DRO Dosimetry Indicator
	Mammography Repeat Analysis

DRO Dosimetry Indicator

Department of Radiation Oncology (DRO)
National Cancer Centre Singapore

What are we measuring and why?

Each patient undergoing radiotherapy gets a therapeutic dose of radiation prescribed by the Radiation Oncologist. Patients are then required to come for 5 to 7 weeks of radiotherapy. Radiation is delivered by a linear accelerator. The dose has to be delivered each day accurately to the tumour site to ensure the effectiveness of the whole treatment. Studies have shown that an accuracy of ±5% in the delivery of absorbed dose to a tumour volume is required for the effective eradication of tumour cells. Under dosage will cause recurrences while over dosage will cause complications.

The most basic measure of dose accuracy is through the use of a radiation daily output check device. This special device is highly stable and robust and is able to detect small changes in radiation output each day. By monitoring the output each day, the Radiation Physicist is able to monitor and if necessary to take proper and timely intervention by adjusting the radiation output. This helps to maintain an optimum level of dose delivery to our patients at all time. The DRO dosimetry indicator is a cumulative report of all our output readings that are used for patient treatment. This covers the output performance of all the linear accelerator machines at DRO.

The pass indicator is calculated as a ratio of the number of radiation output check readings that fall within ±2.5% (pass) to the total number of readings taken.

Causes of failure are often linked to machine fault. Any output reading that falls outside of ±2.5% is immediately reported to the duty Radiation Physicist for corrective action. Patient treatment may only resume when the readings are re-established to within 2.5%.

So far, all our indicator records have been 100% for all the linear accelerators.

What are we doing to maintain our standards so far?

The output for each linear accelerator is maintained by a series of standard radiation dosimetry devices and protocol that we maintain.This includes the use of the International Atomic Energy Agency protocol TRS-398 for absorbed dose to water and participating in IAEA’s annual postal TLD audit. The audit compares a stated dose to the TLD irradiated by us and sent back to IAEA. Many countries participate in this audit.

Our latest audit for 2008 was determined by IAEA to be less than 0.5% of the stated dose.

In addition, we also function as IAEA’s Secondary Standard Dosimetry Laboratory (Radiotherapy) for Singapore. As the SSDL, we hold the secondary standard that is directly traceable to a Primary Standard Dosimetry Laboratory.

Overall, our standards continue to be high and we will continuously strive to uphold a high standard of radiation output for our patient treatment.

By Dr James C L Lee, Chief Radiation Physicist

Mammography Repeat Analysis

What are we measuring and why?

Mammography has evolved to an extent that its usefulness in the detection of early breast cancer is generally recognized throughout the world.

Screen detected cancers are usually small and subtle and their detection relies critically on high quality mammographic technique – mammographic exposure factors, positioning, and patient movement. High quality mammograms must be obtained using the lowest possible radiation dose and all efforts are directed towards minimizing the number of rejected images due to suboptimal technique.

In NCC, we aim to perform mammography at high sensitivity (i.e. detection of breast cancers) while recalling only a small percentage of women for additional testing. The benefits can be realized with our efforts to minimize radiation dose and repeat exposures during mammography.

Repeat rate analysis is performed regularly each month.

Repeats are those patient radiographs that are not diagnostically acceptable and require an additional exposure of the same patient for the same view.

In addition to the total number of films used, the causes of repeats and unnecessary repeats are also evaluated.
Repeat rates are calculated for each of these categories:

• Patient positioning
• Patient motion
• Artifacts
• Over- or underexposed films
• Radiographer
• Film fog
• Equipment malfunctions – radiographic and processing equipment

The radiographers collect their completed films and conduct weekly reviews with their peers and senior radiographer.

The underlying causes of the repeats are further evaluated to recognize trends. The information from the repeat analysis is taken as a learning experience and not a disciplinary one for our radiographers.

Any areas showing significant repeat rates were reviewed to eliminate problems for the future.

How are we doing and how do we compare to best practices?


From 2006 to 3^rd Q 2008, we have consistently maintained repeat rate at an average of 2%.

The American College of Radiology (ACR) Quality Control Manual recommended monthly Repeat Analysis and accepted criteria to be <5%.

* Accreditation scheme for laboratories Technical Note MI 001, October 2007
The SAC Accreditation Programme managed by SPRING Singapore
* American College of Radiology (ACR) Quality Control Manual 1999
BreastScreen Aoearoa National Policy and Quality Standards – Feb 2004
American Association of Physicsts in Medicine (AAPM) recommendations

What we are doing to improve?

In October 2008, we embarked on digital mammography. There was the learning transition from analog to digital imaging. We continued to track the repeat exposures made.

With digital mammography, we have aligned our repeat analysis with MQSA Quality Mammography Standard 900.12(e)(3)(ii) -

If the total repeat or reject rate changes from the previously determined rate by more than 2.0 percent of the total films included in the analysis, the reason(s) for the change shall be determined. Any corrective actions shall be recorded and the results of these corrective actions shall be assessed.





The repeat rate for the 1^st half 2009 averages at 3.5% with changes between 0.5 to 1%.

Today with digital architecture, we are able to review all exposures made.
Our radiographers continue with regular weekly peer reviews of all mammogram exposures made in the department. The larger sampling allows more specific trend charting.

Overall, our repeat rates are comparable to international repeat rates. However, repeat rate should not be construed as a measure of overall quality. Facilities where radiologists are lax in demanding high quality images can have very low repeat rates.

We plan to evaluate extra exposures requested by our radiologist and the reasons for these exposures. This is useful as we review standards in our imaging procotols.