Evaluation of the dose perturbation caused by tungsten shielding in a Fletcher Suit Delclos gynecological applicator during an HDR brachytherapy treatment
Stanley, Thomas R.
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Purpose. To study the effect of tungsten shields present in ovoids on the dose delivered to areas representative of bladder and rectum points, during High Dose Rate (HDR) brachytherapy procedures for cervical cancer. Methods and Materials. A rigid device was constructed to measure the dosimetric aspects of a shielded Fletcher-Suit-Delclos gynecological applicator used in HDR brachytherapy treatments for cervical cancer. Multiple sheets of gafchromic film were positioned posterior to the ovoids and anterior to the central tandem at 6.20 mm intervals for dosimetric analysis in areas representative of the anatomical locations for bladder and rectum. The device, containing the gafchromic films and the shielded Fletcher-Suit Delclos applicator, was then submerged within the confines of a water tank. A treatment plan delivered 650 cGy to a pseudo point A using the phalange as the origin. Dosimetric analysis was then carried out using RIT(TM) software and a measured H&D curve for film calibration. Using a three-dimensional interpolation algorithm in MATLAB, the measured data provided two 9x8x5cm matrices (posterior to the ovoids and anterior to the central tandem, respectively) in which the dose at any point is determinable. Similarly, Nucletron's PLATO(TM) treatment planning system provides users with dosimetric data for any three-dimensionally defined point of interest. A comparison of the measured and computed dose values provided qualitative and quantitative effects of the tungsten shields in the tandem and ovoid applicator. Besides, bladder and rectal points (defined by a physician) for ten different patients were evaluated. The location of each point was then "corrected" to a pseudo origin (the intersection of the central tandem with the ovoids) for correlation with the experimental treatment plan. All points were defined anterior-posterior and superior-inferior using their "corrected" values, as the lateral positions were purposefully varied ± 40 mm from the central axis. Treatment plans accounting for primary attenuation through the tungsten shields were also generated with the rectal and bladder doses computed using the same method. Results. Inherent noise in the system was measured to be roughly 15 cGy, so ten times that value (150 cGy) was set as the threshold dose below which all data was deemed incomparable. Utilizing this limiting factor, measured doses (determined using the gafchromic film) were compared to a treatment plan that accounted for the shields as well as to another plan which ignored any shielding in the ovoids. In the case of bladder points, the computed doses with shielding were 3% less than the computed doses without shielding. The measured doses, however, were up to 28% lower than the computed doses not accounting for shielding. Respective values for rectal points were 15% and 25% implying that the difference between the measured doses and the computed doses without shielding are far more substantial than the planning system's shielding algorithm predicts. It may appear that this algorithm accounts better for rectal than bladder points, but in reality it is a function of the solid angle subtended by the shields in relation to the active source positions in the ovoids. Conclusion. CT/MRI compatible tandem and ovoid applicators do not provide bladder and rectal shielding in ovoids but are becoming very popular in routine clinical practice. Unfortunately, the clinically accepted rectum and bladder tolerance doses have been determined to a large extent using shielded applicators but ignoring their effects. Thus the rectal and bladder tolerance doses determined using a shielded tandem and ovoid applicator needs correction before they can be used in the treatment planning of CT/MR compatible applicators. Present study has given a quantitative estimate of the dose perturbation due to tungsten shields in a tandem and ovoid applicator.