Diode and ion chamber detectors are used in Sun Nuclear dosimetry products. Both detector types offer unique advantages and are selected based on the application.

The goal of radiotherapy QA and Dosimetry is to accurately measure and characterize the treatment beam. Sun Nuclear recognizes that due to their small size, diodes can offer benefits that are in the best interest of this goal. We have invested in detector design for over 25 years. One of the results of this dedication to new technology and research is the SunPoint Diode Detector. In use since 2001, today’s SunPoint Diode Detectors offer superior performance to ion chambers for applications where penumbra and steep dose gradients are being measured. Additionally SunPoint Diode Detectors offer:

• Absolute dose calibration
• Excellent stability
• A long lifetime
• Very small size
• Excellent sensitivity characteristics

The SunPoint name distinguishes Sun Nuclear’s SunPoint diodes from other diodes. Products using SunPoint Diode Detectors include: ArcCHECK, MapCHECK 2, MapCHECK, PROFILER 2, IsoRad, QED, Daily QA 3
and SRS PROFILER.

SunPoint Diode Detector Advantages:

1. Size
   Size
   
   
   SRS, IMRT, VMAT deliver small beamlets around critical structures to the target. When millimeters are so critical in the plan and delivery, small detector size during verification is essential for QA accuracy. Measuring 0.64mm^2 and 0.000019cm^3, SunPoint Diode Detectors are the smallest available detector by orders of magnitude, which results in accurate dose plan QA measurements. Ion chambers may be a gold standard for calibration; however this is not true for patient QA and small field dosimetry. Ion chambers integrate over a much larger area resulting in a loss of accuracy known as dose volume averaging. Dose volume averaging is a characteristic of ion chambers for small field and gradient measurement which is why Sun Nuclear only uses diodes for such applications.
2. Sensitivity (signal)
   Sensitivity (signal)
   
   
   The electron density of silicon is 18,000 times greater than air. Therefore a silicon based diode can be thousands of times smaller than an ion chamber, while its sensitivity can still be 10 times higher. The measurement benefit of this is two-fold. First: a higher signal to noise ratio equals better measurement accuracy and reproducibility. Second: a smaller detector equals better measurement precision. Ion chambers must always be larger than diodes due to their low sensitivity and signal to noise ratio.
3. Sensitivity (accumulated dose)
   Sensitivity (accumulated dose)
   
   
   SunPoint Diode Detectors exhibit consistent sensitivity with accumulated dose. Sensitivity variation is < 0.5%/kGy at 6MV, <1.5%/kGy at 10 MeV. The benefit is infrequent array calibration (< once per year) even when detectors receive different accumulated doses.
4. Sensitivity (dose per pulse)
   Sensitivity (dose per pulse)
   
   
   Unlike older diodes from different manufacturers, SunPoint Diode Detectors sensitivity only changes about ± 1% for a 600-fold changes in dose per pulse. Semiconductor diodes can remain linear with dose per pulse after very high accumulated dose.
5. Stability
   Stability
   
   
   SunPoint Diode Detectors have insignificant radiation degradation in short term and long term use. In a short term reproducibility test of 15 consecutive 60 MU’s measurements, response varied ±0.15%. Quantified over a 261 day (approximately 9 month) period of use, MapCHECK using SunPoint Diode Detectors varied ± 0.2%. Both studies indicate SunPoint Diode Detectors based arrays are more stable than ion chamber based arrays.
6. Fast Setup
   Fast Setup
   
   
   SunPoint Diode Detectors do not require warm-up or the application of bias voltage prior to use. Ion chamber arrays can require up to 60 minutes and 10 Gy prior to use. Warm-up is a result of the design of the instrument. Sun Nuclear’s ion chamber based products do not require warm-up but do require bias voltage.
7. Calibration
   Calibration
   
   
   The calibration for SunPoint diode detector based products is very stable. Users typically calibrate every one to three years using Sun Nuclear’s patented 15 minute WFC method. Sun Nuclear developed and owns a patent on Wide-Field Calibration (WFC). WFC affords users an easy, accurate, and independent calibration method. WFC is used for ion chamber and diode array products and is a benefit to all Sun Nuclear array product users. Every Sun Nuclear array product receives a factory calibration using Sun Nuclear’s Varian 2100C. WFC provides the user the ability to calibrate their Sun Nuclear product with their own Linac, at any time they wish. This allows users to verify the calibration accuracy themselves, and to perform independent research. The accuracy of Sun Nuclear WFC has been proven.
8. Absolute Dose
   Absolute Dose
   
   
   SunPoint Diode Detector based instruments measure the absolute dose accurately with the dose calibration of the reference detector to the standard accelerator output, exactly as an ion chamber device would do.
9. Lifetime
   Lifetime
   
   
   High sensitivity and low change in response give SunPoint Diode Detectors a long life expectancy. SunPoint Diode Detectors do not have a lifetime limit. Lifetime of Sun Nuclear detector arrays (ion chamber or diode) are a result of changing practices and normal electronic and circuit obsolesce and failure. Life expectancy is at least ten years under normal use. After 100 kGy, SunPoint Diode Detector sensitivity is still much higher than that of an ion chamber.

Frequently Asked Questions

1. What is the dose rate dependence (MU/min) of SunPoint Diode Detectors
   What is the dose rate dependence (MU/min) of SunPoint Diode Detectors?
   
   
   The dose rate dependence of the SunPoint Diode Detectors is < 2% from 50 to 600 MU/min. For most applications, including RapidArc, dose rate dependence is not an issue since the dose rate does not change in a very wide range. If the dose rate varies significantly during the measurement session, a median dose rate (such as 160 MU/min) for the dose calibration will reduce the dependence to < +/- 1%.
   
   
   
2. Are SunPoint Diode Detectors n-type or p-type?
   Are SunPoint Diode Detectors n-type or p-type? ^2,5,7,8
   
   
   SunPoint detectors are n-type. Sun Nuclear has manufactured both n and p type diodes and have purposely selected n-type for SunPoint. It is critical to understand that diode performance depends on the individual detector, regardless of n-type or p-type. Several publications demonstrate that n-type diodes can out perform p-type diodes.
   
   
   
3. What are the dependencies for SunPoint Diode Detectors?
   What are the dependencies for SunPoint Diode Detectors?
   
   
   • Temperature.^7,9
     Temperature dependence can be compensated by entering a temperature value, or more accurately by calibrating dose before measurement. The temperature coefficient of SunPoint Diode Detectors remains constant with accumulated dose.
   
   
   • Pressure.
     Ion chamber response is dependent on temperature and pressure, diode response is only dependent on temperature.
   
   
   • Energy.
     Energy dependence for SunPoint Diode Detectors is easily managed with calibration files. For example, 6MV will use a 6MV calibration file; 9MeV will use a 9MeV file.
   
   
   • Field size.²
     Ion chambers have field size dependence for small and intensity modulated fields. SunPoint Diode Detectors can be used in all field sizes. The EDGE Detector, can be used to scan photon (MV) profiles up to 30 x 30cm, and percent depth dose up to 15 x 15cm.
   
   
   • SSD and Depth.
     SunPoint Diode Detectors arrays are proven to accurately measure dose at different SSD’s and depths.6 MapCHECK IMRT QA depths range from 2-15cm. For example, PROFILER 2 is used at varying depths as a substitute for water phantoms.

*Footnotes

Footnotes

1. “Evaluation of a 2D diode array for IMRT quality assurance“, D. Létourneau, et al, Radio. Oncol., 70(2), 199-206 2004 (2004).
2. “A 2-D diode array and analysis software for verification of intensity modulated radiation therapy delivery“, P. A. Jursinic and B.E. Nelms, Med. Phys. 30, 870-879 (2003).
3. “Dosimetric characterization of a large area pixel-segmented ionization chamber“, S. Amerio, et al, Med Phys, 31(2) 414-420 (2004).
4. “Characterization of a 2D ion chamber array for the verification of radiotherapy treatments“, E. Spezi, et al, Phys. Med. Biol., 50, 3361-3373 (2005).
5. “Modeling the instantaneous dose rate dependence of radiation diode detectors“, J. Shi, W.E. Simon, T.C. Zhu, Med. Phys. 30 (9), 2509-2519 (2003).
6. “Performance evaluation of a diode array for enhanced dynamic wedge dosimetry“, T. C. Zhu et al, Med Phys 24, 1173-1180 (1997).
7. “Dose rat and SSD dependence of commercially available diode detectors“, AJ S. Saini and T. C. Zhu, Med. Phys., 31 (4), 914-924 (2004).
8. “Diode in vivo Dosimetry for Patients Receiving External Beam Radiation Therapy“, E. Yorke, et al, AAPM Report No. 87 (TG62), Medical Physics Publishing, College Park, MD, 2005.
9. “Accuracy contra work load in In Vivo Dosimetry“, G. Rikner and E. Grusell, Dept of Med Phys, Uni Hosp, Uppsala, Sweden, www.scanditronix-wellhofer.com.
10. “Novel dosimetric phantom for quality assurance of volumetric modulated arc therapy“, Daniel Létourneau, Julia Publicover, Jakub Kozelka, Douglas J. Moseley and David A. Jaffray, Medical Physics, Vol. 36, No. 5, May 2009
11. “Comparison of two commercial detector arrays for IMRT quality assurance“, Jonathan G. Li,Guanghua Yan,and Chihray Liu, JACMP, Volume 10, Number 2, Spring 2009