Jerome A. Meli

Code of practice for brachytherapy physics: Report of the AAPM Radiation Therapy Committee Task Group No. 56

Recommendations of the American Association of Physicists in Medicine (AAPM) for the practice of brachytherapy physics are presented. These guidelines were prepared by a task group of the AAPM Radiation Therapy Committee and have been reviewed and approved by the AAPM Science Council.

A comparison of solid phantoms with water for dosimetry of 125I brachytherapy sources

Dosimetry of brachytherapy sources is critically dependent on precise measurement of the source-detector distance. A solid phantom can be precisely machined and hence distances can be accurately determined. In this work LiF thermoluminescent chips are used for absolute dose rate measurements in solid water, polymethylmethacrylate (PMMA), and polystyrene. These media are examined for their suitability in the dosimetry of 125I by comparing depth doses in each phantom. Measurements and Monte Carlo calculations show that solid water is equivalent to water for the dosimetry of 125I seeds, however, polystyrene and PMMA are not equivalent to water. Also, photon energy spectra for several depths in each phantom material have been calculated and are used to determine average photon energy and mass energy absorption coefficients as a function of depth.

Dosimetry on transverse axes of 125I and 192Ir interstitial brachytherapy sources

Dose rates along the transverse axes of 125I model 6702, 125I model 6711 and 192Ir 0.2-mm steel sources for interstitial brachytherapy have been measured in a solid-water phantom for distances up to 10 cm using LiF thermoluminescent dosimeters (TLDs). Specific dose rate constants, the dose rates in water per unit source strength 1 cm along the perpendicular bisector of the source, are determined to be 0.90 +/- 0.03, 0.85 +/- 0.03, and 1.09 +/- 0.03 cGy h-1 U-1 for 125I model 6702, 125I model 6711 and 192Ir 0.2-mm steel sources, respectively (1 U = unit of air kerma strength = 1 microGy m2 h-1 = 1 cGy cm2 h-1). In older and obsolete units of source strength (i.e., mCi apparent), these are 1.14 +/- 0.03, 1.08 +/- 0.03, and 4.59 +/- 0.15 cGy h-1 mCi-1 (apparent). Currently accepted values of specific dose rate constant for 125I sources are up to 20% higher than our measured values which are in good agreement with the results of our Monte Carlo simulations. But for 192Ir there is good agreement between our measured value of the specific dose rate constant and currently accepted values. The radial dose function for 125I model 6702 is found to be consistently larger than that for 125I model 6711, with an increasing difference as the distance from the source increases. Our measured values for the radial dose function for 125I sources are in good agreement with the results of our Monte Carlo simulation as well as the measured values of Schell et al. [Int. J. Radiat. Oncol. Biol. Phys. 13, 795-799 (1987)] for model 6702 and Ling et al.(ABSTRACT TRUNCATED AT 250 WORDS)

Determining Pion, the correction factor for recombination losses in an ionization chamber.

The 1983 AAPM protocol for the determination of absorbed dose from high-energy photon and electron beams recommends using Pion (the reciprocal of collection efficiency), as determined by the two-voltage technique, to correct for recombination losses in ionization chambers. Methods and data for the determination of ionization chamber collection efficiencies are scattered throughout the literature. The present work consolidates the available information, rectifies certain omissions, and provides several convenient and readily implemented methods for determining Pion. Computer programs, quadratic approximations, and data tables are presented to facilitate the determination of Pion for continuous, pulsed, and pulsed-swept beams.

Influence of the variation of energy spectra with depth in the dosimetry of 192Ir using LiF TLD

Absolute dose measurements around low activity seeds used in brachytherapy are difficult to perform with ionisation chambers. The physical size of the chamber does not allow good resolution close to the seeds and the ionisation current generated is very low. Small thermoluminescent dosimeters (TLDs) overcome these problems but the energy dependence of their response should be considered. In this work, dose in polystyrene was measured at several distances from the high activity 192Ir source (370 GBq) of a remote afterloading device using an ionisation chamber and LiF TLD chips. These data show that over a range of 1-10 cm from the source the sensitivity of LiF varies by up to 8.5%. This is attributed to the higher response of LiF to the lower photon energies, and to the shift of the photon spectrum to lower energies with increasing depth. The sensitivity of LiF to 192Ir was also calculated by weighting the energy-dependent response of LiF by the Monte Carlo calculated photon spectra. The calculations give a similar change in sensitivity with distance from the source.

Dosimetry on transverse axes of sup 125 I and sup 192 Ir interstitial brachytherapy sources

Dose rates along the transverse axes of 125I model 6702, 125I model 6711 and 192Ir 0.2-mm steel sources for interstitial brachytherapy have been measured in a solid-water phantom for distances up to 10 cm using LiF thermoluminescent dosimeters (TLDs). Specific dose rate constants, the dose rates in water per unit source strength 1 cm along the perpendicular bisector of the source, are determined to be 0.90 +/- 0.03, 0.85 +/- 0.03, and 1.09 +/- 0.03 cGy h-1 U-1 for 125I model 6702, 125I model 6711 and 192Ir 0.2-mm steel sources, respectively (1 U = unit of air kerma strength = 1 microGy m2 h-1 = 1 cGy cm2 h-1). In older and obsolete units of source strength (i.e., mCi apparent), these are 1.14 +/- 0.03, 1.08 +/- 0.03, and 4.59 +/- 0.15 cGy h-1 mCi-1 (apparent). Currently accepted values of specific dose rate constant for 125I sources are up to 20% higher than our measured values which are in good agreement with the results of our Monte Carlo simulations. But for 192Ir there is good agreement between our measured value of the specific dose rate constant and currently accepted values. The radial dose function for 125I model 6702 is found to be consistently larger than that for 125I model 6711, with an increasing difference as the distance from the source increases. Our measured values for the radial dose function for 125I sources are in good agreement with the results of our Monte Carlo simulation as well as the measured values of Schell et al. [Int. J. Radiat. Oncol. Biol. Phys. 13, 795-799 (1987)] for model 6702 and Ling et al.(ABSTRACT TRUNCATED AT 250 WORDS)

Interseed effects on dose for 125I brachytherapy implants

Dose calculations in multiseed brachytherapy implants are done by adding the contribution of each individual seed and by assuming that radiation from each seed is unaffected by the presence of the other seeds. To test the validity of this assumption, dose measurements with various configurations of multiseed implants of 125I model 6702 and 125I model 6711 sources were performed. For a linear configuration of three 125I model 6702 seeds at 1-cm separation, with their transverse axes coincident, doses at distances of 3.05 and 5.09 cm from the center along the transverse axis were found to be about 8% lower than the sum of doses from the three individual seeds. However, for three seeds at 1-cm intervals with their longitudinal axes coincident, doses at 3.05 and 5.09 cm distances from the center along the longitudinal axis were found to be about equal to the dose sums from individual seeds. These initial experiments indicated that the magnitude of the interseed effect depends upon the orientation of the seed relative to each other in an implant. To evaluate the importance of this interseed effect for multiseed configurations of 125I model 6702 and 125I model 6711 seeds, dose rates at various distances from a two-plane implant (each plane containing a 3 x 3 array of sources in a 1-cm spacing square grid) were measured in a Solid Water phantom with LiF TLDs. These measurements were carried out in two different planes at different orientations relative to the implant. The average values of the interseed effect at distances ranging from 1 to 7 cm outside the implant were observed to be about the same for 125I model 6702 and model 6711 sources. The mean value of the interseed effect was 6% and the maximum was 12%. On the whole, the interseed effect reduces the dose at the periphery of the iodine implant by 6%.

The effect of lead, gold, and silver backings on dose near 125I seeds

Brachytherapy for ocular melanoma uses 125I seeds backed by a gold shield. Conflicting results are reported in the literature on the effect of the gold on dose close to the seeds. In this work, a small lucite jig was constructed such that the seed-to-detector separation remained fixed as high-Z materials of lead, silver, and gold were moved in and out of position behind the seed. The jig was clamped in place in the water filled tank of a beam scanning system. The response of two p-type silicon diodes was measured at several distances from the seed with and without the high-Z backings. The response with the high-Z backing relative to water, found to be the same for each diode and the same for lead and gold, decreased from about 1.01 at 1.5 mm to about 0.92 at 20 mm. It has been suggested in the literature that L-shell fluorescent x rays of approximately 10 keV from the gold backing might contribute significantly to the dose within 7 mm of the seed. To test this, the response with the gold backing relative to water was measured with an aluminum cap of 1-mm wall thickness covering the diode. The cap transmits about 70% of the 125I influence but is essentially infinitely thick to 10-keV photons. The relative response (gold/water) was the same with and without the cap showing that the contribution of 10-keV x rays is negligible. Compared to water, the silver backing was found to enhance the diode response by about 14% between 5 to 10 mm from the seed.

The effect on dose of kilovoltage x-rays backscattered from lead

Dose enhancement on the backscatter side of a soft tissue/high Z material interface is known to exist for megavoltage x-ray beams. Caused by an increase in backscattered electron fluence, the enhancement persists for short distances upstream of the interface, equal to the range of these electrons. Since photon interaction cross sections are small, there is little photon backscatter at these energies. Consequently, beyond the range of the backscattered electrons, the dose upstream is unaffected by the presence of the interface. A similar dose enhancement has been reported for kilovoltage beams. In this case, due to the very low energy of the backscattered electrons, the enhancement persists for a very short upstream distance. Since photon interaction cross sections at keV energies are relatively large, there is also a substantial backscattered photon fluence. This experimental work investigates the effect of these photons on dose at distances upstream from a water/lead interface beyond the range of the backscattered electrons. Measurements of ionization charge, as a function of interface distance and field size for 60, 100, and 250 kV beams, were made with a parallel plate chamber at a fixed depth. A significant underdose was found upstream of the interface compared to a homogeneous water medium. For example, with the 100 kV beam and a 15 x 15 cm2 field the measured underdose is 23% at 3 mm and 14% at 1.5 cm upstream of the interface. The effect decreases with field size. In fact, for a 2 x 2 cm2 field the upstream dose in unaffected by the interface. Detailed results for this and the other two beams are presented along with backscatter factor measurements for lead. An explanation for the observed underdose is also presented.

Output factors and dose calculations for blocked x-ray fields

Output factors for blocked fields have been measured in a polystyrene phantom for four collimator field sizes and two blocking schemes using 6-MV x rays. For all measurements the phantom surface was at the calibration source-surface distance (SSD) because, as is shown, the calculation of dose to any point in a phantom at an arbitrary SSD can be expressed in terms of the output factor for the field size at the calibration distance. It is found that output factors are a function of both the surface field size of the blocked field and the collimator field size. Specifically, the output factor for a blocked field is less than that for the collimator field size used but greater than that for an unblocked field of the same surface field size formed by collimator settings only. A method is proposed for utilizing these data to calculate the output factor for any collimator and blocked field size. The validity of the method is checked by using it to calculate dose to a point in a phantom and comparing this to the measured dose.