BackgroundThe radiation field on most megavoltage radiation therapy units are shown by a light field projected through the collimator by a light source mounted inside the collimator. The light field is traditionally used for patient alignment. Hence it is imperative that the light field is congruent with the radiation field.MethodA simple quality assurance tool has been designed for rapid and simple test of the light field and radiation field using electronic portal images device (EPID) or computed radiography (CR). We tested this QA tool using Varian PortalVision and Elekta iViewGT EPID systems and Kodak CR system.ResultsBoth the single and double exposure techniques were evaluated, with double exposure technique providing a better visualization of the light-radiation field markers. The light and radiation congruency could be detected within 1 mm. This will satisfy the American Association of Physicists in Medicine task group report number 142 recommendation of 2 mm tolerance.ConclusionThe QA tool can be used with either an EPID or CR to provide a simple and rapid method to verify light and radiation field congruence.
We conclude that the 54Gy/60Gy dosed patients have almost the same tumor response as the standard 60 Gy treatment. A dosimetrically similar supine plan obviates the need for a prone delivery.
Purpose: An Arteriovenous malformation (AVM) is an abnormal connection between an artery and a vein bypassing a capillary. AVMs if left untreated can bleed and cause other neurological complications including death. Stereotactic radiosurgery is used as an alternative to conventional surgery to obliterate the AVM nidus (the central core of the AVM). SRS is generally restricted to small volume (less than 5cc) AVMs. Typically single fraction SRS doses of 1600–2000cGy are delivered to the contoured volume of the AVM while ensuring sharp gradients at the interface of AVM and normal brain. We discuss a novel strategy of dosing large volume AVMs (5–20cc) using a differential dose scheme that delivers 1600cGy at the rim and 2000cGy at the nidus of the AVM. Methods: Accuray CyberKnife (G‐3), Multiplan V3.5 treatment planning system. Patient CT, MRI and other imaging studies are sent to the Multiplan planning system. The neurosurgeons and the radiation oncologist at our site contour the AVM and the relevant anatomy. The contoured AVM volume is contracted by 2.5mm, and a rim is created by subtracting the previously contracted structure from the AVM. A treatment plan is generated using iterative planning,using custom planning structures. The treatment plan delivers 2000cGy to the core of the AVM, and 1600cGy at the rim, while keeping the dose gradient at the AVM normal brain interface high, enabling us to spare surrounding brain tissue. Results: Post treatment follow‐up imaging done at 3, 6 and 12 months shows a gradual diminishing and obliteration of the nidus. Conclusion: We conclude that this scheme of differential dosing enables us to treat larger volume AVMs effectively and safely.
Purpose:
To illustrate 25% reduction in CyberKnife prostate SBRT treatment times using a water filled rectal balloon.
Methods:
We perform prostate SBRT using a 3800cGy in 4 fraction regimen prescribed between 51% 59% iso‐dose lines to 95% of PTV using a CyberKnife System. The resultant heterogeneous dosimetry is analogous to HDR dosimetry. Our patients are treated in a feet first supine position to decrease treatment couch sag and also to position the prostate anatomy closer to the robot. CT imaging is performed with a Radiadyne Immobiloc rectal balloon filled with 45‐50cc water placed firmly inside the patient's rectum. A treatment plan is developed from this CT study using Multiplan. The patient is treated every other day for 4 days using the rectal balloon for each fraction. Gold fiducials previously implanted inside the prostate are used for tracking by the CyberKnife system.
Results:
Critical structures comprise the usual GU anatomy of bladder, rectum, urethra, femoral‐heads along with emphasis on doses to anterior rectal wall and rectal mucosa. The water filled rectal balloon localizes the rectum, which enables the physician to accurately contour both anterior rectal wall, and rectal mucosa. The balloon also has a gas release valve enabling better patient comfort. Rectum localization enables the CyberKnife system to make fewer corrections resulting in fewer treatment interruptions and time lost to re‐adjustment for rectal motion, bowel filling and gas creation. Effective treatment times are reduced by 25% to approximately 45 minutes. Adoption of the balloon has required minimal change to our planning strategy and plan evaluation process.
Conclusion:
Patient follow‐up comparisons show no difference in effectiveness of treatment with and without balloons We conclude that rectal balloons enhance patient comfort and decrease effective treatment times.
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