SunPoint Diode Detectors
Contents
1. Penumbra measured by diode Penumbra measured by ion chamber Small size more precision less averaging PTW 729 5 0 x 5 0 25 0 5 0 0 125 3 3 al SunPoint Relative scale size comparison of 2D array detectors Diode advantage 1 Smaller active area The SunPoint Diode Detectors used in Sun Nuclear products measure only 0 8 x 0 8mm 0 64mm Table 1 clearly illustrates that both the 4mm RK chamber similar in size to the MatriXX detector and the 5 5mm 0 125 cc chamber similar in size to the 729 detector significantly over estimate the penumbra region by 1 7mm to 2 4mm 59 to 83 SunPoint MatriXX Detector 729 Detector Penumbra over estimation is due to the large size of the chamber As seen in Table 2 the active detection area of a SunPoint Diode Detector used in Sun Nuclear products is approximately 1 25 the area of a MatriXX detector 15 90mm7 and 1 39 the area of a 729 detector 25 00 mm Active Area Diode advantage 2 Thinner active thickness and volume The electrode spacing of the ion chambers in MatriXX and the 729 is more than 150 times larger than the active thickness of the SunPoint Diode Detector If the SSD is 100cm there could be a 1 difference in beam intensity between the top and bottom of a 5mm thick f Q chamber The diode is taking a point measurement at exactly the specified depth the ion c chambers are taking an average measurement along a depth range2. Additionally the volume of the SunPoint Diode Detector is only 0 000019cm This is approximately 4 200 times smaller than a MatriXX chamber and 6 500 times smaller than a 729 chamber 0 2 O gt Diode advantage 3 Better sensitivity A silicon pn junction diode is about 18 000 times more sensitive than the air cavity ion chamber The SunPoint Diode Detector has an active volume more than 4 200 times smaller than the gt ion chambers used in MatriXX and 729 yet its overall sensitivity to radiation is still about 10 2 times higher 2 Q Y Diode Advantage 4 Less Drift The diodes used in Sun Nuclear products have less drift than the detectors used in the Matrixx and 729 Users typically calibrate SunPoint Diode Detectors only once per year using Sun Nuclear s patented Wide Field Calibration The calibration takes only 15 minutes using the easy to follow steps in the SNC Patient software See Case Study 2 for more details on drift Drift CASE STUDY 1 Dose modeling The errors in Figure 1 a are due to the fact that the radiation beam that was modeled was not reflective of the actual beam delivered to the MapCHECK The beam model in a was based on scanning data from a large volume chamber If a large chamber is used to collect the beam data for planning system beam modeling the result will be planned dose maps that do not accurately portray the actual beam and these inaccuracies will not be caught if the plan
3. maps are also verified using large chambers Diode detectors are small enough to detect these dose volume averaging errors as demonstrated in a below After determining scenario a was unacceptable this Figure 3 IMRT plan errors detected by MaoCHECK a Original model 79 passed 6 mm detector WARNING CASE STUDY 2 Reproducibility institution re modeled their small fields with a 2mm detector and when the beam was re measured with the MapCHECkK dose map b resulted and the pass rate rose from 79 to gt 95 QA measurements with large chambers will not reveal these errors in fact it may hide the error with an erroneously high pass rate especially if the beam was modeled with a similarly large chamber b New model gt 95 passed 2 mm detector lonization chambers have large measurement volumes and are NOT designed for accurate small field or steep dose gradient QA Small beam models often model dose gradients and beam shape incorrectly Attempting to discover such errors with ion chambers will extend the measurement error rather than reveal it In published reports short term and long term SunPoint Diode Detectors reproducibility has been shown to be better than ion chamber arrays Because ion chambers are typically desired for their stability this is a significant finding and further strengthens the argument for using diode detectors Short Term Reproducibility MapCHECK 0 15 15 consecutive measu
4. SUN NUCLEAR Benefits Analysis corporation SunPoint Diode Detectors SumPoint PRECISION QA PATIENT MACHINE ROUTINE DOSIMETRY The Right Detector for the Right Application SunPoint Diode Detectors Sun Nuclear manufactures products using diodes and ion chambers Both offer unique advantages based on the application Unlike some vendors who insist ion chambers are the only acceptable detector for every application Sun Nuclear recognizes that diodes offer benefits that are in the best interest of the patient For this important reason SunPoint Diode Detectors are selected as the dosimeter of choice for many Sun Nuclear products These benefits include Size SRS IMRT and VMAT deliver small beamlets to avoid critical structures and hit the target When millimeters are so critical in the plan and delivery they are also essential for measurement accuracy Measuring only 0 64mm and 0 000019cm SunPoint Diode Detectors are smaller than ion chambers by orders of magnitude which results in accurate dose plan QA measurements lon chambers may be a gold standard for dosimetry calibration however this is not the case for patient QA lon chambers integrate over a much larger area resulting in a loss of accuracy known as Dose Volume Averaging Dose Volume Averaging is characteristic in ion chambers for small field and gradient measurement which is why Sun Nuclear only uses diodes for such applications Sens
5. aging If QA is performed on the same and stereotactic QA gt beam with ionization chambers gradients will continue to be underestimated hiding a critical dose non conformity error When ion chambers are used for both beam modeling and QA the planned dose and measured dose may agree because they BOTH under and or over estimate dose to the target volume and surrounding structures The QA results may be favorable but they are not satisfying the objective of revealing inconsistencies between the conceptual planned dose and actual delivered dose Table 1 Comparison of penumbra width of a 2 x 2cm field at 1 5cm depth for a 6MV beam Manufacturer Sun Nuclear Detector SunPoint Active detector size mm 0 8 x 0 8 Measured penumbra width mm 2 9 Measured flatness 1 0 Figure 1 lon Chamber Diode Film 100 gt 2 x 2cm field profile measurements with various detectors Table 2 Comparison of active area volume and sensitivity Manufacturer Sun Nuclear IBA Detector array MapCHECK MatrixX Active dimension mm 0 8 x 0 8 4 5 O Active area mm 0 64 15 9 Active thickness mm 0 03 59 Active volume cm 0 000019 0 08 Sensitivity nC Gy 32 0 2 4 Diameter 4 IBA PTW PTW RK PinPoint 0 125 4 0 2 0 5 5 4 6 4 1 5 3 4 2 3 4 6 1 Figure 2 Scanning Active volume ion chamber gt 6000x that of diode SunPoint Active volume Diode Detector 0 000019cm
6. ctor is obtained for the center diode By multiplying this absolute dose calibration factor to the array calibration factor of each diode SunPoint Diode Detectors can measure absolute dose FAQ 4 What is the lifetime of the SunPoint Diode Detectors SunPoint Diode Detectors has been in use since 2002 and prototypes are still in clinical use today SunPoint Diode Detectors have a lifetime of more than 100 kGy which for most centers will easily equate to more than 10 years FAQ 5 What happens if one of my diode detectors fail Diodes rarely fail It is a misconception that diodes are more likely to fail than chambers The diode detector is the simplest component on the detector array If there is a failure it is almost always a channel measurement failure and will most likely be in the measurement electronics which are common to all array types including ion chamber arrays These electronic components are far more complex than a diode detector Sun Nuclear includes a software feature to turn off a defective channel This feature will not likely be used due to a defective diode however it may be used if there is a channel measurement failure FAQ 6 Is a diode more likely to fail than an ion chamber No A user is actually more likely to have an ion chamber failure because the leakage requirements are much more stringent for an ion chamber due to its much lower sensitivity and high voltage Humidity and microscopic dust will have a muc
7. diodes can perform better than p type diodes 2578 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 Lifetime High sensitivity and good stability give SunPoint Diode Detectors an extremely long life expectancy The lifetime of Sun Nuclear instrument arrays ion chamber or SunPoint Diode Detector based are a result of changing practices and normal electronic and circuit obsolescence 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 Benefits of SunPoint Diode Detectors Smaller Active Area Better Sensitivity Thinner Active Thickness amp Volume Less Drift No Dose Volume Averaging Why chambers are wrong for IMRT VMAT Sophisticated treatments such as VMAT IMRT and SRS are collections of small beamlets with very steep dose gradients penumbra measuring 1 x 1cm or smaller Because these dose gradients are meantto tightly conform to patientanatomy and tumor volume accurate and precise measurement of the dose gradients is of the highest importance Special care must be taken when comparing planned dose to measured dose If the planned dose is modeled with an ion chamber the dose gradients will be underestimated due to dose volume aver
8. h larger impact on ion chambers than on diodes FAQ 7 Is energy dependence an issue for SunPoint Diode Detectors No With MV beams used in IMRT the response change of the diodes with energy is negligible for the same energy beam SunPoint Diode Detectors works perfectly if it is calibrated with each energy taking only about 15 minutes FAQ 8 What impact will temperature have on SunPoint Diode Detectors Any changes in temperature in the measurement environment will not be important since the SunPoint Diode Detectors can be calibrated in absolute dose immediately before the measurement session FAQ 9 Will get reproducible results with SunPoint Diode Detector based instruments Yes Please see the case study on page 6 Document references 1 Evaluation of a 2D diode array for IMRT quality assurance 8 Diode in vivo Dosimetry for Patients Receiving External Beam D L tourneau et al Radio Oncol 70 2 199 206 2004 2004 Radiation Therapy E Yorke et al AAPM Report No 87 TG62 Medical Physics Publishing College Park MD 2005 2 A 2 D diode array and analysis software for verification of intensity modulated radiation therapy delivery 9 Accuracy contra work load in In Vivo Dosimetry P A Jursinic and B E Nelms Med Phys 30 870 879 2003 G Rikner and E Grusell Dept of Med Phys Uni Hosp Uppsala Sweden www scanditronix wellhofer com 3 Dosimetric characterization of a large area pixel segme
9. itivity The electron density of silicon is 18 000 times greater than air Therefore a silicon 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 lon chambers must always be larger than diodes due to their low sensitivity and signal to noise ratio Sensitivity as a function of accumulated dose SunPoint Diode Detectors exhibit consistent sensitivity with accumulated dose Sensitivity variation is lt 0 5 kGy at 6MV lt 1 5 kGy at 10 MeV The benefit is infrequent array calibration lt once per year even when detectors receive different accumulated doses The Right Detector for the Right Application Sensitivity as a function of dose per pulse Unlike other diodes from different manufacturers the SunPoint Diode Detector sensitivity only changes about 1 for 600 fold changes Semiconductor diodes can remain linear with dose per in dose per pulse pulse after very high accumulated dose 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 measurements response varied 0 15 Quantified over a 261 day period of use MapCHECK using SunPoint Diode De
10. nted ionization chamber 10 Comparison of two commercial detector arrays for IMRT quality S Amerio et al Med Phys 31 2 414 420 2004 assurance Jonathan G Li Guanghua Yan and Chihray Liu JACMP Volume 10 4 Characterization of a 2D ion chamber array for the verification of Number 2 Spring 2009 radiotherapy treatments E Spezi et al Phys Med Biol 50 3361 3373 2005 11 I mRT MatrixX User Manual 5 Modeling the instantaneous dose rate dependence of radiation diode 12 User Manual 2D Array Seven29 T10024 Version 1 1 detectors J Shi W E Simon T C Zhu Med Phys 30 9 2509 2519 2003 13 Data provided by Ron Watts Ph D 6 Performance evaluation of a diode array for enhanced dynamic 14 US Patent No 6 125 335 wedge dosimetry T C Zhu et al Med Phys 24 1173 1180 1997 7 Dose rate and SSD dependence of commercially available diode detectors AJ S Saini and T C Zhu Med Phys 31 4 914 924 2004 Clinical testimony gt gt For domestic plan verification the expected two dimensional distribution of the array signals is calculatediviarconvolutionsofithesplannedidoserdistributionsobtained from the treatment planning system with two dimensional response function of a single chamber B Poppe et al referring to PTW 2D chamber arrays Two dimensional ionization chamber arrays for IMRT plan verification Medical Physics April 2006 Vol 33 a 4mm chambe
11. r is small enough to accurately measure the profile asslongrasithesieldysizeuis not smaller than 4x4cm S Amerio et al original collaborators to the Matrixx Dosimetric characterization of a larger area pixel segmented ionization chamber Medical Physics February 2004 Vol 31 SunPoint Diode Detectors gt gt The Right Detector for the Right Application SUN NUCLEAR aA Peda Cour corporation Melbourne Florida 32940 tel 1321 259 6862 Your Most Valuable QA and Dosimetry Tools web www sunnuclear com SunPointW11032011
12. rements MatrixXX Unknown Requires 15 minute warmup 729 0 50 Long Term Reproducibility MapCH ECK 0 20 9 months totaling approximately 50Gy MatriXX 1 30 7 month period 729 1 4month period Frequently asked questions gt gt FAQ 1 Do SunPoint Diode Detectors have a high instantaneous dose rate dependence No SunPoint Diode Detectors exhibit a sensitivity variation of only 1 over a 600 fold change in instantaneous dose rate diode sensitivity change from 75cm to 250cm SSD in an open beam and also underneath the primary collimator FAQ 2 Will have to calibrate the diode array frequently No Sun Nuclear uses proprietary radiation hardened diodes Our diode sensitivity change is less than 0 5 kGy with a 6 MV beam and 1 5 kGy with a 10 MeV beam A Sun Nuclear user typically calibrates a diode array every 1 to 3 years for a typical patient load The patented array calibration procedure is easily performed in only 15 minutes by the user with a software wizard There is NO need to send the unit back to the manufacturer for calibration As seen in Case Study 2 ion chambers could have greater long term drift than diodes There are many factors that contribute to detector stability such as material stability electronic components etc FAQ 3 Can SunPoint Diode Detectors measure in absolute dose Yes By following a one minute calibration routine in the SNC Patient software the absolute dose calibration fa
13. s Temperature 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 9 Pressure lon chamber response is dependent on temperature and pressure while diode response is only dependent on temperature Energy Energy dependence for SunPoint Diode Detectors is easily managed with calibration files For example 6MV will 9MeV will use a 9MeV file use a 6MV calibration file Field size lon chambers have field size dependence for small and intensity modulated fields SunPoint Diode Detectors can be used in all field sizes The Sun Nuclear EDGE Detector utilizing a single SunPoint Diode Detector can be used to scan photon MV profiles up to 30 x 30cm and percent depth dose up to 15 x 15cm For array products using SunPoint Diode Detectors such as MapCHECK calibrate the array at the chosen depth if the field size is greater than 25 x 25 cm and deeper than 10 cm SSD and Depth SunPoint Diode Detector based arrays are proven to accurately measure dose at different SSD s and depths MapCHECK IMRT QA depths range from 2 15cm and PROFILER 2 is used at varying depths as a substitute for water phantoms n type and p type Diode performance depends on the individual detector regardless of n type or p type Several publications demonstrate that n type
14. tectors varied 0 2 Both studies indicate SunPoint Diode Detector based arrays are more stable than ion chamber based arrays 12410 Fast Setup SunPoint Diode Detector based instruments do not require warm up or the application of bias voltage prior to use lon chamber arrays can require up to 60 minutes and 10Gy 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 Calibration The calibration for SunPoint Diode Detector products is very stable Users typically calibrate every one to three years using Sun Nuclear s patented 15 minute Wide Field Calibration WFC method 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 in house linear accelerator or Cobalt 60 source 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 clinically proven worldwide in thousands of cancer centers SumPoint PRECISION QA Performance specifications of SunPoint Diode Detectors are the best in the industry Dependencie
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