Med Phys. 2016 Jan;43(1):314. doi: 10.1118/1.4938262.

Commissioning optically stimulated luminescence in vivo dosimeters for fast neutron therapy

Lori A. Young

Department of Radiation Oncology, University of Washington, Seattle, Washington 98195

Fei Yang

Sylvester comprehensive Cancer Center, University of Miami, Miami, Florida 33124

Davis Woodworth

Department of Physics, University of Reno, Reno, Nevada 89557

Zephyr McCormick

Department of Physics, University of California, Santa Barbara, California 93106

George Sandison

Department of Radiation Oncology, University of Washington, Seattle, Washington 98195

 

Abstract:

Purpose: Clinical in vivo dosimeters intended for use with photon and electron therapies have not been utilized for fast neutron therapy because they are highly susceptible to neutron damage. The objective of this work was to determine if a commercial optically stimulated luminescence (OSL) in vivo dosimetry system could be adapted for use in fast neutron therapy.

Methods: A 50.5 MeV fast neutron beam generated by a clinical neutron therapy cyclotron was used to irradiate carbon doped aluminum oxide (Al2O3:C) optically simulated luminescence dosimeters (OSLDs) in a solid water phantom under standard calibration conditions, 150 cm SAD, 1.7 cm depth, and 10.3×10.0 cm field size. OSLD fading and electron trap depletion studies were performed with the OSLDs irradiated with 20 and 50 cGy and monitored over a 24-h period to determine the optimal time for reading the dosimeters during calibration. Four OSLDs per group were calibrated over a clinical dose range of 0–150 cGy.

Results: OSLD measurement uncertainties were lowered to within 2%–3% of the expected dose by minimizing the effect of transient fading that occurs with neutron irradiation and maintaining individual calibration factors for each dosimeter. Dose dependent luminescence fading extended beyond the manufacturer’s recommended 10 min period for irradiation with photon or electron beams. To minimize OSL variances caused by inconsistent fading among dosimeters, the observed optimal time for reading the OSLDs post irradiation was between 30 and 90 min. No field size, wedge factor, or gantry angle dependencies were observed in the OSLDs irradiated by the studied fast neutron beam.

Conclusions: Measurements demonstrated that un certainties less than 3% were attainable in OSLDs irradiated with fast neutrons under clinical conditions. Accuracy and precision comparable to clinical OSL measurements observed with photons can be achieved by maintaining individual OSLD calibration factors and minimizing transient fading effects. C 2016 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4938262]

 

 

Supplemental Information:

The University Washington Medical Center is the only Radiation Oncology Center in the United States offering fast neutron therapy for the treatment of radio-resistant tumors that do not respond well to conventional photon therapy. Individuals with inoperable salivary gland tumors comprise the largest population of patients treated by the Clinical Neutron Therapy System (CNTS) as well as some aggressive sarcomas, melanomas, renal cell, thyroid cancers, and other late stage metastatic disease sites.

Fast neutron irradiation is produced by accelerating a spectrum protons with a maximum energy of 50.5 MeV using a Scanditronix MC50 cyclotron.  The protons are generated from hydrogen gas in an ion source that strips electrons from the hydrogen molecule.  These protons are steered onto a 10.5 mm thick beryllium target with copper backing to produce the fast neutron beam.  The beamline is mounted on a gantry that revolves around the patient couch 360° about the radiation isocenter located 150 cm away from the target.  Patient specific treatment fields are created with a multi-leaf collimation system with a total of 40 leafs.

 

 

LY FIG1Figure 1.  CNTS with solid water phantom for OSLD calibration.

 

For conventional photon and electron external beam therapy, several devices such as thermoluminescent dosimeters (TLDs), diodes, metal-oxide semiconductor field-effect transistors (MOSFETs), and OSLDs are currently in use for early detection and mitigation of potential treatment errors. In this study, Al2O3:C OSLDs were investigated as a possible in vivo dosimeter for fast neutron therapy. OSLDs were primarily selected because they are not destroyed by neutron irradiation unlike commercially available MOSFETs and diodes. OSLDs can also be read multiple times with minimal signal loss.

This work showed a notable difference in OSLD response following irradiation by fast neutrons in comparison to photons and electrons. Differences in OSLD sensitivity and time dependent fading signal response were observed in dosimeters irradiated with neutrons, This phenomenon may be attributed to the emission of high linear energy transfer (LET) heavy particles when neutrons interact with the media in the solid water phantom and by-products of neutron activation. These differences in OSLD response may also apply to their use as an in vivo dosimeter with other heavy particle irradiation such as carbon ions for treating cancers.

To achieve adequate accuracy and precision it was necessary to assign unique low dose and high dose calibration factors for each OSLD that needed to be checked periodically. Careful attention is needed to read the dosimeters at the same time interval as conducted during calibration to minimize the differences in the fading effects in fast neutron irradiated OSLDs due to neutron activation causing the emission of γ-rays, protons and alpha particles. Significant differences in calibration factors were observed for neutrons versus photons for the same dose exposure. Typical differences observed in calibration factors are plotted in Figure 2. The raw counts detected after OSLDs irradiation by photons is about 2.5 times greater than neutrons for the same absorbed dose.

LY FIG2

Figure 2.   Comparison of calibration factors for OSLDs irradiated with photons versus neutrons at relatively high and low doses.

 

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