Azam Niroomand-Rad

Radiochromic film dosimetry: Recommendations of AAPM Radiation Therapy Committee Task Group 55

Recommendations of the American Association of Physicists in Medicine (AAPM) for the radiochromic film dosimetry 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.

Radiation dose pertubation at tissue-titanium dental interfaces in head and neck cancer patients

Purpose : To determine the dose perturbation effects at the tissue-metal implant interfaces in head and neck cancer patients treated with 6 MV and 10 MV photon beams. Methods and Materials : Phantom measurements were performed to investigate the magnitude of dose perturbation to the tissue adjacent to the titanium alloy implants with (100 μ and 500 μ thick) and without hydroxylapatite (HA) coating. Radiographic and radiochromic films were placed at the upper (and lower) surface of circular metal discs (diameter x thickness : 15 x 3.2, 48 x 3.2, 48 x 3.8 mm 2 ) in a solid water phantom and were exposed perpendicular to radiation beams. The dosimeters were scanned with automatic film scanners. Using a thin-window parallel-plate ion chamber, dose perturbation were measured for a 48 x 3.2 mm 2 disc. Results : At the upper surface of the tissue-dental implant interface, the radiographic data indicate that for 15 x 3.2 mm 2 uncoated, as well as 100 μ coated discs, dose perturbation is about +22.5% and +20.0% using 6 MV and 10 MV photon beams, respectively. For 48 x 3.2 mm 2 discs, these values basically remain the same. However, for 48 x 3.8 mm 2 discs, these values increase slightly to about +23.0% and +20.5% for 6 MV and 10 MV beams, respectively. For 48 x 3.2 mm 2 discs with 500 μ coating, dose enhancement is slightly lower than that obtained for uncoated and 100 μ coated discs for each beam energy studied. At the lower interface for 15 x 3.2 mm 2 and 48 x 3.2 mm 2 uncoated and 100 μ coated discs, dose reduction is similar and is about -13.5% and -9.5% for 6 MV and 10 MV beams, respectively. For 48 x 3.8 mm 2 discs, dose reduction is about -14.5% and -10.0% for 6 MV and 10 MV beams, respectively. For 48 x 3.2 mm 2 discs with 500 μ coating, the dose reduction were slightly higher than those for uncoated and 100 μ coated discs. Conclusions : For the beam energies studied, dose enhancement is slightly larger for the lower energy beam. The results of dose perturbation were similar for 100 μ coated and uncoated discs. These results were slightly lower for the 500 μ coated discs but are not clinically significant. The dosimetry results obtained from radiochromic films were similar to the ones obtained from radiographic film. The dose enhancement results obtained from ion chamber dosimetry are higher than those obtained from film dosimetry. The ion chamber data represent the data at true tissue-titanium interface, whereas the ones obtained from film dosimetry represent the data at film-titanium interface.

Air cavity effects on the radiation dose to the larynx using Co-60, 6 MV, and 10 MV photon beams

PURPOSE To determine the perturbation effect in the surface layers of lesions located in the air-tumor tissue interface of larynx using 60Co, 6 MV, and 10 MV photon beams. METHODS AND MATERIALS Thermoluminescent dosimeters (TLDs), were embedded at 16 measurement locations in slab no. 8 of a humanoid phantom and exposed to two lateral-opposed beams using standard small 7 x 7 cm fields. Similarly, radiographic and radiochromic films were placed between slabs no. 7 and no. 8 of the humanoid phantom and exposed to two lateral-opposed radiation beams. The dosimeters were irradiated with 60Co, 6 MV, and 10 MV photon beams. Computer tomography (CT) treatment planning without inhomogeneity correction was performed. RESULTS At the tissue-air interface, the average measured percentage dose (% dosem) is about (108.7 +/- 4.8)% with TLD data, (96.8 +/- 2.5)% with radiographic film data, and (100.8 +/- 4.9)% with radiochromic film data. Similarly, in the central part of the cavity, the % dosem is (98.4 +/- 3.1)% with TLD data, (94.3 +/- 3.3)% with radiographic film data, and (91.7 +/- 5.0)% with radiochromic film data. Using the CT-based generated dose distribution (without inhomogeneity correction), the average calculated percentage dose (% dosec) is (98.7 +/- 1.0)% at the tissue-air interface and 98% in the central part of the air cavity. CONCLUSION For the beam energies studied, the variation from the % dosem at the tissue-air interface for a given dosimetry technique is relatively small [< 5% (TLD), < 3% (radiographic), and < 5% (radiochromic)] and therefore should not be significant in clinical settings. The variation from the % dosem at the tissue-air interface is more significant for lower energies [8% (60Co), 7.3% (6 MV)]. This variation is about 4.3% for 10 MV photon beam, therefore, while our institutional practice favors lower energy (60Co to 6 MV) for node-negative glottic cancers, physical/dosimetric evidence offers no disadvantage to the use of higher energy photons.

Measured dose to ovaries and testes from Hodgkin's fields and determination of genetically significant dose

PURPOSE To determine the genetically significant dose from therapeutic radiation exposure with Hodgkins fields by estimating the doses to ovaries and testes. MATERIALS AND METHODS Phantom measurements were performed to verify estimated doses to ovaries and testes from Hodgkins fields. Thermoluminescent LiF dosimeters (TLD-100) of 1 x 3 x 3 mm3 dimensions were embedded in phantoms and exposed to standard mantle and paraaortic fields using Co-60, 4 MV, 6 MV, and 10 MV photon beams. RESULTS Our results show that measured doses to ovaries and testes are about two to five times higher than the corresponding graphically estimated doses for Co-60 and 4 MVX photon beams as depicted in ICRP publication 44. In addition, the measured doses to ovaries and testes are about 30% to 65% lower for 10 MV photon beams than for their corresponding Co-60 photon beams. CONCLUSION The genetically significant dose from Hodgkins treatment (less than 0.01 mSv) adds about 4% to the genetically significant dose contribution to medical procedures and adds less than 1% to the genetically significant dose from all sources. Therefore, the consequence to society is considered to be very small. The consequences for the individual patient are, likewise, small.

Film dosimetry of small elongated electron beams for treatment planning

The characteristics of 5, 7, 10, 12, 15, and 18 Mev electron beams for small elongated fields of dimensions L X W (where L = 1, 2, 3, 4, 5, and 10 cm; and W = 1, 2, 3, 4, 5, and 10 cm) have been studied. Film dosimetry and parallel-plate ion chamber measurements have been used to obtain various dose parameters. Selective results of a series of systematic measurements for central axis depth dose data, uniformity index, field flatness, and relative output factors of small elongated electron beams are reported. The square-root method is employed to predict the beam data of small elongated electron fields from corresponding small square electron fields using film dosimetry. The single parameter area/perimeter radio A/P is used to characterize the relative output factors of elongated electron beams. It is our conclusion that for clinical treatment planning square-root method may be applied with caution in determining the beam characteristics of small elongated electron fields from film dosimetry. The calculated and estimated relative output factors from square-root method and A/P ratio are in good agreement and show agreement to within 1% with the measured film values.

Wedge factor dependence on depth and field size for various beam energies using symmetric and half‐collimated asymmetric jaw settings

The depth- and field-size dependence of the in-phantom wedge factor have been determined for a Cobalt-60 (Co-60) teletherapy unit and four medical linear accelerators with 4-, 6-, 10-, and 18-MV x-ray beams containing 15 degrees-60 degrees (nominal) lead, brass, and steel wedge filters. Measurements were made with ionization chambers in solid water or water with a source-skin distance of 80 or 100 cm. Field sizes varied from 4 x 4 cm up to a maximum allowable size for each wedge filter. Measurements were performed for symmetric and half-collimated asymmetric fields at depth of maximum dose, 5- and 10-cm depths. For half-collimated fields, wedge factor reference points were located at a fixed off-axis distance from the collimators rotational axis. These systematic measurements on wedges indicate that the wedge factor dependence on depth and field size is a function of beam energy as well as the design of the treatment head and wedge filters. Significance of the results reported herein are discussed for the most commonly used treatment depths and field sizes with various beam energies and wedge filters.

Effects of beam spoiler on radiation dose for head and neck irradiation with 10-MV photon beam

PURPOSE To determine the effects of a lucite beam spoiler on the dose distribution to points inside and outside the primary beam for head and neck irradiation with a 10-MV photon beam. METHODS AND MATERIALS Build-up and depth-dose measurements were performed with a parallel-plate ionization chamber for 5 x 5, 10 x 10, and 15 x 15-cm field sizes using lucite spoilers with two different thicknesses at two different lucite-to-skin distances (LSD) for a 10-MV x-ray beam. Corrections were applied to account for finite chamber size. Beam profiles and isodose curves were obtained at several depths using film dosimetry. Beam uniformity was determined from uniformity indices. Peripheral doses (PD) were measured at the surface and at 1.5- and 2.5-cm depths using film dosimetry and a parallel-plate ionization chamber. Measurement points were positioned at the edge of a 10 x 10-cm field and at distances extending to 5.0 cm away. The treatment planning data for the 10-MV x-ray beam were modified to account for the effects of the beam spoiler when treating head and neck patients. RESULTS The spoiler increased the surface and build-up dose and shifted the depth of maximum dose toward the surface. With a 10-MV x-ray beam and a 1.2-cm-thick lucite at 15 cm LSD, a build-up dose similar to a 6-MV x-ray beam was achieved. The beam uniformity was altered at shallow depths. The peripheral dose was enhanced particularly at the surface and at the points close to the beam edge. The effects of the beam spoiler on beam profile and PD were reduced with increasing depths. CONCLUSION The lucite spoiler allowed use of a 10-MV x-ray beam for head and neck treatment by yielding a build-up dose similar to that of a 6-MV x-ray beam while maintaining skin sparing. The increase in PD was at superficial depths and was reduced at points away from the edge; therefore, it is clinically nonsignificant. Spoiling the 10-MV x-ray beam resulted in treatment plans that maintained dose homogeneity without the consequence of increased skin reaction or treatment volume underdose for regions near the skin surface.

Physical aspects of total body irradiation of bone marrow transplant patients using 18 MV x rays.

The physical aspects of a total body irradiation (TBI) treatment are described. Patients seated in a special chair with their legs bent backwards are irradiated anteriorly and posteriorly (AP/PA). The chair reduces patient movement and facilitates positioning patients during 9 fractions of TBI over a 3-day period. The dose to lower extremities are monitored and raised to the total body dose. A conventional linear accelerator in a standard size treatment room is used to deliver 18 MV x-ray beams at a dose rate of approximately 20 cGy/min at a 350 cm treatment distance. Results of dose distribution, field flatness, dose uniformity, in vivo and in vitro dosimetry, and boost irradiation techniques are described.

Education and training of medical physics in Iran: The past, the present and the future

PURPOSE The aim of this study was to investigate the current status of education and training programs in medical physics in Iran. METHODS A questionnaire was designed and sent to 274 IAMP (Iranian Association of Medical Physicists) members focusing on these two topics: the educational situation (course syllabus, number of faculty members, number of PhD and MSc students and sub-fields offered in the department) and the professional situation (work experience, workplaces of medical physicists, postgraduate degrees that were granted and the amount of therapy and imaging equipment). RESULTS Medical physics education in Iran is provided at 14 universities at master and doctorate levels. All medical physics departments offer an MSc program and 6 of them offer a PhD program. Most medical physics faculty (24%) work in the radiotherapy physics sub-specialty. Also, about 95 medical physics students graduate every year. There are six major peer-reviewed Iranian journals that publish medical physics papers in English. In addition, there are 74 radiotherapy machines including Co-60 and LINACs (LINear ACcelerators) across Iran as of 2013. CONCLUSIONS The curriculum of medical physics programs (MSc and PhD) in Iran must be improved to include long-term clinical courses in the four major sub-specialties of radiotherapy, medical imaging, nuclear medicine and radiation protection. It is hoped that clinical medical physicists will go through nationally-accredited exams before assuming independent clinical responsibilities. Moreover, the work situation of the medical physics profession in Iran should be clear and the government authorities must recognize importance of this interdisciplinary field in medicine.

Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator

A set of standard wedge filters has been modified for use with half-collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half-collimated field coverage (up to 20 x 30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off-axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.