Impact of magnetic fields on dose measurement with small ion chambers illustrated in high-resolution response maps

PURPOSE

Dosimetry of ionizing radiation in the presence of strong magnetic fields is gaining increased relevance in light of advances for MRI-guided radiation therapy. While the impact of strong magnetic fields on the overall response of ionization chambers has been simulated and measured before, this work investigates the local impact of the magnetic field on dose response in an ion chamber. High-resolution 1D and 2D response maps have been created for two small clinical thimble ionization chambers, the PinPoint chambers 31006 and 31014 (Physikalisch Technische Werkstaetten Freiburg, Germany).

METHODS

Working on the Imaging and Medical Beam Line of the Australian Synchrotron an intense kilovoltage radiation beam with very low divergence, collimated to 0.1 mm was used to scan the chambers by moving them on a 2D motion platform. Measured current and beam position were correlated to create the response maps. Small neodymium magnets were used to create a field of about 0.25 T. Chamber axis, magnetic field, and beam direction were perpendicular to each other. Measurements were performed with both orientations of the magnetic field as well as without it. Chamber biases of 5 and 250 V in both polarities were used.

RESULTS

The local distribution of the response of small thimble-type ionization chambers was found to be impacted by a magnetic field. Depending on the orientation of the magnetic field, the chamber response near the stem was either enhanced or reduced with the response near the tip behaving the opposite way. Local changes were in the order of up to 40% compared to measurements without the magnetic field present. Bending of the central electrode was observed for the chamber with the steel electrode. The size of the volume of reduced collection near the guard electrode was impacted by the magnetic field. As the here investigated beam and field parameters differ from those of clinical systems, quantitatively different results would be expected for the latter. However, the gyroradii encountered here were similar to those of a 6-7 MV MRI linac with a 1.5 T magnet.

CONCLUSIONS

Magnetic fields impact the performance of ionization chambers also on a local level. For practical measurements this might mean a change in the effective point of measurement, in addition to any global corrections. Further knowledge about the local response will help in selecting or constructing optimized chambers for use in magnetic fields.

Commissioning of a PTW 34070 large-area plane-parallel ionization chamber for small field megavoltage photon dosimetry

PURPOSE

This study investigates a large-area plane-parallel ionization chamber (LAC) for measurements of dose-area product in water (DAPw ) in megavoltage (MV) photon fields.

METHODS

Uniformity of electrode separation of the LAC (PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high-resolution microCT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm2 field were acquired. A 6 MV calibration coefficient, ND,w,LAC , was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAPw . Additional calibrations in 10 × 10 cm2 fields at 6, 10, and 18 MV were performed.

RESULTS

Electrode separation is uniform and agrees with specifications. Volume-averaging leads to a signal increase proportional to ~1/cos(α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off-axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm2 field. ND,w,LAC was 163.7 mGy cm-2 nC-1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm2 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV.

CONCLUSIONS

The LAC requires only small correction factors for DAPw measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm2 fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over-response. The accuracy of relative point-dose measurements in the field's periphery is an important limiting factor for the accuracy of DAPw measurements.

Chiari medial displacement osteotomy

The results of 36 Chiari medial displacement osteotomies of the pelvis were studied to determine the efficacy of the operation in the treatment of subluxated and/or painful dysplastic hips. Fifteen of 16 osteotomies performed for congenital hip dysplasia provided satisfactory coverage, pain relief, and increased function. The results were less consistent in patients with myelodysplasia, spasticity, and Perthes disease. The Chiari osteotomy is a technically exacting procedure capable of providing adequate femoral head coverage. This coverage facilitates pain relief and increased function. This procedure is most suited for those individuals with a painful hip subluxation in whom a concentric reduction is not possible.

The use of banked autologous blood in patients undergoing surgery for spinal deformity

The cases of fifty-two patients who underwent sixty elective spinal fusions for spinal deformity were studied to evaluate the efficacy of the use of banked autologous blood to replace operative loss of blood. The patients ranged in age from ten to forty-nine years. Each patient began to take 325 milligrams of ferrous sulphate, three times a day, as soon as surgery was scheduled, and was evaluated weekly at the Shepeard Community Blood Bank. If a patient's hemoglobin level was more than eleven milligrams per 100 milliliters, either a whole unit of blood or a half-unit was drawn at each visit. An average of 3.3 units of blood (range, 1.5 to 6.0 units) was obtained and was stored for as long as forty-two days. Either citrate phosphate dextrose with adenine (CPDA-1) or adenine, dextrose, and mannitol (ADSOL) was used as a preservative. In 85 per cent of the procedures only autologous blood was required for transfusion. This method proved to be simple, safe, and very well accepted.