Q: Why does the MapCHECK use diode detectors?
Q: Can the MapCHECK perform MLC QA?

Q: Is a gantry attachment available?

Q: Why does the MapCHECK use diode detectors?
A: Diode detectors have been the subject of much interest for their small size and instantaneous dose response.  Various dose response dependencies have been eliminated or minimized over the years and diodes are now a mature technology that offers several benefits over ionization chambers. 

The chief advantage of the MapCHECK detector over an ionization chamber is the 0.8 mm x 0.8 mm size of the MapCHECK detector.  It is widely known that small detectors give better pinpoint dose readings.  This is because the small size has less dose area to average into a single reading.  A MapCHECK detector is very small and the variation in dose intensity across it is statistically insignificant.  A chamber on the other hand is always going to be larger.  Even the very smallest chambers have an interior diameter of 2mm.  These chambers are double the MapCHECK detector size and are only available as a single detector, not as an array of 445 detectors.  Ionization chamber arrays use chambers that are even larger, specifically 5mm x 5mm x 4mm.  Within one of these chamber’s 25.0mm² area the variation in dose intensity is much more significant – however the chamber is limited to return a single dose value – the average dose value.  This is extremely important for the measurement of highly modulated IMRT fields.  The ion chamber will average dose data across penumbra regions, offering artificially low values near the top of a gradient, and artificially high values at the bottom of a gradient.  The net result is a flattened penumbra.  The dose data returned by a MapCHECK detector is not averaged across the beam gradients and more accurately reflects the beam output with no flattening effect.  The detectors used in the MapCHECK array have even pointed out errors in the commissioning of planning systems with chambers that are too large.

For up to date details on the importance of small detectors in measuring IMRT fields and penumbras, see:
"The use of film dosimetry of the penumbra region to improve the accuracy of intensity modulated radiotherapy" M. Arnfield, etal, Medical Physics, 32(1) p12 2005
View Paper

Q: Can the MapCHECK perform MLC QA?
A: The MapCHECK can perform efficient MLC QA by checking for the reproducibility of a delivered plan.  A delivered plan is a collection of many MLC movements w/ radiation between the movements.  If the movements are not right, the dose distribution will not be right.  Any saved measurement can be used as a QA template for MLC checks.  For instance, once a specific IMRT plan has been validated, there is a known pass rate based on the defined percent difference and distance-to-agreement criteria.  This creates a unique opportunity for future comparisons.  If the plan irradiated 411 detectors, and 97% pass with criteria of 3% difference and 3mm DTA, then if the same plan is delivered a week, month, or six months into the future the same results should be achieved.  Furthermore, the 3% of detectors that do not pass are illustrated and identified, and those same detectors should not pass during any future comparison.  Using this technique focuses on MLC QA from end to end.  Rather than evaluate the MLC leaf by leaf, this technique looks at the big picture.  As a means to further isolate problems that can be uncovered by running the reproducibility test described above, Sun Nuclear plans to include more specific and quantifiable MLC QA functions in the MapCHECK in the future.

Q: Is a gantry attachment available?
A: The MapCHECK can be mounted in the same gantry attachment that the PROFILER™ and Daily QA Check 2™ use.  This however limits the SSD to less than 100cm and does not allow for the proper build-up or rigidity needed for stringent IMRT QA.  A new Isocentric MapCHECK Gantry Attachment is available that holds the MapCHECK along with the proper build-up at 100cm SSD.  This is a robust fixture that is very rigid, which makes it an excellent tool for checking sag and performing other Isotropic tests.

Q: Are composite measurements supported by the MapCHECK?
A: The belief that the verification of individual beams is a more stringent test than composite beams is widely supported.  However, composite measurements where the beam axis is perpendicular to the MapCHECK detector array are supported by the MapCHECK.   With the MapCHECK on the treatment couch, you can instruct the planning system to keep the gantry angle fixed during all fields, and execute all fields during one MapCHECK measurement.  Alternatively, you can use the Isocentric MapCHECK Gantry Attachment to deliver the beams with the MapCHECK mounted in the gantry at the proper depth and SSD.  Either method would require you to import the corresponding composite QA plan and compare.

Q: Does MapCHECK replace the PROFILER?
A: No. The MapCHECK was developed for a single specific purpose: to measure and compare IMRT plans.  This focused goal led the design and construction of MapCHECK in a direction that is fundamentally different from the PROFILER.

Key differences are:
1. The MapCHECK uses integrator circuitry that does not enable real-time beam imaging, and has a dose limit per measurement of approximately 300 cGy. The PROFILER uses dose/rate circuit design, enabling unlimited dose measurement and real-time analysis, which is especially useful for any type of beam imaging, service, or maintenance requirements. Profiler can save each profile measurement with analysis every 70 ms, which can be used to check the beam stability at start up, and beam symmetry delivered by the MLC for small MU deliveries. For more details, please refer to the above paper by E. Lief, et al and the paper by M. Bieda et al: “Microanalysis and temporal characterization of small MU segments in IMRT”, Med. Phys., Vol. 29, No. 6, p. 1304, June 2002.

2. PROFILER spacing is 5mm along x or y axis. MapCHECK spacing is 7mm along the diagonal, 10mm along x or y axis.

Although there may appear to be similarities, there are key differences that do not make the MapCHECK a suitable substitute for the PROFILER™. The design of each product has been essentially optimized for two unique applications.

Q: What type of cable is used with the MapCHECK?
A: The MapCHECK uses the same cable and power supply as the PROFILER and DAILY QA CHECK 2.  It is a 25 Meter power/data cable with 8 conductors.

Q: What Treatment Planning Systems (TPS) will be supported?
A: All major Treatment Planning Systems are supported. If you have a custom or unusual planning system, please contact Sun Nuclear and we will attempt to accommodate your system. We have already written interfaces to several custom systems.

Q: How is the MapCHECK calibrated?
A: Array calibration is performed with a built-in software application that guides the user through Sun Nuclear’s wide field calibration (US Patent # 6125335.) Six separate exposures calibrate the entire detector array using detector substitution.  Calibration takes about 15 minutes per energy.  Published papers suggest the average clinic will need to recalibrate their MapCHECK array about once per year.

Q: Can MapCHECK be used to measure the absolute dose?
A: Yes. Dose calibration is performed in a 10 x 10 field with the array at a depth where the dose is known. This establishes a dose calibration factor on the center detector, which is then transferred to the other array detectors via the sensitivity correction factors that are relative to the center detector.  Absolute dose can be calibrated for as often as is desired and takes about 25 seconds. 

Q: The MapCHECK compares IMRT treatment plans without film. Is film required for reimbursement?
A: No. From the ACR Bulletin, April 2001, Volume 57, Issue 4, titled New Intensity-modulated Radiation Therapy Codes for Hospital Outpatient Procedures:

"The accuracy of dose delivery must be documented for each coarse of treatment by irradiating a phantom that contains either calibrated film to sample the dose distribution or an equivalent measurement system to verify that the dose delivered is the dose planned."