contribution of dose from each phase to every voxel in the region of interest. This was tracked by a deformable registration program throughout all of the respiratory phases of the 4D CT scan. A dose heterogeneity index, defined as the ratio between (D20-D80) and the prescription dose, was introduced to numerically illustrate the impact of respiratory motion on the dose distribution of treatment volume. A respiratory cycle range of 4-8 s and randomly distributed beam-on timing were assigned to simulate the patient respiratory motion during the free-breathing treatment. The results showed that the respiratory cycle period and radiation beam-on timing presented limited impact on the target dose coverage and slightly increased the target dose heterogeneity. This motion impact tended to increase the variation of target dose coverage and heterogeneity between treatment fractions with different radiation beam-on timing. The target dose coverage and heterogeneity were more susceptible to the radiation beam-on timing for patients with long respiratory cycle (longer than 6 s) and large breast motion amplitudes (larger than 0.7 cm). The same results could be found for respiratory cycle up to 8 s and respiratory motion amplitude up to 1 cm. The heart dose distribution did not change significantly regardless of respiratory cycle and radiation beam-on timing.

A dose verification method using a monitor unit matrix for dynamic IMRT on Varian linear accelerators

Dosimetry verification is an important step during intensity modulated radiotherapy treatment (IMRT). The verification is usually conducted with measurements and independent dose calculations. However, currently available independent dose calculation methods were developed for step-and-shoot beam delivery methods, and their uses for dynamic multi-leaf collimator (MLC) delivery methods are not efficient. In this study, a dose calculation method was developed to perform independent dose verifications for a dynamic MLC-based IMRT technique for Varian linear accelerators. This method extracts the machine delivery parameters from the dynamic MLC (DMLC) files generated by the IMRT treatment planning system. Based on the parameters a monitor unit (MU) matrix was separately calculated as two terms: direct exposure from the open MLC field and leakage contributions, where the leaf-end leakage contribution becomes more important in higher dose gradient regions. The MU matrix was used to compute the primary dose and the scattered dose with a modified Clarkson technique. The doses computed using the method were compared with both measurement and treatment planning for 14 and 25 plans respectively. An average of less than 2% agreement was observed and the standard deviation was about 1.9%.

Evaluation of the substitution of Ir-192 seed ribbons for wires in Paris system using dose nonuniformity ratio

The substitution of Ir-192 seed ribbons for wires in the Paris system of interstitial implants was re-evaluated using the dose nonuniformity ratio. The dose nonuniformity ratio, which is based on volumetric data, measures the dose nonuniformity of the implant quantitatively. The lower the dose nonuniformity ratio value, the smaller the dose nonuniformity, and the better is the dose homogeneity for the implant. Implants configured in a single-plane and double-planes in the form of squares or triangles using Ir-192 wires or seed ribbons were considered. The difference between the particular reference dose rate that yielded a minimum in the dose nonuniformity ratio curve for wire implants and seed implants is about 4 cGy/hr. The difference between the minimum dose nonuniformity ratio value representing the optimal dose homogeneity of the implants is about 5%. The dose homogeneity may be considered better for implants configured using seed ribbons.

Breast skin doses from brachytherapy using MammoSite® HDR, intensity modulated radiation therapy, and tangential fields techniques

Skin doses from brachytherapy using MammoSite® HDR, Intensity Modulated Radiation Therapy (IMRT), and conventional tangential fields techniques were compared. For each treatment technique, skin doses were measured using paired thermoluminescent dosimeters placed on the patients skin: (i) directly above the balloon catheter during MammoSite® HDR brachytherapy treatments and (ii) 4 cm inside the treatment borders during the IMRT and conventional breast treatments. The mean dose measured was about 58% of the prescription dose for the patients treated using the MammoSite® technique. On the other hand, for patients treated with IMRT and tangential fields, the mean dose was found to be about 69% and 71% of the corresponding prescription dose. This study suggests that in breast cancer radiation treatments the MammoSite® HDR technique reduces skin doses compared to IMRT and tangential field techniques. PACS numbers: 87.53.Jw, 87.53.Xd, 87.66.Sq

Various wedge isodose angles for treatment planning.

Various wedge isodose angles or simply wedge angles smaller than the nominal wedge angle were created by combining the isodose distributions generated from a single physical wedge with the isodose distributions of the open field for the 8-MV photon beam. The particular wedge angle generated depends on the weights imposed on these isodose distributions. The relationship between these weights and the wedge angle were examined and found to be nonlinear. The difference between the wedge angles defined at 10 cm depth and those defined using the 50% isodose curve is less 6 degrees. The present data was fitted using two proposed empirical equations.

Determination of Basal Dose Rates in Paris System of Interstitial Implants

The isodose distributions are related to the source configurations in the Paris system of interstitial implants. In this system, the basal dose rates are determined at those points identified relative to the implant. Once these points have been located, the basal dose rates are merely the summation of the dose rate contributions from all the sources. By applying the symmetry properties of the implant, the number of basal dose rate computations and the number of dose rate summations are reduced. The reduction in both the number of computations and the number of summations has facilitated the determination of the average basal dose rate. After the average basal dose rate has been determined, the reference dose rate that is used clinically is calculated. Examples of determining the basal dose rates for a few interstitial implants are presented.

High Energy Photon Beam Percent Depth Dose Curves

The percent depth dose of 8 MV and 18 MV photon beams were measured for various field sizes. The percent depth dose curves exhibited crossed over at selective depth in phantom. Hence, the percent depth dose increases at shallower depth, is independent at crossed over depth, and decreases at deeper depth with decreasing field size. This peculiar phenomenon, which cannot be explained using the primary and scattered radiation concept, should be noted.

Cyberknife stereotactic radiosurgery and radiation therapy treatment planning system

CyberKnife is an image-guided stereotactical dose delivery system designed for both focal irradiation and radiation therapy (SRT). Focal irradiation refers the use of many small beams to deliver highly focus dose to a small target region in a few fractions. The system consists of a 6-MV linac mounted to a robotic arm, coupled with a digital x-ray imaging system. The radiation dose is delivered using many beams oriented at a number of defined or nodal positions around the patients. The CyberKnife can be used for both intracranial and extracranial treaments unlike the Gamma Knife which is limited to intracranial cases. Multiplan (Accuray Inc., Sunnyvale, CA) is the treatment planning system developed to cooperate with this accurate and versatile SRS and SRT system, and exploit the full function of Cyberknife in high-precision radiosurgery and therapy. Optimized inverse treatment plan can be achieved by fine-tuning contours and planning parameters. Precision is the newest version of Cyberknife treatment planning system (TPS) and an upgrade to Multiplan. It offers several new features such as Monte Carlo for multileaf collimator (MLC) and retreatment for other modalities that added more support for the Cyberknife system. The Cybeknife TPS is an easy-to-use and versatile inverse planning platform, suitable for stereotactic radiosurgery and radiation therapy. The knowledge and experience of the planner in this TPS is essential to improve the quality of patient care.