Diagnostic : Answers :

071. A

072. A

073. c

074. E

075. c

076. D

077. E

078. D

20

Historically, the Nuclear Regulatory Commission (NRC) and Agreement States required patients receiving radionuclide therapy to remain hospitalized until the retained activity in the patient was less than 1 ll 0 MBq (30 mCi) or the dose rate at 1 m from the patient was less than 0.05 mSvlh (5 mremlh). However, in 1997 the NRC amended its regulations con-cerning radionuclide therapy patients through the issuance of new rules that appeared in the Federal Register on January 29, 1997. These NRC regulations, revised 10CFR 35.75 effective May 1997, allow for the release from medical confinement of patients if the expected total effective dose to individuals exposed to the patient is not likely to exceed 5 mSv.

For a controlled or restricted area, such as a radiopharmacy, surveys of ambient radiation levels must be performed daily, and wipe tests for removable radioactive contamination must be performed weekly. For unrestricted, uncontrolled areas, radiations surveys are required weekly, and wipe tests are required monthly.

The FWHM is the distance between the two arms of the curve corresponding to half the peak intensity.

The two gamma energies correspond to 419 - 24 7 ke V - 172 ke V and 247- 0 keY= 247 keV These two photon energies are emitted in cascade. The question asked to identify the emission energies. If the question had requested the peaks detected on a gamma camera, this would have included a third peak corresponding to 172 + 24 7 = 419 ke V, the sum peak of the two gamma photons.

NET counts per minute is given by the total of the cpm - background = l 000 cpm - 40 cpm = 960 cpm. Using the detector efficiency to convert from cpm to dpm (disintegrations per minute) yields 960/0.8 = 1200 dpm. When 1200 dpm is converted to disintegrations per second ( 1 dps = 1 Bq), then 1200 dpm /60 s/min = 20 dps = 20 Bq.

Currently BGO is used by General Electric; LSO by Siemens; GSO by Philips; Nal was used in the PENN PET system sold by GE and Philips earlier. Ceramics are used as CT detectors only.

Axial (depth) resolution is the ability of the ultrasound beam to separate two objects lying in tandem along the axis of the beam. The pulse length equals the wavelength of the beam times the number of wavelengths used. The axial resolution is defined as 1 I 2 of the pulse length. The highest frequency (10 MHz) has the shortest wavelength and a continuous beam has an infinite pulse length.

The greatest doppler shift is at the highest frequency with the beam parallel to the direction of blood flow.

Raphex 2008

079. A

080. B

081. A

082. D

083. A

084. E

085. B

086. c

087. E

Raphex 2008

Diagnostic ! Answers!

Attenuation (dB per em) is approximately proportional to frequency. Increased frequency results in a decreased depth of penetration and results in a shorter wavelength (frequency is inversely proportional to wavelength). Axial resolution is improved with higher frequency due to the shorter spatial pulse length. Higher ultrasound frequency has limited penetration depth, allowing higher pulse repetition frequency. The speed of sound is constant in tissue. It is independent of the frequency.

The attenuation in dB is given by: Attenuation (dB)= 0.5 dBIMHz-cm x 5 MHz x 4 em= 10 dB.

dB= 10 log (1;110 ) = 10 log (I/ 10 ) = 1

1/10 = 10

Coded aperture is a method of imaging with scintillation cameras without using standard collimators.

Deterministic effects are characterized by a dose threshold and increase in severity with dose. Cancer is a stochastic, not a deterministic effect, as it is thought that any dose of radiation can induce a cancer and the severity of the cancer is not dependent upon dose. Radiation-induced deterministic effects, such as skin erythema and desquamation, may occur within several days or weeks following irradiation. Other than the rare cases of skin damage resulting from prolonged fluoroscopy, the doses used in diagnostic radiology seldom cause deterministic effects.

21

Diagnostic ! Answers !

088. c

089. E

090. D

091. B

092. E

22

The human doubling dose, which is the dose to double the incidence of mutations in the offspring of irradiated people, has been estimated at l to 2 Gy. Radiation does not produce unique mutations, but only increases the incidence of known mutations that appear in the population. It is thought that any dose, however small, can induce mutations, including the radiation emitted by radioisotopes in the earth's crust. The results of epidemiologic studies are consistent with the conclusion that people are less sensitive than rodents for the induction of mutations by radiation.

As tabulated and published by the Report of Biological Effects of Ionizing Radiation (BEIR VII) and based on the linear, no-threshold dose-response model, current "best" estimate (BEIR VII value) for the lifetime risk factor for radiogenic fatal cancer from high radiation dose given over a short period of time (high dose rate) is approximately 800 excess cancers/million people/rem, rounded off to 1 000 excess cancers/million people/rem, or 1 00,000 excess cancers/million people/Sv.

The results of epidemiologic studies indicate that the fetus is most sensitive to the induction of mental retardation if the irradiation occurs during the 8th to the l51h week of gestation.

In a recent controversial review article [Brenner DJ and Hall EJ, (2007), "Computed tomography- an increasing source of radiation exposure." N Eng/ J Med 357(22):2277-2284] that addressed the increase in the use of CT, it was estimated that based on the use of CT from 1991 through 1996, 0.4% of all cancers in the United States may be attributable to the radiation from CT studies. However the authors went on to state that "by adjusting this estimate for current CT use, this estimate might now be in the range of 1.5 to 2.0%."

Individuals with potentially fatal wounds from the explosion should first be treated for their injuries before efforts are made to decontaminate them since it is likely that their level of exposure to radiation is not immediately life-threatening. If large numbers of people appear at a hospital emergency room following the detonation of a "dirty bomb," it may be best, so as not to overwhelm the facility, to only admit injured people into the hospital. Individuals who were not injured should be washed down in outdoor showers and then transported to a non-medical facility for further evaluation. The greatest concern for people within 200 miles of the explosion of a radiation dispersal device would likely be an increased risk for cancer and'genetic effects. It is unlikely that the dose associated with a radiation-dispersal device would be great enough to cause death from an acute radiation syndrome for most people i.n this range. Unless there is evidence that the bomb included high-energy y-ray emitting radioisotopes, it is reasonable to handle the victims using standard hospital gowns and gloves since the amount of isotope and the energy of the emitted radiation are likely to be low. There should always be regulation of people entering and exiting from the area where the victims of the accident are being treated so as not to contaminate other areas in the hospital with radioisotopes.

Raphex 2008

093. B

094. A

095. E

096. D

Raphex 2008

Diagnostic ! Answers!

NCRP Report No. 116 recommends (but does not regulate) that the maximum permitted effective dose (the dose to each organ multiplied by the tissue weighting factor) for a radiation worker is 50 mSv per year. Infrequent exposure for a member of the general public is 5 mSv per year. A minor is permitted exposure to radiation if it is received as part of their education (e.g., a 15-year-old student performing research in a radiology lab as part of a school project) and as long as the annual dose is held below 1 mSv. Continuous exposure for members of the public is limited to 1 mSv per year. A radiation worker may receive up to 500 mSv per year to the hands and feet.

"OH (hydroxyl radicals) are responsible for two--thirds to three-fourths of the damage produced by the indirect action of radiation resulting from the radio lysis of water.

The GSD is the average gonadal dose adjusted for the child expectancy of people who make up that population. The GSD for a radiology procedure performed only with post-menopausal women would always be zero, regardless of the number of people imaged, since none of these people is capable of conceiving a child. Radon inhalation would not affect the GSD since the a-particles produced by the radon daughter products located in the lungs would not irradiate the gonads. Although the current GSD may be somewhat greater due to the increased use of CT, the most recent survey estimated the average GSD associated with the performance of diagnostic radiology procedures to be 0.3 mSv.

The annual estimated dose resulting from exposure to cosmic rays, radioisotopes in the earth's crust, and inhaled/ingested radioisotopes, including radon, is approximately 3 mSv.

23

Therapy : Answers:

Tl. C

T2. D

TJ. B

T4. C

TS. C

T6. D

T7. C

T8. C

T9. E

TIO. B

Til. D

Tl2. B

24

TMR at ~x is 1.0 by definition for any photon energy, since it is the ratio of dose at depth d to dose at dmax

PDD increases with increasing SSD because it has two components: attenuation and inverse square. The inverse-square component decreases as distance increases.

MU = (doselfraction)I(SAD0 utput X TMR) = 1501(1.086 x 0.867) = 159.

MU =(dose/fraction) I (SSDoutput X PDD/100) = 150/(1.052 X 0.771) = 185.

The maximum tissue dose will be at depth dmax (1.6 em). The dose at this point is the entrance+ exit dose:

150(PDDdmax I PDDmidptane) + 150(PDDexit I PDDmidplane) = 150(100177.1) + 150(56.8177.1) = 305 = 300 X 1.017.

Neck dose= axis dose x (TMR d5 I TMR d9) = 3600 x (0.937/0.844) = 3997 cGy.

(Note: If neck point is not on the same plane as the isocenter, the inverse-square factors will almost cancel out.)

The depth of dmax is greater for 18 MV photons.

The maximum dose occurs at dmax This effect is field size dependent; for example, it would be 22 em for 8x8 em fields. It is also energy dependent: as energy increases, the thickness increases.

Any factor that increases the POD will decrease the total dose at dmax compared with the total dose at midplane. Treating at SSD rather than SAD gives a slightly higher POD.

POD increases with increasing SSD because of the change in the inverse-square factor. POD (130 em SSD, d8) = PDD(lOO em SSD, d8) x [(100 + 8)/(100 + dmax)f

X [(130 + dmax)/(130 + 8)F. For 6 MY, dmax is 1.6 em.

Percent depth dose is a combination of two factors: patient attenuation (which is independent of distance but varies with depth) and inverse-square falloff between dmax and depth. The longer the SSD, the smaller the inverse-square factor. Thus, POD increases with increasing SSD.

A rule of thumb for calculating C, the side of the equivalent square, is: C = 4 x area/perimeter= 4(a x b)/2(a +b). Equivalent squares enable the data for any rectangular field (TMR, POD, etc.) to be obtained from one table of square-field data.

Raphex 2008

Til. C

Tl4. B

TIS. D

Tl6. D

Tl7. A

TIS. A

Tl9. B

T20. B

T21. A

T22. C

T23. C

Raphex 2008

Therapy : Answers:

The equivalent square CxC of a rectangular field has the same PDD and TMR as the rectangle. It is smaller in area than the rectangle (i.e., CxC < 8x30, in this case), since it is the field with the same scatter contribution on the beam axis. A useful rule of thumb is that C = 4 (area/perimeter). The use of"equivalent square" enables PDD and TMR tables to be simplified to only square fields, rather than tabulating the many rectangular fields in use.

A universal wedge acts as a beam hardener, resulting in a slightly more penetrating depth dose. It also attenuates the beam along the central axis, which in turn requires more monitor units to deliver the same dose. Since the universal wedge is high up in the linac head, it does not increase scatter dose the way a conventional wedge does. The wedge transmission factor is a function of field size for both dynamic and universal wedges.

The "hinge angle" (between the axes) is 90. Wedge angle= (180- Hinge angle)/2 = (180- 90)/2 = 45. The smaller the hinge angle, and the closer the fields, the larger the wedge angle required.

There is a hot spot of 11 0% in the anterior of the PTV indicating that the plan is "underwedged." Increasing the wedge angle will reduce the hot spot anteriorly.

The lateral wedges compensate for the dose falloff across the volume from the open field. The greater the weight of the open field, the greater the actual difference in dose across the volume, and hence the greater the wedge angle required to compensate for this gradient. The difference between the two plans would be seen in the exit dose of the open beam, and the entrance doses of the wedged fields (i.e., femoral head vs. small bowel dose).

As SSD decreases, the patient will receive more scatter from the collimators. Skin dose increases as field size increases. Bolus is generally used to remove skin sparing and to bring the skin dose up to I 00%. Oblique incidence increases the skin dose.

Beam spoilers create electrons that scatter into the beam, and increase dose in the build-up region without removing skin sparing, as bolus would do. They are used in situations where the build-up characteristics of a lower photon energy are desirable, but a higher energy is needed for greater dose homogeneity, e.g., breast tangents with a very large separation (greater than 26 em).

The divergence of the spine field is tan- 1 [(25/2)/ 1 00] = tan- 1 0.125 = 7.

The formula for the gap (g) to be left on the skin between adjacent light fields of collimator settings C 1 and C2, if the match depth is dcm, is:

g = (d/SAD) x (C 1 + C2)/2 = (5.5/ 100) x (30 + 24)/2 = 1.5 em.

25

Therapy ! Answers !

T24. D

T25. C

T26. B

T27. C

T28. B

T29. E

TlO. A

Tll. C

T32. D

Tll. D

T34. A

TlS. B

26

Divergence= tan- 1(9/100) =5 for each field. To eliminate divergence, the RPO gantry angle = 60 + I 80 - (2 x divergence) = 230.

The attenuation of a 6 MV photon beam, for a lOxlO em field, is about 3.5% per em.

As photon energy increases, attenuation decreases, resulting in a greater POD at a given depth.

Flattening filters cannot flatten the beam equally at all depths; they tend to produce "horns" toward the edges of the beam at dmax and to underfiatten at d20.

Dmax for a single 18 MV beam occurs at 3.5 em, but the 95% isodose in the build-up region is closer to the skin. With parallel-opposed beams, the exit dose from the opposite beam brings the depth of the 95% dose even closer to the skin. Thus the depth of ~ax is not a reliable guide for the depth at which an adequate clinical dose is delivered.

The patient would be further away from the isocenter, and the dose would be lower by the inverse-square factor of (100/101.5f

TMRs are a measure of attenuation only, whereas PODs comprise attenuation and inverse square components.

The HVL (in Al or Cu) defines the penetrability of a low-energy X-ray beam. Different combinations ofkVp and filtration can produce beams with the same HVL, and hence the same depth dose characteristics. The SSD also affects the POD and is important for superficial x-ray units that typically treat at short SSDs.

For A- C, the opposite is true.

Spatial resolution in the caudal direction for CT-generated digitally reconstructed radiographs (DRRs) is compromised when scanning with large slice thicknesses. The disadvantage of using a small slice thickness is the increased size of the dataset, and hence possibly more work involved in contouring structures.

The greatest attenuation difference occurs for the lowest energy and the medium with the greatest effective depth difference. 10 em lung is approximately equivalent to 3 em muscle tissue, or 7 em missing tissue. 5 em of dense bone is approximately equivalent to 8 em muscle, or 3 em extra tissue.

In the other cases, the density on the CT is not representative of the density at the time of treatment. However, the physicist may determine that the correction is not necessary if it has a small effect on the dose distribution.

Raphex 2008

T36. C

T37. C

T38. D

T39. B

T40. C

T41. B

T42. D

T43. D

T44. D

T45. C

T46. B

T47. B

T48. A

T49. C

Raphex 2008

Therapy ! Answers !

The effect of blocking is greatest in a situation with the greatest scatter dose, i.e., at lower energy and greater depth.

Five HVLs are equivalent to 3% transmission. The additional dose is due to scatter from the surrounding tissue.

By similar triangle geometry: Field size at extended SSD =Extended SSD Field size at l 00 em 1 00

Therefore: Extended SSD = (75/40) x 100 = 187.5 em.

B .. 1 . 1 Sizeonskin _ 88 y Simi ar tnang e geometry: s . - ] OO .

Ize at 1socenter

The dose is due to internal scatter, head leakage, and scatter from the collimators and wedge.

2 Gy out of 40 Gy is 5%. This occurs at about 2 em from the field edge.

To a good approximation, the inverse-square law can be used to calculate the output at extended distance. For 1.0 cGy/MU at 100 em SSD, the dose rate at 4.5 m is 1.0 x (1.0/4.5)2 = 0.049 cGy/MU.

As SAD increases, PDD increases, thus increasing dose homogeneity for parallel-opposed fields.

Gamma Knife SRS is generally prescribed to 50% of the maximum dose.

Average MLC transmission is generally Jess than 2%, with transmission between adjacent leaves generally less than 3%. A 5 HVL block transmits about 3%.

The graph shown is a cumulative DVH (as opposed to a differential DVH). It depicts the fraction of the volume (y-axis) that receives at least the dose values given on the x-axis. For example, 100% of the volume receives at least 30% of the prescribed dose; 75% of the volume receives at least 50% of the dose; 25% of the volume receives at least 75% of the dose.

The lTV (internal target volume) is the expansion on the CTV (clinical target volume) due to organ motion. The PTV (planning target volume) is an expansion around this to account for setup error and gross patient motion.

With IMRT, 6 MV photons can give good results even for large depths (e.g., for a prostate). However, the integral dose outside the PTV will generally be greater for lower energy.

27

Therapy : Answers:

TSO. E

TSI. E

T52. C

TSJ. B

T54. A

TSS. A.

T56. B

T57. C

T58. D

T59. B

T60. A

The accuracy of leaf positioning affects the width of the gaps in an IMRT field. The dose delivered is sensitive to small variations in gap width. Therefore, the dose is sensitive to errors in leaf position. This is true for step-and-shoot IMRT as well as for sliding-window IMRT.

Dose differences within 3% are bard to achieve in high-gradient regions due to spatial uncertainties in dose calculation and measurement. The DTA criteria is designed to compensate for small spatial errors in high-gradient regions.

At diagnostic energies, the probability of photoelectric interactions increases as Z3, which magnifies the difference in attenuation between bone and tissue. In MV beams, the Compton effect predominates, with virtually no photoelectric effect, and shows differences in electron density rather than Z.

Because the beam area is larger, there is more scatter dose in a cone beam scan, which reduces contrast and overall image quality. Blurring due to respiration and patient motion can also decrease image quality in a cone beam scan. Motion artifacts are possible in a spiral CT, but these do not lead to blurring.

Gadolinium is a ferromagnetic agent that is useful in the imaging of a variety of lesions. It is always important to verify that adequate gadolinium was delivered, and looking at the intensity of uptake (i.e., brightness) ofthe nasal mucosa can help confirm this.

CT and MRI resolution depend on field of view (FOV), but are usually I mm or less. PET scan resolution is limited to a few millimeters, mainly because of the finite range of the positrons, and also because the 511 ke V annihilation gammas are not exactly antiparallel.

Tumors and soft-tissue differences in the brain are usually more easily seen in MR images.

The range of 6 MeV electrons is 3 em in tissue, or 2 em tissue+ 1 em tissue-equivalent lung. Since lung density is 1 I 4 that of soft tissue, 1 em in tissue is approximately equivalent to 4 em in lung. Thus, the total range is 2 + 4 = 6 em.

When electrons interact with high Z components in the collimation system (scattering foils, collimator jaws, etc.), bremsstrahlung, and a smaller number of characteristic x-rays, are produced. Interactions with tissue also produce bremsstrahlung, but about one order of magnitude less. The bremsstrahlung "tail" increases with increasing energy, but is usually in the range of 1% to 5%. -

The rule of thumb is that the MeV /3 - MeV /4 is the approximate depth, in centimeters, of the 90% isodose level. This depends on the linac manufacturer and the design of the collimator system.

28 Raphex 2008

T61. D

T62. D.

T63. C

T64. D

T65. C

T66. A

T67. C

T68. C

T69. D

T70. D

T71. C

Raphex 2008

Therapy : Answers:

All the isodose curves move up towards the skin.

Care should be taken when using small inserts in larger cones, especially for higher energy. The depth of the 90% isodose can be significantly less due to lack of scatter from the blocked area. If the depth is adequate, the output for the small insert will need to be measured; and the skin dose and build-up characteristics should be expected to change also.

MU = dose/(output x PDD/ 100) = 200/(1.13 x 0.93).

The range of electrons in lead is approximately 2 MeV/mm. The range in Cerrobend is 20% greater; i.e., (12/2) x 1.2 = 7 mm.

Electrons alone would treat the volume adequately, and would deliver the least dose to the cord and the contralateral parotid. The reason for adding photons is to reduce the skin dose, and the trade-off is increased cord dose, which must still be kept below tolerance. Adding photons does not increase the depth that can be covered homogeneously.

X-ray contamination in an electron beam is mostly along the central axis and angling the beams reduces X-ray dose to the patient. Angling the beams also produces a more uniform dose distribution over a larger area.

A= A0 exp -(0.693 x 7/17) = 0.75 A0.

The measured dose rate will be lower by the ratio of the exposure rate constants, i.e., 3.26/8.25 = 0.40. Also, whereas 160 mCi (64 mg Ra eq) is a reasonable loading for a Fletcher-Suit type applicator, 160 mg Ra eq would be unusually high.

Units of Ci and Bq are related to the number of disintegrations per unit time. Air kerma rate (AKR) has units of dose rate. Air kerma strength (AKS) = AKR x d2

Assume a point source of 240 mg Ra eq at the center of the container. Exposure rate at 10 em= [activity (mg Ra eq) X rR.] /d2

= (240 X 8.25)/102 = 19.8 R/h. 4 TVLs will reduce this to 1.98 mR/h.

The dose distribution around a seed is "anisotropic," i.e., lower along the seed axis, due to self-absorption, than perpendicular to the axis. Most treatment planning systems treat the seed as a "point source," as the orientation is generally unknown. However, this would overestimate the average dose rate at a given distance from the source, and is corrected by using the anisotropy correction, to give the average dose rate around an actual seed.

29

Therapy ! Answers !

T72. B

T73. D

T74. D

T7S. A

T76. D

T77. B

T78. D

T79. C

TSO. B

T81. D

T82. A

T83. C

Total dose= Initial dose rate X Tmean = 1600 cGy, where Tmean = T 112 * 1.44. After two half-lives, the dose rate has dropped to 0 .25 x Initial dose rate. The total dose delivered from this point on will therefore be 0.25 x 1600 cGy, and the dose delivered during the first two half-lives will be (1 - 0.25) x 1600 = 1200 cGy.

In 3 years 25 mCi will decay to 25 x exp -(0.693 x 3 x 365/60) = 8 x 10-5 mCi. The dose rate at 1 ern, assuming a point dose, is: 1.45 x 8 x 10-5 = 0.12 mR/h. This exposure rate is so low that it poses no hazard to staff. Even films taken immediately after an implant show no sign of fogging, since the dose rate at 1 m from the patient is generally close to background.

The ovoid closest to the cassette will appear smaller. However, this is a small difference and may not be helpful in distinguishing the ovoids.

The distanced is given by: d2 = x2 + y2 + 2 2. Thus d2 = 32 + 42 +52 = 50; d = 7.1

Radioactive materials must be correctly packaged and labeled according to DOT regulations. They cannot be carried on public transportation.

A sensitive detector is the fastest and most reliable way to verifY that no sources have been left in the patient or dropped in the bed during removal. This should be done as soon as the sources have been removed because of the serious consequences to the patient if sources are left in place longer than intended. The removed sources should be moved away from the patient's immediate area during the measurement. The sources must be counted before return to the manufacturer, but this does not have to be done immediately at the patient's bedside.

Doubling the distance reduces the exposure by a factor of 4. A lead apron (typically containing 0.5mrn Pb) is ineffective for 137Cs photons (660 keV, HVL 5.5 mm Pb).

The transport index is the maximum dose rate (in mR/h) at 1 m.

The half-life of 103Pd is 17 days, so 131 Cs delivers the dose faster, requiring a higher initial air kerrna rate. The higher energy makes the dose distribution more homogeneous and is more forgiving if the seed placement is not exactly as planned.

30 Raphex 2008

T84. A

T85. C

T86. A

T87. C

T88. A

T89. D

T90. D

T91. D

T92. B

T93. D

T94. E

T95. B

Raphex 2008

Therapy ! Answers!

Because ofthe inverse-square law, the PDD at a given depth from the surface of an applicator increases as the diameter increases.

Some brachytherapy planning systems allow iiiteractive optimization, in which an isodose distribution can be locally shaped by dragging an isodose line with the cursor, hence altering dwell weights in real-time. This can be a useful tool; however, a small displacement of the 1 00% isodose cloud can have a large effect on the dose at point A. This is due to the inverse-square law, which has a powerful effect at the short distances encountered in brachytherapy. Point A is 2 em from the tandem at the closest point. If all the sources were at 2 em, the increase would be (2.2/2.0)2 = 1.21; i.e., 21%. However, the source distances range from 2 em to about 5 em, depending on the actual geometry, so the actual difference is closer to 10%. One can use this optimization tool to shape the isodose cloud, but it is usual to renormalize the plan to deliver the prescribed dose to point A.

The product of activity x time (Ci x seconds) is constant. This product should be verified before each treatment, as part of the QA.

When the new source strength is entered at the treatment unit, the times for stored plans are automatically adjusted for the new source strength. Checking that the product of source strength and time are constant for a plan is part of the pre-treatment QA.

An air pocket pushes the volume of tissue to be treated away from the balloon, thus potentially underdosing it. The recommended maximum volume is 10% of the PTV

The scattering foil is used in the electron mode to create a large, flat beam.

Neutrons are created by high-energy photons (and to a lesser extent electrons) incident on the high Z components in the head of the linac.

The chamber is calibrated at an accredited calibration lab at standard temperature and pressure (22C and 760 mm Hg). The correction factor corrects the reading back to what it would have been under calibration conditions. Dose is related to charge collected per unit mass of gas in the chamber, and the mass changes as the gas expands or contracts, assuming the chamber is not sealed.

31

Therapy ! Answers !

T96. C

T97. E

T98. C

T99. B

TIOO. A

32

Parallel-plate chambers can also be used.

Using concrete for primary shielding will also shield for neutrons. Lead is not a good attenuator for neutrons because of its high Z value, and must be supplemented by additional neutron shielding, typically borated polyethylene.

At 6 MY, Compton interactions predominate, and attenuation per unit thickness is proportional to the mass density of the material. Thus, the equivalent thicknesses of concrete and steel are in approximately the inverse ratio of their densities.

Lead is inefficient as a neutron moderator, but it is placed in the door downstream from the borated polyethylene to attenuate the capture gammas. The neutron dose is about I 0 times higher for photons than for electrons. The threshold for neutron production is 8 MeV, but supplemental shielding is not required below I 0 MeV

1 0 HVLs attenuate by a factor of 1 I 210 = I I I 024. B is 5 HVLs, and C and D are both 2 TVLs = 11100 (regardless of the material used).

Raphex 2008