Anna Ralston

Small field diode correction factors derived using an air core fibre optic scintillation dosimeter and EBT2 film

There is no commercially available real-time dosimeter that can accurately measure output factors for field sizes down to 4 mm without the use of correction factors. Silicon diode detectors are commonly used but are not dosimetrically water equivalent, resulting in energy dependence and fluence perturbation. In contrast, plastic scintillators are nearly dosimetrically water equivalent. A fibre optic dosimeter (FOD) with a 0.8 mm(3) plastic scintillator coupled to an air core light guide was used to measure the output factors for Novalis/BrainLab stereotactic cones of diameter 4-30 mm and Novalis MLC fields of width 5-100 mm. The FOD data matched the output factors measured by a 0.125 cm(3) Semiflex ion chamber for the MLC fields above 30 mm and those measured with the EBT2 radiochromic film for the cones and MLC fields below 30 mm. Relative detector readings were obtained with four diode types (IBA SFD, EFD, PFD, PTW 60012) for the same fields. Empirical diode correction factors were determined by taking the ratio of FOD output factors to diode relative detector readings. The diodes were found to over-respond by 3%-16% for the smallest field. There was good agreement between different diodes of the same model number.

A prototype scintillation dosimeter customized for small and dynamic megavoltage radiation fields

A prototype plastic scintillation dosimeter has been developed with a small sensitive volume, rapid response and good dosimetric performance. The novelty of this design is the use of an air core light guide to transport the scintillation signal out of the primary radiation field. The significance of this innovation is that it eliminates the Cerenkov background signal that is generated in conventional optical fibres. The dosimeter performance was compared to existing commercial dosimeters in 6 MV and 18 MV photon beams and 6 MeV and 20 MeV electron beams, in both static and dynamic fields. The dosimeter was tested in small static fields and in dynamically delivered fields where the detector volume is shielded, while the stem is irradiated. The depth dose measurements for the photon beams agreed with ionization chamber measurements to within 1.6%, except in the build-up region due to positional uncertainty. For the 6 MeV and 20 MeV electron beams, the percentage depth dose measurements agreed with the ionization chamber measurements to within 3.6% and 4.5%, respectively. For field sizes of 1 cm x 1 cm and greater, the air core dosimeter readings agreed with diamond detector readings to within 1.2%. The air core dosimeter was accurate in dynamically delivered fields and had no measurable stem effect. The air core dosimeter was accurate over a range of field sizes, energies and dose rates, confirming that it is a sensitive and accurate dosimeter with high spatial resolution suitable for use in megavoltage photon and electron beams.

Clinical application of the OneDose™ Patient Dosimetry System for total body irradiation

The OneDose Patient Dosimetry System (Sicel Technologies) is a new dosimeter based on metal oxide semiconductor field-effect transistor technology and designed for the in vivo measurement of patient dose during radiotherapy. In vivo dosimetry for total body irradiation (TBI) is challenging due to the extended treatment distance, low dose rates and beam spoilers. Phantom results confirm the suitability of the dosimeter for TBI in terms of inherent build-up, post-irradiation fading, accuracy, reproducibility, linearity and temperature dependence. Directional dependence is significant and should be taken into account. The OneDose dosimeters were also trialed in vivo for two TBI patients and the dose measured compared to conventional dosimeter measurements using an ionization chamber and thermoluminescent dosimeters (TLD), with agreement to within 2.2% and 3.9%, respectively. Phantom and patient results confirm that the OneDose patient dosimetry system is a practical and convenient alternative to TLDs for TBI in vivo dosimetry. For increased confidence in results with this dosimeter, we recommend that two dosimeters be used for each site of interest.

Extracorporeal irradiation for malignant bone tumors

PURPOSE Extracorporeal irradiation (ECI) has been used selectively in the management of primary malignant bone tumors since 1996. We report our techniques for ECI and the short-term oncologic and orthopedic outcomes. METHODS AND MATERIALS Sixteen patients with primary malignant bone tumors were treated with ECI from 1996 to 2000. The median age was 14 years. The histologic diagnoses were Ewings sarcoma (11), osteosarcoma (4) and chondrosarcoma (1). The treated sites were femur (7), tibia (4), humerus (2), ilium (2), and sacrum (1). Following induction chemotherapy in Ewings sarcomas and osteosarcoma, en bloc resection of the tumor and tumor-bearing bone was performed. A single dose of 50 Gy was delivered to the bone extracorporeally using either a linear accelerator (9 cases) or a blood product irradiator (7 cases). The orthopedic outcome was recorded using a standard functional scale. RESULTS At a median follow-up of 19.5 months, there were no cases of local recurrence or graft failure. One patient required amputation due to chronic osteomyelitis. For the 10 patients with follow-up greater than 18 months, the functional outcomes were graded good to excellent. CONCLUSION The short-term oncologic and orthopedic results are encouraging and suggest that ECI provides a good alternative for reconstruction in limb conservative surgery in selected patients. This technique should only be used in a multidisciplinary setting, where careful follow-up is available to assess the long-term outcomes.

Over-response of synthetic microDiamond detectors in small radiation fields

The recently commercialized PTW microDiamond detector (T60019) has been designed for use in small radiation fields. Here we report on the measurement of relative output ratios for small fields using five microDiamond detectors. All of the microDiamond detectors over-responded in fields smaller than 20 mm, by up to 9.3% for a 4 mm field. The over-response was independent of accelerator type and choice of collimation. The over-response was slightly larger than that observed in silicon diodes. Since all five microDiamond detectors showed the same over-response the corrections presented here should be transferable to other examples of the microDiamond detector, provided that the detector meets the manufacturing specifications and the beam characteristics are comparable.

Thyroid dose in children undergoing prophylactic cranial irradiation

PURPOSE To determine the radiation dose received by the thyroid gland as a result of prophylactic cranial irradiation (PCI) in childhood leukemia and the factors influencing that dose. METHODS AND MATERIALS The dose to the thyroid resulting from simulated cranial irradiation with parallel opposed lateral fields of an adult anthropomorphic (ART) phantom with both 6 MV X-rays and Cobalt-60 gamma-rays was measured using thermoluminescent dosimeters (TLDs). The dependence of thyroid dose on the distance of the field from the thyroid and the proportions of thyroid dose from stray radiation (leakage, scatter from jaws, etc.) and tissue scattered radiation were measured. The effects of a shadow tray and shielding blocks were also determined. Calculation of thyroid dose using the Clarkson scatter integration method was performed for 6 MV X-rays to compare with the measured doses. In vivo thyroid dose estimates were made using TLD measurements for three children receiving PCI with 6 MV X-rays. RESULTS Using open, unshielded fields, the thyroid region of the phantom received 1.2-1.4% of the prescribed cranial dose for 6 MV X-rays and 1.5-1.7% for Cobalt-60. For both treatment units, stray radiation accounted for approximately two thirds of the thyroid dose and tissue scatter accounted for the remaining one third. The thyroid dose increased as the field moved closer to the thyroid, with an increasing proportion of the dose due to tissue scatter. Placement of a thyroid shielding block on a shadow tray reduced the thyroid dose by only 20% compared with the open, unshielded setup. Thyroid dose from 6 MV using open fields was affected by the orientation of the collimator. When the inferior field edge was defined by the lower jaw, the dose was reduced by 27% compared with the upper jaw. Good correlation of dose to the thyroid region was obtained between phantom measured doses, in vivo measured doses and calculation of dose using the Clarkson method. CONCLUSION For PCI doses of 1800 or 2400 cGy in the adult phantom, the dose to the thyroid was 20-40 cGy (1-2%). For small children this could rise to approximately 5% of the prescribed dose, of which half was due to stray radiation. As the thyroid in children is very sensitive to radiation and the dose-response curve for thyroid tumor induction is linear, attempts to shield the thyroid during cranial irradiation are mandatory. Cobalt-60 units should not be used, as the thyroid dose was higher than using 6 MV X-rays. Collimator orientation and the use of shadow trays and shielding were important factors in determining thyroid dose.

Small field in-air output factors: The role of miniphantom design and dosimeter type

PURPOSE The commissioning of treatment planning systems and beam modeling requires measured input parameters. The measurement of relative output in-air, Sc is particularly difficult for small fields. The purpose of this study was to investigate the influence of miniphantom design and detector selection on measured Sc values for small fields and to validate the measurements against Monte Carlo simulations. METHODS Measurements were performed using brass caps (with sidewalls) or tops (no sidewalls) of varying heights and widths. The performance of two unshielded diodes (60012 and SFD), EBT2 radiochromic film, and a fiber optic dosimeter (FOD) were compared for fields defined by MLCs (5-100 mm) and SRS cones (4-30 mm) on a Varian Novalis linear accelerator. Monte Carlo simulations were performed to theoretically predict Sc as measured by the FOD. RESULTS For all detectors, Sc agreed to within 1% for fields larger than 10 mm and to within 2.3% for smaller fields. Monte Carlo simulation matched the FOD measurements for all size of cone defined fields to within 0.5%. CONCLUSIONS Miniphantom design is the most important variable for reproducible and accurate measurements of the in-air output ratio, S(c), in small photon fields (less than 30 mm). Sidewalls are not required for fields ≤ 30 mm and tops are therefore preferred over the larger caps. Unlike output measurements in water, S(cp), the selection of detector type for Sc is not critical, provided the active dosimeter volume is small relative to the field size.

Effect of scatter material on diode detector performance for in vivo dosimetry

The accuracy of Scanditronix EDE, EDP10 and EDP20 diodes for entrance dosimetry of complex fields has been assessed using static and dynamic multileaf collimator test fields on phantoms. Specifically, surrounding scatter material size and composition have been investigated. The EDP10 and EDP20 diodes incorporate steel build-up caps. The effect of varying the diameter of the wax scatter discs on the dosimeter response showed a systematic detector under-response for the smaller discs with errors up to 4.1% relative to full scatter conditions. In static fields, all diodes over-respond at a field size of 1 x 1 cm(2). Diodes with non-water-equivalent build-up material exhibit over-response of up to 10.8%. In dynamic fields, diodes over-respond when there is an increased contribution from phantom scatter and under-respond in shielded regions due to low dose rate and beam hardening. For high dose regions, all diodes over-respond with the greatest over-response of 3.8% observed with a 6 mm sliding window field. The EDE diode with a 6 cm scatter disc correlated best with the reference dosimeter. A diode design with minimal non-water-equivalent components and the addition of a 6 cm diameter water-equivalent disc for scatter material is recommended for the in vivo dosimetry of 6 MV complex fields.

Extracorporeal irradiation — novel use of a blood product irradiator

A CS-137 blood product irradiator (BPI) is used for extracorporeal irradiation of bone grafts. For dose verification purposes two bone phantoms were constructed of plaster of Paris and irradiated in the BPI. The first was a hollow cylinder measuring 15 cm /sX 3.3 cm to simulate cortical bone, filled with paraffin wax to simulate yellow bone marrow. The second was a concave ellipse 11.5 cm /sX 9.5 cm /sX 2.5 cm to simulate a section of ilium. The absorbed dose was measured with radiochromic film in the phantoms and in water for comparison with the manufacturer’s calibration certificate. The relative dose distribution in the bone phantoms was measured using Li-F thermoluminescent dosimeters and normalized to a reference point near the centre of each phantom. The doses measured in water matched the calibration certificate within 4%. The doses measured at the cylindrical and elliptical phantom reference points were 49.9 Gy and 52.3 Gy respectively, compared to the nominal dose of 50.0 Gy. The relative doses within the cylindrical phantom ranged from 88% to 100% along the central axis of the wax cylinder, from 89% to 100% along the plaster/wax interface, and from 90% to 103% along the outer surface of the plaster cylinder. The relative doses within the elliptical phantom ranged from 100% to 108% along the inside surface, from 99% to 107% along the outer surface, and from 98% to 103% through the centre of the plaster ellipse. These data agree well with the isodose plot provided by the manufacturer.

On the measurement of dose in-air for small radiation fields: choice of mini-phantom material.

The purpose of this work was to determine the effect of choice of mini-phantom material on the measurement and calculation of in-air output factors (Sc) in small fields. Monte Carlo simulations in conjunction with a theoretical determination of Sc were used to validate previously reported measurements. Options for alternative mini-phantom materials were compared. A 6 MV beam from a Varian Novalis linear accelerator operating in stereotactic (SRS) mode was modelled. Phase-space data were used to determine the theoretical value of Sc. To validate previously reported Sc measurements the data were used to model the fibre-optic detector and brass mini-phantom. The impact of mini-phantom material was investigated by comparing the energy spectra of electrons entering the detector volume as a function of field size, and comparing the simulated Sc-measurement to the theoretical calculation. In order to determine factors leading to changes in Sc with field size, the origins of particles in the beam as incident on the mini-phantom were determined. Sc values derived from simulated measurements using a brass mini-phantom on a fibre-optic detector agreed with the measured Sc to within 0.7%. For simulation of measurement for all other mini-phantom materials, Sc values agreed with the theoretically calculated values to within 0.6%. The dominant processes responsible for a decrease in Sc with field size is occlusion of the focal and primary collimator contributions, while the secondary scatter, from the flattening filter and cone collimators, has minimal effect. The secondary electron spectrum is affected by the choice of mini-phantom material, but is almost independent of field size. For cone-collimated small fields in the Novalis beam (<30 mm), the decrease in Sc with field size is primarily due to collimation of the focal radiation beam and scatter from the primary collimator. A fibre optic detector with either a brass, gold or lead mini-phantom with at least d(max) equivalent height is suited to measure Sc for small SRS fields. The use of materials with higher electron/physical density can be used to reduce the size of the mini-phantom and reduce spatial averaging.