RELACIÓN DE VISITAS REALIZADAS EN LA COMUNIDAD DE MADRID 8

Table 3.12 and Figure I.1 (Appendix I) provide a comparison of mean organ and tissue absorbed doses from the TLD measurements for the three CT examinations assessed. Without taking into account the relative radiosensitivities of the organs and tissues, the highest absorbed doses are to the bone surface, brain and eye lenses for the CT brain examination. Despite the angling of the gantry, the eye lenses still receive a significant dose. The highest thyroid absorbed dose is from the CT chest examination and this is dependent on each individual scan and the selection of collimation as the thyroid may or may not be within the scan volume. Even when it is outside the scan volume it is likely to receive a dose from scattered radiation, over-beaming and over-ranging in the case of helical examinations when the chest is being imaged. As expected, organs within the scan volume receive higher absorbed doses than those outside the scan volume.

Lens Absorbed Dose

The lens of the eye does not need to be considered for stochastic effects. However, it is useful to consider the deterministic effects, such as cataracts, which have a threshold of

CHAPTER 3. EXPERIMENTAL DOSIMETRY

Table 3.12: TLD measured organ and tissue absorbed doses in an anthropomorphic phantom of a 10 year old child for different CT exam- inations.

Organ/Tissue Absorbed Dose (mGy)a

Mean±2σ

CT Brain CT Chest CT Abdomen/Pelvis

Bone Marrow 6.1±0.3 2.4±0.1 3.2±0.1 Colon 0.026±0.001 0.52±0.03 8.0±0.4 Lung 0.46±0.02 9.9±0.4 4.1±0.2 Stomach 0.068±0.003 4.8±0.2 8.9±0.4 Breast 0.27±0.01 9.1±0.4 1.29±0.06 Gonads 0.0127±0.0006 0.080±0.004 3.7±0.2 Bladder 0.0111±0.0006 0.080±0.004 6.9±0.3 Oesophagus 0.48±0.02 9.2±0.4 3.3±0.1 Liver 0.118±0.005 10.5±0.5 8.5±0.4 Thyroid 1.67±0.07 10.9±0.5 0.24±0.01 Bone Surface 5.0±0.2 3.4±0.2 3.6±0.2 Brain 33.6±1.4 0.22±0.01 0.033±0.001 Salivary Glands 2.7±0.1 1.32±0.06 0.117±0.005 Skin 2.0±0.3 1.38±0.09 1.7±0.1 Total Remainder 0.49±0.02 4.4±0.2 4.9±0.2 Adrenals 0.094±0.005 5.1±0.3 6.9±0.3 ET Region 1.67±0.07 10.9±0.5 0.24±0.01 Gall Bladder 0.054±0.003 2.7±0.2 8.6±0.5 Heart 0.51±0.02 8.6±0.4 1.02±0.05 Kidney 0.064±0.003 1.80±0.09 8.3±0.4 Lymph Nodes 0.53±0.02 2.5±0.1 2.8±0.1 Muscle 0.53±0.02 2.5±0.1 2.8±0.1 Oral Mucosa 2.7±0.1 1.32±0.06 0.117±0.005 Pancreas 0.084±0.004 6.4±0.3 8.7±0.4 Prostate 0.0087±0.0007 0.057±0.005 5.8±0.4 Small Intestine 0.023±0.001 0.36±0.02 8.5±0.4 Spleen 0.156±0.007 9.2±0.4 7.9±0.4 Thymus 0.47±0.02 9.3±0.4 0.81±0.04 Uterus/Cervix 0.0126±0.0009 0.115±0.008 6.6±0.4 Eye Lenses 19.3±0.9 0.191±0.009 0.040±0.002 Testes 0.0115±0.0007 0.029±0.003 0.99±0.05 Ovaries 0.0139±0.0008 0.131±0.007 6.4±0.3 a

Errors are expressed as two standard deviations of the combined random and systematic errors.

CHAPTER 3. EXPERIMENTAL DOSIMETRY

Table 3.13: TLD measured absorbed doses to soft tissue in the colon consist- ing of the upper large intestine (ULI) and lower large intestine (LLI) and the bone locations for the red bone marrow (RBM) and bone surface (BS) mea- surements in an anthropomorphic phantom of a 10 year old child for different CT examinations.

Organ/Tissue Absorbed Dose to Soft Tissue (mGy)

CT Brain CT Chest CT Abdomen/Pelvis

Ascending colon (ULI) 0.03 0.34 8.50

Transverse colon (ULI) 0.03 1.04 8.28

Descending colon (LLI) 0.03 0.33 8.63

Rectosigmoid colon (LLI) 0.01 0.12 6.04

Pelvis (RBM/BS) 0.01 0.10 6.06 Ribs (RBM/BS) 0.49 9.09 4.66 Skull (RBM/BS) 41.09 0.14 0.03 Spine – middle (RBM/BS) 0.16 7.73 4.52 Spine – lower (RBM/BS) 0.02 0.32 5.59 Scapulae (RBM/BS) 0.55 8.05 0.59

Table 3.14: Values of effective dose for standard paediatric CT examina- tions calculated from TLD measurements of absorbed dose using the ICRP 60 (ET LD,60) and ICRP 103 (ET LD,103) formalisms. The percentage difference be- tween the values is also provided.

Examination ET LD,60 (mSv) ET LD,103 (mSv) (ET LD,103-E60)/E60

CT Brain 1.8 1.4 -22%

CT Chest 4.3 5.0 16%

CT Abdomen/Pelvis 5.0 4.8 -4%

about 0.5 Gy (ICRP, 2011). The TLD measured absorbed dose to the lens was 19 mGy for the CT brain examination. This is similar to the absorbed dose (15±2 mGy) mea- sured in another study (Heaney and Norvill, 2006) for adult CT head examinations with gantry angulation. Despite angling the gantry as a dose saving technique, the lens dose remains relatively high although well below the deterministic threshold. This dose may have resulted from scattered radiation or over-beaming, where the penumbral region of the X-ray beam is not used for image formation. This ensures uniform exposure of the MDCT detectors, but results in exposure of the patient that does not contribute to the data acquisition. The portion of the beam missing the detectors is greater for smaller collimations as was the case for the brain examination.

Directly Irradiated Organs

Measured absorbed doses for multiple TLDs located in directly irradiated large organs varied throughout the organ volume. This is due to differences in photon attenuation for measurement locations that are superficial compared with those that are deeper within

CHAPTER 3. EXPERIMENTAL DOSIMETRY

an organ and/or relative to the body and other structures (e.g. bony) within it. The individual measured absorbed doses in the brain for the scan of the head region ranged from 25.9 to 41.2 mGy. This variation in dose is also attributable to non-uniformity in the exposure because of gantry angulation. The range of absorbed doses in the lung for the chest examination was 8.4 to 11.4 mGy and in the liver was 7.3 to 9.8 mGy for the abdomen/pelvis examination. It is therefore necessary to use multiple measurement locations across larger organs, so that the mean absorbed dose adequately reflects the heterogeneity in dose deposition.

Partially Irradiated Organs

Organs on the periphery of the scan volume can have a significant variation in absorbed dose across the organ due to partial irradiation. For example, as already discussed, the measurement points in the kidney for the chest examination resulted in absorbed doses of 2.8, 1.7 and 0.9 mGy (cranial to caudal). For the brain and abdomen/pelvis examinations where the kidney was either completely outside the scanned volume or located directly within it, the absorbed doses were more consistent. For the brain examination the three kidney measurement points were all 0.1 mGy and, cranial to caudal, the kidney absorbed doses were 8.3, 8.2 and 8.4 mGy for the abdomen/pelvis examination. Averaging of multiple measurement points is necessary for organs which are partially irradiated.

Selecting the location of an organ or tissue can have a considerable effect on the measured absorbed dose when the organ or tissue is located on the edge of the scan volume. The thyroid is on the periphery of the scan range for the chest examination and therefore may or may not be directly exposed depending on operator technique. For the chest examination performed on the anthropomorphic phantom, the dose to the thyroid (10.9 mGy) appears to indicate that it is in the primary beam. However, this dose may also be due to over- beaming and/or over-ranging.

It is clear from Figure 3.8 that absorbed doses of equivalent magnitude to those in the directly irradiated scanned volume were measured outside of the imaged range. The dotted lines on these figures represent the estimated additional scan length due to over-ranging (van der Molen and Geleijns, 2007) necessary for data interpolation in helical scans. The additional length at the beginning and end of the imaged volume correlates well with absorbed doses measured beyond the imaged volume. Over-ranging is greater for larger beam collimation and higher pitch, which may reduce scan times, but at the cost of increased dose in the over-ranging region (Fujii et al., 2009).

The extrathoracic (ET) region is included as a remainder organ in ICRP 103. It was dif- ficult to determine a location in the anthropomorphic phantom for this tissue and instead the thyroid absorbed dose was used as a substitute. This may be a poor approximation when the scan volume is adjacent to this region since the thyroid may be in the scan range, while the ET region is not, but it will still be allocated a dose as though it were ex-

CHAPTER 3. EXPERIMENTAL DOSIMETRY

posed to the primary beam. This is a recognised limitation of substituting organ absorbed doses.

Out-of-Field Doses

Out-of-field doses have been measured for tissues and organs outside the scanned volume for all three CT examinations (Figure 3.8). There is a gradual reduction in dose with distance from the scanned volume for each examination. The extent of out-of-field doses is greatest for the brain examination where the mAs (current exposure time product) is more than three times higher than either body examination and where the gantry was angled towards the body for the examination. It is interesting to note that organs and tissues outside of the scanned volume for the brain examination, such as the lung, breast, oesophagus, thymus, heart and liver, have all received a small absorbed dose. This may result from the uncertainty in measurement for the TLDs at low doses or may arise from scattered radiation. The increased photon flux and/or gantry angulation may have resulted in a higher scattered radiation dose to distant organs for this examination. Alternatively, since the brain is located at a distal part of the body, there is greater body length for the scattered dose to be deposited and hence measured, compared with an examination of the chest or abdomen/pelvis.