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A teaching and information resource on ventilation/perfusion imaging in Nuclear Medicine, primarily for the differential diagnosis

of Pulmonary Thrombo-Embolism (PTE/PE), especially where the images are complicated by the presence of other diseases

such as pneumonia and chronic obstructive airways disease (COPD/COAD).


Radiation Dose Facts

 
Now that there is irrefutable objective evidence that diagnosing Pulmonary Embolism (PE) by Ventilation/Perfusion Single Photon Emission Computed Tomography (V/Q SPECT) imaging is at the very least equivalent in efficacy to the radiological procedure known commonly as CTPA, it is incumbent on referring Doctors to consider the very large difference in radiation dose delivered to their patients from the two technologies.
 
Where access to V/Q SPECT exists, it is in the best interests of your patients, especially females of reproductive age, to be referred for that study rather than CTPA, if you suspect PE. The radiation doses received in the female breasts from CTPA can be the equivalent of that patient having 137 chest x-rays or 23 mammograms. This in turn could lead to a lifetime attributable risk of radiation-induced cancer of 1 in 300, or 0.3% [data taken from: Smith-Bindman R, et al. “Radiation Dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer”. Arch Intern Med. 2009;169(22):2078-2086]. By comparison, a V/Q SPECT study delivers about 1/300 of the breast dose. [International Commission on Radiological Protection, ICRP Publication #80 “Radiation dose to patients from Radiopharmaceuticals”. Pergamon, 1999]. An important point, often overlooked by non radiation specialists is that the dose on the V/Q procedure can be modified to suit special patient circumstances, merely requiring longer imaging time to get the same quality data; whereas the dose from CTPA is fully committed and dictated by the machine’s characteristics.
 
The Smith-Bindman paper cited above is particularly pertinent as it has emanated from an academic radiological institution in the USA, and is in effect a survey of four large institutions being “a retrospective cross-sectional study describing radiation dose associated with the 11 most common types of diagnostic CT studies performed on 1119 consecutive adult patients”. In the discussion, they state:” Neither physicians nor patients are generally aware of the radiation associated with CT, its risk of carcinogenesis, or the importance of limiting exposure among younger patients. It is important to make both physicians and patients aware that this risk exists.”
A second review publication on dosimetry most relevant to V/Q SPECT is worth noting. [Radiation dosimetry and safety issues in the investigation of pulmonary embolism Schembri GP, Miller AE, Smart R; Semin Nucl Med  2010 Nov;40(6):442-54]

The abstract follows:

When considering the investigation of the patient with possible pulmonary embolism, one needs to balance the likelihood of disease and the diagnostic utility of the test against the risks associated with the investigation. Both computed tomography pulmonary angiography (CTPA) and the ventilation/perfusion (V/Q) scan involve exposure to ionizing radiation. The effect of low-level ionizing radiation remains an issue of some controversy. CTPA delivers a greater effective dose and, in particular, greater doses to breast tissue, than the V/Q scan (typically 10-70 mGy for CTPA vs <1.5 mGy for V/Q to breast). Since breast tissue is particularly radiosensitive in younger women, the V/Q study has an advantage over CTPA in this group. In the pregnant patient, fetal exposure has been raised as a concern. In fact, there is typically only low fetal exposure from either study (<1 mGy). The CTPA does deliver less fetal exposure, particularly in the first trimester, but the difference between CTPA and V/Q scan is small when compared with the difference in dose to maternal breast from the 2 investigations. The "as low as reasonably achievable" (ie, ALARA) principle favours the use of V/Q scans in young women, assuming the diagnostic power of the 2 tests is comparable. CTPA requires a contrast injection that can cause adverse reactions in a small number of patients. No significant risk, however, has been demonstrated with the radiopharmaceuticals involved in V/Q scans.

 

Radiation Dosimetry in Pulmonary Embolism Detection

 

A/Prof Lee Collins AM

Director Medical Physics, Westmead Hospital NSW

lee.collins@swshs.health.nsw.gov.au

 

 

The current discussions regarding the relative benefits and problems of CTPA vs V/Q scanning for diagnosis of pulmonary embolism revolve around two main themes. Which provides the better diagnostic value, and what radiation dose is delivered to the patient in the process? This note covers the latter issue, although neither should be seen in isolation.

 

Dosimetry Factors

 

For V/Q scanning, the dosimetry is relatively uncomplicated, and is affected mainly by administered activity in each phase. The radiopharmaceutical used has a role, but given that the macroaggregates all have similar dosimetry and use 99mTc , the variations then come from the ventilation agents – Technegas, aerosols and gases. The main biological factor we need to consider is whether a female patient is pregnant or not. The dosimetry is independent of the imaging device.

 

The situation with CTPA is very different. There is a wide range of factors which can be varied. Some of these, and a brief effect for each, are:

 

  • kVp – lower kVp can reduce dose
  • mAs – a simple linear relationship
  • scan slice width – thin slices can increase dose
  • whether CT automatic exposure control (AEC) is used
  • number of slices, or scan volume – again, a simple relationship, but also will determine which organs are irradiated
  • scan pitch – lower pitch can increase dose
  • number of scan runs – pre- and post-contrast runs will effectively double the dose
  • breast shielding – use can decrease breast dose
  • the CT scanner – doses vary with manufacturer and model

 

This all means that the prediction of CTPA doses is difficult if not impossible, and the reader of the literature needs to be aware that stated doses can vary widely.

 

For both modalities, the dose to the patient can be expressed as organ absorbed dose (in mGy), and as effective dose to the body as a whole (in mSv). Strictly speaking, it is equivalent dose to an organ in mSv rather than absorbed dose which should be used; however for photons the two are numerically equal. The International Commission on Radiation Protection (ICRP) also uses a tissue weighting factor WT to describe the relative radiation sensitivity of various organs – the total of all WT values=1. Each organ equivalent dose is multiplied by its WT value, and the sum over all exposed organs is known as the effective dose, with the unit of Sv, or more commonly mSv.

 

Until recently (2), WT for the breast was 0.05, based on an assumed risk factor for fatal cancer. The highest weighting factor of 0.20 was for the gonads, based on assumed risks of hereditary effects as well as tumour induction and infertility impairment. All factors including breast are gender-averaged.

 

In 2007, the ICRP issued new Recommendations (3). These do not suggest any fundamental change in the system of radiation protection, but do take into account knowledge gained in the last 20 years. Although there has been no evidence that radiation exposure to the parent causes excess hereditable disease in the offspring the ICRP continues to believe that radiation can cause mutations to reproductive cells, but that the risk of hereditable diseases has been overestimated. As a result, the tissue weighting factors for gonads could be considerably reduced. The weighting factor for gonads was reduced to 0.08. However, the weighting factor for breast was increased to 0.12, based on a re-evaluation of epidemiological studies in exposed populations, in particular atomic bomb survivors.

 

What are the implications of this? Firstly breast now becomes the highest weighted organ, along with red marrow, colon, lung, stomach and all “remainder tissues”. Gonads have the second highest weighting. This has the effect of significantly increasing effective dose in cases where breast exposure is a significant component. Two good examples of this are in cardiac diagnosis and treatment, and the diagnosis of pulmonary embolism. In the latter case, this raises the question of the relative radiation risk of each technique, as well as the diagnostic value.

 

When looking at comparative doses between CTPA and V/Q scanning, there are two aspects to be considered – patient effective and relevant organ dose, but also foetal dose should the patient be pregnant.

 

In the following table of doses, we will consider two CTPA protocols – one what might be called “standard” (Toshiba Aquilion 16) and the other using lower kVp  (Toshiba Prime)– both used at Westmead Hospital. Breast dose reduction by use of breast shields is considered separately for the higher kVp technique. CT automatic exposure control is included only in the low kVp case.  The perfusion scan administered activity used is the most common (or mode) from an Australian/NZ diagnostic reference activity study (4).

 

Doses are from published sources, or calculated using references shown. The published effective dose references (5, 6) have not yet been updated for ICRP 103 tissue weighting factors, however the result would be little changed.

 

 

Modality

Agent and

Administered

Activity

CT Protocol

DLP

mGy.cm

Breast Dose

(mGy)

Foetal Absorbed

Dose (mGy)

~3 months

Effective dose

(patient) mSv

V/Q – perfusion

99mTc MAA

200 MBq

NA

NA

1.0 6

0.8 9

2.2 6

V/Q – ventilation

Technegas

40 MBq 1

NA

NA

0.3 6

Uterus

<0.02 6

 

0.6 6

V/Q - ventilation

99mTc DTPA aerosol 1

60 MBq

NA

NA

0.1 5

0.3 9

0.4 5

CTPA – full lung fields 7

NA

120 kVp, 150 mAs, 1mm slices, pitch 0.937, no breast shielding used, no AEC, single phase,  scan length 240 mm,

Toshiba Aquilion 16 slice

510

25

Uterus

0.03

11

CTPA – full lung fields + breast shields 7,8

NA

120 kVp, 150 mAs, 1mm slices, pitch 0.937, breast shielding used, no AEC, single phase,  scan length 240 mm,

Toshiba Aquilion 16 slice

457

12.5

Uterus

0.03

11

CTPA – full lung fields + low kVp 7

NA

100 kVp, auto mAs, 0.5 mm slices, pitch 0.81 7, no breast shielding, single phase, scan length 240 mm,

Toshiba Prime 80 slice

 

460

(scanner calculation)

18

Uterus

0.02

8.1

 

 

 

Notes and References:

 

  1. Activity actually inhaled by the patient (approximate, depends on patient status).
  2. International Commission on Radiological Protection, ICRP Publication 60 “1990 Recommendations of the ICRP”, Pergamon, 1991
  3. International Commission on Radiological Protection, ICRP Publication 103 “The 2007 Recommendations of the ICRP”, Elsevier, 2007
  4. Botros GM, Smart RC, Towson JE “Diagnostic reference activities for nuclear medicine procedures in Australia and New Zealand derived from the 2008 survey” ANZ Nuclear Medicine 40:4, 2-11, 2009
  5. International Commission on Radiological Protection, ICRP Publication 53 “Radiation dose to patients from radiopharmaceuticals”, Pergamon, 1988
  6. International Commission on Radiological Protection, ICRP Publication 80 “Radiation dose to patients from radiopharmaceuticals – Addendum to ICRP 53”, Elsevier, 1999
  7. ImPACT CT Patient Dosimetry Calculator Version 1.04, ImPACT, St Georges Hospital, London (using ICRP103 tissue weighting values)
  8. McLean ID et al, “Radiation safety report – breast dose and effects of shielding”, internal report, Medical Physics Dept. Westmead Hospital 2005 (showed 42% dose reduction for medium breast)
  9. Russell JR, Stabin MG, Sparks RB et al. “Radiation absorbed dose to the embryo/fetus from radiopharmaceuticals” Health Physics 73(5):756-769, 1997