Chapter 6: Simulation and Treatment Planning

35. A patient is being treated isocentrically with four fields (equally weighted) to their pelvis using 10× photons. The AP SSD is 90 cm, Right Lateral SSD is 78 cm, PA SSD is 90 cm and left lateral SSD is 82 cm. The total tumor dose per fraction is 180 cGy. What information will you need before you can look up the factors to complete the monitor unit calculation? A. Field size B. Patient position C. Location of patient tattoos D. Gantry angles

A

16. Which of the following are considered when designing fields and beam arrangements? (Choose all correct answers). A. Total dose B. Beam energy C. Patient setup

A, B, and C. When designing fields and beam arrangements, one must consider dose and fractionation scheme, isocentric or isometric setup, modality (photons or electrons), beam energy, number of fields needed to decrease dose to normal tissue, fixed or rotational beams, field weighting, and beam attenuation in tissue. Bone attenuates radiation beam more than tissue, while lungs attenuate less.

18. Which of the following beam modifying devices would not be used to compensate for missing tissue in a patient? A. Cerrobend blocks B. Bolus C. Compensating filters D. Wedges

A. Compensating filters compensate for missing tissue. The filter is made of tissue-equivalent material to a thickness equal to that of tissue missing. Compensating filters allow for improved dose distribution while preserving skin-sparing effect. Bolus is tissue equivalent material placed directly on patient skin which can be used to compensate for missing tissue or increase dose to skin. Bolus must be flat on skin and air gaps must be eliminated. Wedges may also be added to compensate for missing tissue.

24. The _______________ is the projection of what the treatment field will look like, from the point of view from the origin of the beam. A. Beam's eye view B. Isodose distribution C. Dose volume histogram D. Field size view

A. Fields and beam arrangements are evaluated with several tools. The beam's eye view (BEV) is the projection of what the treatment field will look like, from point of view from the origin of beam. Isodose distributions are graphical representation of how dose is deposited within the tissue. Dose volume histograms (DVH) are graphical representation of volume of organ vs. dose received.

8. Thermoplastic immobilization devices are used to immobilize the: A. Head and neck B. Chest C. Abdomen D. Pelvis

A. Immobilization devices are used to immobilization the patient's anatomy being treated. Historically, tape was used. Thermoplastic immobilization devices are used for immobilization of the head and neck areas. Chemical molds may be used for immobilizing chest, abdomen or pelvis, as are vacuum molds. With IMRT treatment, abdominal compression devices are used when treating chest and abdomen to immobilize tumors in this area.

1. Which of the following is a negative contrast agent? A. Air B. Iodine C. Barium sulfate D. Nonionic venous contrast

A. Negative contrast shows up dark on images, as it absorbs less radiation than tissue. Oxygen and air are most common types. Positive contrast shows up light or white on images, as it absorbs more radiation than tissue. Compounds with high atomic numbers are positive contrast agents. Barium sulfate and iodine are the common types of positive contrast agents.

29. Percent depth dose increases with: A. Increased energy B. Decreased field size C. Increased depth D. All of the above

A. Percent depth dose increases with increased energy, increases with increased field size, decreases with increased depth, and decreases with increased SSD.

3. Prior to the administration of intravenous contrast, you should assess the patient for: A. Shellfish allergy B. Lung function C. Joint pain D. GI complications

A. Prior to administration, it is important to check kidney function prior, as well as questioning the patient for allergies to prior contrast and shellfish.

17. Which of the following tumors would best be treated with an arc treatment? A. Central lung tumor B. Inflammatory breast cancer C. Ewings sarcoma D. Nasopharyngeal tumor

A. Rotational fields are useful for central, well-defined tumors. The isocenter is set past tumor (past-pointing) to ensure point of highest dose is in the target.

15. Which of the following is true regarding the forward planning technique? A. Plan is optimized after fields are designed B. Computer design treatment fields based on set criteria C. Is used for IMRT planning D. Is only used for brachytherapy planning.

A. Treatment planning can be forward or inverse. With forward planning, the beams are designed and then changed to optimize the plan. With inverse planning, the computer program designs beams, based on criteria set at beginning of planning session; used for IMRT treatment planning.

13. What volume accounts for the patient's physiologic movement, such as breathing? A. Clinical target volume B. Internal margin C. Gross target volume D. Motion volume

B

14. What term describes critical structures in or near the treatment field? A. Internal target volume B. Organs at risk C. Irradiated volume D. Internal margin

B

21. What device would be used in order to compensate for missing tissue as well as increase the dose to the skin? A. Compensating filter B. Bolus C. Wedge D. Hand block

B

4. The hyoid bone is located at what vertebral level? A. C1 B. C3 C. T1 D. T3

B

5. The SSN corresponds to vertebral level: A. C3 B. T3 C. T6 D. L3

B

7. What positioning device is used to move a patient's small bowel out of pelvic treatment fields? A. Shoulder assistance straps B. Bellyboard C. Wingboard D. Thermoplastic device

B

12. What is not included in the CTV? A. GTV B. Margin for motion C. Microscopic disease D. All of the above are included in the CTV

B. After acquiring images, tumor, target, and organs at risk are identified by radiation oncologist and planning dosimetrist and/or physicist. Volumes used in treatment planning are: • Gross tumor volume (GTV): tumor itself • Clinical target volume (CTV): GTV+margin to include anatomy which may have microscopic disease • Planning target volume (PTV): CTV+margin for setup uncertainty or patient movement - Internal margin (IM): margin to account for patient's physiologic movement - Internal target volume (ITV): CTV+IM • Treated volume: anatomy receiving prescription dose • Irradiated volume: area that receives dose significant to normal tissue tolerance • Organs at risk (OAR): critical structures in or near treatment field

28. Backscatter factor is the: A. Ratio of dose with modifier in beam to dose without modifier in beam B. Ratio of dose at dmax in phantom to dose at dmax in air C. Target-to-axis distance D. Ratio of absorbed dose at depth to dose at dmax

B. Backscatter factor (BSF) is the ratio of dose at dmax in phantom to dose at dmax in air, and may be referred to as peak scatter factor. BSF is dependent of energy and field size but independent of SSD. Sp (phantom scatter factor)=ratio of BSF for given field to BSF for reference field (usually 10×10 cm). Attenuation factor is the ratio of dose with modifier in beam to dose without modifier in beam and used for wedges, block trays, and compensators. Percent depth dose is the ratio of absorbed dose at depth to dose at dmax.

20. Multileaf collimators (MLC) are made of: A. Lipowitz metal B. Tungsten C. Lead D. Brass

B. Multileaf collimators are computer controlled blocking in the treatment machine used to customize treatment fields. Made of tungsten, these devices can move during treatment to modulate treatment beam.

27. The ________ factor is the ratio of dose at dmax for field size to dose at dmax for standard field size. A. Attenuation B. Output C. Back scatter D. Tissue maximum

B. Output factor is the ratio of dose at dmax for field size to dose at dmax for standard field size (typically 10×10 cm). Output factor may also be referred to as Sc (collimator scatter factor) and is typically 1 cGy/MU at dmax with 10×10 cm field. Output factor increases with increased field size.

31. The ratio of scattered dose at depth in phantom to scatter dose at depth in air is: A. Tissue-air ratio (TAR) B. Scatter-air ratio (SAR) C. Tissue maximum ratio (TMR) D. Tissue phantom ratio (TPR)

B. Tissue-air ratio (TAR) is the ratio of dose at depth in phantom to dose at depth in air. Scatter-air ratio (SAR) is the ratio of scattered dose at depth in phantom to scatter dose at depth in air and used for irregularly shaped fields. Tissue maximum ratio (TMR) is the ratio of dose at depth to dose at dmax, while tissue-phantom ratio (TPR) is the ratio of dose at depth to dose at reference depth.

33. Calculate the gap required for adjacent fields (Field A—length 12, Field B—length 20) treated at 100 cm SSD with 6 MV photon beams to a treatment depth of 5 cm. A. 0.5 cm B. 0.8 cm C. 1.3 cm D. 1.5 cm

B. To calculate the skin gap of adjacent photon fields.

23. Calculate the hinge angle when using 30° wedges in a wedge pair field arrangement. A. 60° B. 120° C. 180° D. 240°

B. Wedge pairs are when two fields use wedges to modify the isodose distribution. The hinge angle is angle between two fields. The formula for hinge angle=180°−(2×wedge angle). When using wedge pairs, heels are placed together.

22. What is a limitation of using wedges in a patient's treatment plan? A. Increased time needed for treatment planning B. Field size limitations C. Limitations in gantry angles that can be used D. Little variety in available wedge angles

B. Wedges are used to alter isodose distribution in patient or compensate for sloping surfaces. Physical wedges come in 15°, 30°, 45°, and 60° angles and have field size limitations. The heel of wedge is the thicker part of wedge, while the toe is the thinner portion. When compensating for a sloping surface, the heel is placed towards area missing tissue. Dynamic wedges are when the jaw of collimator moves during treatment to simulate a wedge in treatment field.

34. Calculate the monitor units needed for a patient being treated AP/PA to their abdomen with a 12×16 cm field, SSD=100 cm, treated midplane to a depth of 7 cm, Sc=1.08, Sp=1.007, %dd=76 %, tumor dose to target is 180 cGy A. 90 MU B. 101 MU C. 109 MU D. 118 MU

C

19. Blocks are ____ HVL thick. A. 1 B. 3 C. 5 D. 7

C. Blocks must transmit less than 5 % of the treatment beam, thus are 5 HVL thick. Hand blocks are non-divergent standardized blocks, which are placed in slotted block tray and placed clinically. They are useful for emergency treatments. Custom blocks are made of cerrobend, or Lipowitz metal, and are custom to patient and field. These divergent blocks can be positive (block center of field) or negative (block outside of field).

2. Which of the following is not true regarding iodine contrast? A. Can be administered intravenously B. Kidney function must be checked prior to administration C. Is ionic only D. Patients allergic to shellfish should not receive iodine contrast

C. Iodine contrast can be ionic (increased risk for allergic reactions) or non- ionic. It is administered intravenously to visualize vessels and kidneys or through catheter into bladder.

30. What is the Mayneord's factor for a patient who was originally treated at 100 cm SSD to a depth of 5 cm with a 10× photon beam, but now needs to be treated at 110 cm SSD? A. 0.990 B. 0.993 C. 1.003 D. 1.010

C. Mayneord's factor is used for SSD calculation when the patient is treated with a different SSD:

9. When obtaining images through conventional simulation, what is true regarding mA? A. Controls contrast of the image B. Represents the quality of the beam C. Controls density of the image D. All of the above are true

C. Milliamperage (mA) is the quantity of beam and controls overall density. Kilo voltage potential (KvP) is the quality of beam and controls contrast. When taking images, follow the 15 % rule—an increase of 15 % of KvP should also include a decrease of mAs by 50 %.

10. What is not a step of CT simulation? A. Scanning the area of interest per physician order B. Imaging with slices of 2-8 mm C. Taking orthogonal images D. Ensuring the patient anatomy and simulation marks are included in field of view as the image is reconstructed.

C. Orthogonal images are reconstructed after the CT simulation is completed.

6. An example of a positioning device is: A. Thermoplastic mask B. Chemical mold C. Head holder D. Vacuum mold

C. Positioning devices are used to help patient keep position each day and can be used for multiple patients. Examples are head holders, knee sponges, shoulder assistance straps, and wing boards. Belly boards are used to move the patient's small bowel out of pelvic treatment fields.

36. The practical range of a 12 MeV electron beam is: A. 3 cm B. 4 cm C. 6 cm D. 12 cm

C. Range of electrons is based on energy. The practical range (Rp) is the depth electrons travel in tissue, and is calculated by MeV of beam/2. The depth of 80 % isodose curve=MeV of beam/3, while the depth of 90 % isodose curve=MeV of beam/4.

26. Calculate the equivalent square for a 6×12 cm field size. A. 4×4 cm B. 6×6 cm C. 8×8 cm D. 10×10 cm

C. The equivalent square of treatment fields is used for dose calculations. This takes into account different scattering properties of different shaped fields. Equivalent square of field=4× (area of field/perimeter of field).

11. After CT simulation, what must be documented by the radiation therapist? A. Gantry angle B. Collimator angle C. Field size D. Patient setup

D. Following the CT simulation procedure, the radiation therapist should document information to reproduce patient setup (position, tattoo location, positioning aids, and immobilization devices used), patient measurements, and special instructions.

32. TAR is independent of: A. Energy B. Field size C. Depth D. SSD

D. TAR increases with increased energy and increased field size, decreases with increased depth, and is independent of SSD. This factor is used in SAD calculations. TAR=BSF when measured at dmax.

25. What is not included in the radiation therapy treatment prescription? A. Dose per fraction B. Beam-modifying devices C. Treatment volume D. Patient position

D. The radiation therapy treatment prescription must include treatment volume and dose, fractionation scheme (number of fractions, dose per fraction and scheduling of fractions), information regarding treatment (mode and energy of radiation and beam modifying devices).