Purpose: Comparing the energy deposition characteristics of beta‐emitter P‐32 and photon emitter Pd‐103 to evaluate potential improvement in prostate brachytherapy dosimetry.Materials /Methods: Energy deposited by an activity A after total decay is calculated by 1.44T1/2AE, where E is the energy emitted by a single disintegration. The total energy deposited in the target will be 1.44T1/2AE F, where F is the absorbed fraction (AF). Absorbed fraction is defined as the fraction of emitted energy absorbed in the target. It increases monotonously with target volume. Monte Carlo calculated AF data for uniformly distributed spherical targets, 1</sup> for P‐32 and clinically obtained AF, 2</sup> from Pd‐103 permanent prostate brachytherapy are used in this study. Results: A unity activity (1 mCi) of P‐32 will deliver 7.38 joules, while the same activity of Pd‐103 will deliver 0.26 joules. The total energy deposited in a volume of 20 cc will be 6.90 joules for P‐32 and 0.11 joules for Pd‐103. As the volume of the target increases to 60 cc, the total energy deposited for P‐32 will be 7.06 joules, and 0.14 joules for Pd‐103 respectively. Conclusion: From the preliminary comparison, P‐32 is about 50 to 60 times more efficient in utilizing energy than Pd‐103. This high efficiency will reduce total activity needed to achieve desired dosage in the target. Also, the fact that most energy is absorbed locally in the target suggests that the dose to healthy tissue outside the target can be significantly reduced. Further study is needed in order to fully realize these potential dosimetric advantages.
Purpose: To establish correlation between radiation post‐survey and planning dosimetric parameters in permanent prostate brachytherapy. Combined with patient separation, it is possible to calculate the dose rate at patient surface and exposure rate 1 meter away from the patient. Materials /Methods : The transrectal ultrasonography (TRUS) images are obtained using the B&K unit (Analogic Corporation) in 5 mm steps. Planning goals are: for prostate V 100 95%, D 90 100%, and for prostatic urethra D 10 150%. Dose calculations are performed using VariSeed (Varian Medical System) planning system. The dose rate is obtained by dividing the absolute dose by 1.44T 1/2 , where T 1/2 is the half ‐life of implanted isotope. The center of the prostate is determined by locating a point on the D column in the ultrasound template at the midline and the mid‐section of the prostate. The distance from the center of the prostate to patient surface is about half of the patient separation. The dose rate is also calculated using AAPM TG‐43 point source model, g(r)/r 2 , using the same dosimetric parameters for planning. Comparisons are made between point source and VariSeed (discrete multiple sources) results. Results: For distances larger than 8.0 cm from the center of the prostate, the dose rate by discrete multiple sources can be accurately approximated by AAPM TG‐43 point source model calculation within 5.0% for I‐125 and Pd‐103, for Cs‐131 within 10.0%. In addition, depending on patient separation, dose rate for Cs‐131 can be more than 30 times higher than that for Pd‐103, 10 times higher than that for I‐125. Conclusion: Planning dosimetric parameters along with patient separation can be used to calculate the post‐survey results at the patient surface using AAPM TG‐43 point source model. The exposure rate 1 meter away from the patient can be obtained by applying the inverse square factor.
Purpose: Compare the integral target dose to further understand their clinical suitability. Materials /Methods: Dose rate for MammoSite and Axxent S700 source due to an isotropic point source can be calculated by: . The dose is the products of D(r) and Δt, where Δt is the dwell time. Δt is obtained from PLATO planning system using 40820 U (10 Ci) source strength. Assuming Axxent S700 produces comparable air kerma strength with that of a 10 Ci Ir‐192 source, the same treatment time is used. Target for both systems is a 1.0 cm thickness spherical shell from the surface of the balloon; the integral target dose is obtained by integrating the point source model over this spherical shell. Dosimteric parameters for MammoSite are obtained from AAPM TG43. For Axxent S700, data are obtained from published data 1 . Radial dose function for Axxent S700 (50 keV) is curve‐fitted with a polynomial to facilitate calculation. Results: For a treatment of 340 cGy, MammoSite delivers 3.80 joules for balloon radius of 2.0 cm, increases to 6.66 joules for balloon radius of 3.0 cm, while Axxent S700 delivers 0.98 joules and 1.32 joules respectively. MammoSite delivers 3.9 times more than Axxent S700 at balloon radius of 2.0 cm. This number increases to 5.0 for balloon radius of 3.0 cm. Conclusion: The integral target dose increases with balloon size for both systems. So is the integral target dose difference between the two. The difference in energy deposition is attributed to the differences in physical characteristics, such as, dose rate constant and radial dose function. Non‐target integral dose evaluations for both systems are underway. Further, equivalent uniform dose (EUD) for both systems can be calculated, when air cavities are present.
Purpose: To explore the feasibility of measuring and analyzing beam profiles for diagnostic x-ray beams with a 2D ion chamber array. Method and Materials: Diagnostic x-ray beam profiles were measured using PTW seven29 ionization chamber array at 100 SSD with 0.5 cm buildup. Acuity simulator (Varian Medical Systems Inc.) was used for generating diagnostic x-rays. Fluoroscopy mode was used throughout the study. Blades were used to shape the fields while the wires were set completely out of those filed. For 115 kVp beam energy, field size 10×10 was used and for 75 kVp beam energy, field size 20×20 was used. For 10×10 field size, the operating parameters were: 80 mA, 15 ms. Approximately 60 seconds measurement time was used to improve signal-to-noise ratio and for 20×20 field size: 50 mA, 5 ms with 30 seconds measurement time. Beam flatness and symmetry were analyzed using MultiCheck and VeriSoft software (PTW Inc.) Results: The flatness (Percentage Dose Difference) for 10×10 field is 6.0%, and for 20×20 field is 4.0%. The symmetry (Maximum Dose Ratio) for 10×10 is 1.05, and for 20×20 is 1.03. Conclusion: Diagnostic x-ray beam profiles were measured with PTW seven29 2D ion chamber array. The device came with a calibration certificate for 60Co. and was designed for dose measurements in radiation therapy. It could be used in relative dose mode to provide a straightforward way for verifying beam flatness and symmetry for diagnostic x-ray beams. The data obtained could be added to form the basis for a periodic QA program for such units. Further study is needed to investigate the relative advantages and limitations of this method.
Purpose : To explore the feasibility of measuring virtual source‐to‐skin distance and mean‐square angular spread for broad electron beams with a 2D ion chamber array. Method and Materials : The virtual SSD (S vir ) and mean‐square angular spread (θ vir 2 ) for 6, 9, 12, 16, and 20 MeV electron beams from a Clinac 21EX (Varian Medical Systems) Linac were measured using a 2D ion chamber array PTW seven29 (PTW Inc.). Cone size 20 by 20 was used throughout the measurement to ensure side‐scatter equilibrium. The virtual SSD was measured using FWHM method (based on measurements of variation of field size with the nominal source‐to‐detector distance). The PTW seven29 was exposed perpendicularly to the beam axis at nominal SSD of 100, 105, 110,115, 120, and 125 cm. Using MultiCheck software (PTW Inc.), the FWHM and the penumbra of each beam profiles were obtained. A back projection of FWHM with nominal SSD giving straight line, the virtual SSD was deduced from the slope and the intercept of this line. At the same time, back projection of penumbra with air gap giving a straight line, the mean‐square angular spread was calculated from the slope of this line along with published parameter. Results : The measured virtual SSD and mean‐square angular spread for broad electron beams were listed below:(see PDF for table). Conclusion : Both the virtual SSD and mean‐square angular spread for broad electron beams were measured with PTW seven29 2D‐Array. It provided the necessary parameters to perform clinical broad beam electron dosimetry, especially in a filmless department.
To explore the possibility of using Compton scattering method to characterize the energy spectra of clinical beams. This method is experimentally tested on a Co/sup 60/ radiation therapy machine and independently verified using Monte Carlo (MCNP4B) simulation. Measured spectrum and MCNP simulated spectrum including detector response for 90/spl deg/ scattered beam show good agreement.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)