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# Treatment Planning

### Radiation Therapy Treatment Planning

Question | Answer |
---|---|

Radiation Therapy prescription | Communication tool between the radiation oncologist and the treatment planning and delivery team (medical dosimetrist and radiation therapist) and provides the information required to administer the appropriate radiation treatment (W/L, pg. 493). |

Components of RT prescription | Defines the treatment volume, intended tumor dose, number of treatments, dose per treatment, and frequency of treatment. Also stated are the type and energy of radiation to be used, beam-shaping devices and any other appropriate factors (W/L, pg. 493). |

Dose (or absorbed dose) | Refers to the energy deposited at a specific point in a medium. The dose is measured at a specific point (in a patient or phantom) and is commonly measured in Gray (Gy). (W/L, pg. 493). |

Depth | The distance beneath the skin surface where the prescribed dose is to be delivered (W/L, pg. 494) |

Separation | The measurement of the patient’s thickness from the point of beam entry to the point of beam exit (W/L, pg. 494). |

SSD | The distance from the source of photons to the patient’s skin surface (W/L, pg. 494). |

SAD | The distance from the source of photons to the machines isocenter (W/L, pg. 494). |

Isocenter | The intersection of the axis of rotation of the gantry and the axis of rotation of the collimator for the treatment unit (W/L, pg. 494). |

Field Size | Physical dimensions set on the collimators of the therapy unit that determine the size of the treatment field at a reference distance (W/L, pg. 494). |

Depth of Maximum Equilibrium (Dmax) | The depth at which electronic equilibrium occurs for photon beams. Dmax is the point where the maximum absorbed dose occurs for single field photon beams and depends mainly on the energy of the beam (W/L, pg. 496). |

Output | The dose rate of the machine; it is the amount of radiation “exposure” produced by a treatment machine or source as specified at a reference field size and a specified reference distance. (W/L, pg. 496). |

Output Factor | Ratio of the dose rate of a given field size to the dose rate of the reference field size (W/L, pg. 496). |

Gap Formula | Gap = (L1/2 × d/SSD1) + (L2/2 × d/SSD2) (W/L, pg. 516). |

Given Dose | Given Dose= (TD/PDD) × 100 (W/L, pg. 509). |

Tumor Dose (Dmax Dose) | TD= (Given dose x PDD at depth of calculation)/100 (W/L, pg. 509). |

Mayneord F-factor | (SSD1+ d)^2 / (SSD1+ Dmax )^2 ×(SSD2 )+ Dmax )^2/(SSD2+ d)^2 *New PDD = Old PDD x Mayneord F-factor (W/L, pg. 508). |

Inverse Square Law | I1/I2 =(D2/D1 )^2 (W/L, pg. 497). |

Sterling Formula | Equivalent Square= (4(L ×W)) / (2(L +W)) (W/L, pg. 498). |

Inverse Square Correction Factor (ISCF) [for SSD Set-ups] | ISCF = (Reference distance + Dmax)^2 / (Treatment distance + Dmax)^2 (W/L, pg. 508). |

ISCF (for Extended Distance Set-ups) | ISCF = (Reference source calibration distance)^2 / (Treatment SSD +Dmax)^2 *(Reference source calibration distance = Reference distance + Dmax for the energy) (W/L, pg. 508). |

ISCF (for SAD Set-ups) | (Reference distance / Source Calculation Point Dose [SCPD])^2 *Reference distance = 100; SCPD = The set up SSD + Depth (W/L, pg. 508). |

Time/MU Calculations for SSD Set-ups | MU/Time=(Prescribed Dose)/ (RDR*ISCF*Sc*Sp*PDD/100*Other) (W/L, pg. 504) |

Monitor Unit Calculations for SAD (Isocentric) Set-ups (TAR) | MU= (Prescribed Dose) / (RDR×ISCF×Sc×TAR×Other factors) (W/L, pg. 511). |

Monitor Unit Calculations for SAD (Isocentric) Set-ups (TMR) | MU= (Prescribed Dose) / (RDR×ISCF×Sc×Sp×TMR×Other factors) (W/L, pg. 511). |

Monitor Unit Calculations for SAD (Isocentric) Set-ups (TPR) | MU= (Prescribed Dose) / (RDR×ISCF×Sc×Sp×TPR×Other factors) (W/L, pg. 511). |

Hinge Angle | HA = 180 - 2(wedge angle) (RT Essentials, pg. 135) |

Wedge Angle | WA = 90 – (Hinge angle/2) (RT Essentials, pg. 135) |

Electron Beam Mean Energy | EO = C4R50; EO = Mean Energy; C4 = 2.4 MeV; R50 = Depth of 50% dose in cm (W/L, pg. 554). |

Practical Range (Er) in cm Electron Beam in Tissue | Er = MeV/2 (W/L, pg. 554). |

Electron 80% Isodose line | MeV/3 (W/L, pg. 555). |

Electron 90% Isodose line | MeV/4 (W/L, pg. 555). |

Activity Full Strength Source | A = (0.66mg/cm) x (active length of source in cm) |

Activity Half Strength Source | A = (0.33mg/cm) x (active length of source in cm) |

Patient is to be treated with AP/PA fields (2:1). Total dose is 200 cGy. What is the dose to each field? | 200 cGy/3 = 66.7 cGy AP = 66.7 x 2 = 133.4 cGy. PA = 66.7 x 1 = 66.7 cGy. |

Patient is to be treated with total dose of 180 cGy in a four field arrangement AP, PA, RL and LL (2:1:1.5:1.5). What is the dose to each field? | AP field: 30 x 2 = 60 cGy PA field: 30 x 1 = 30 cGy RT Lat field: 30 x 1.5 = 45 cGy LT Lat field: 30 x 1.5 = 45 cGy |

Calculate the Gap: Field 1- Length = 17 cm, Width = 6 cm, Depth = 3 cm, SSD = 92 cm; Field 2- Length = 15 cm, Width = 12.5 cm, Depth = 3 cm, SSD = 91 cm. | (17/2 x 3/92) + (15/2 x 3/91) (8.5 x .0326) + (7.5 x .0324) (.2771 + .0243) Gap= .52 |

What is the field size on a film if the collimator setting is 7 cm X 19 cm and the magnification factor is 1.33x? | 9 X 25; 1.33 x 7 cm = 9.31 cm; 1.33 x 19 cm = 25.27 cm |

A patient has a tumor 3.5 cm wide at a depth of 4 cm. What will be the necessary field width at the skin surface to cover the tumor volume plus a 1 cm margin on each side using a linear accelerator and isocentric set-up? | (5.5/100) = (x/96); 100x = 528; x = 5.28. |

The dose rate on a linear accelerator is 102.4 cGy/Min at 100 cm. What is the dose rate at 85.5 cm? | (102.4/x) = (85.5/100)^2; x = 140 cGy/Min. |

The optimum hinge angle for a 60 degree wedge would be: | 180 - 2(60) = 60 degrees. |

Calculate the Gap: Field 1- Length = 10 cm, Width = 10 cm, Depth = 5 cm, SSD = 100 cm; Field 2- Length = 15 cm, Width = 10 cm, Depth = 4 cm, SSD = 100 cm. | (10/2 x 5/100) + (15/2 x 4/100); Gap= .55cm. |

Calculate the equivalent square for a field size of 10 cm X 15 cm. | (4(10x15) / (2(10+15) = 12. |

What are the 80% and 90% isodose lines for a patient treated with a 16 MeV electron beam? | 80% = 16/3 = 5.3 cm; 90% = 16/4 = 4 cm. |

A patient is treated at 100 cm SSD with 6 MV photons. Collimator setting is 15 cm x 15 cm. There is no blocking. 300 cGy per fraction is to be delivered at Dmax. What is the dose delivered at a depth of 5 cm? | (300 x 87.9)/100 = 263.7 cGy. (See Table 24-6 W/L pg. 519) |

A patient is prescribed a dose of 180 cGy at a depth of 10 cm with 10 MV photons at 100 cm SSD. The PDD is 60%. Calculate the dose to the depth of maximum dose. | 180/0.60 = 300 cGy. |

What is the angle between the central rays of the two beams when using a wedged pair? | Hinge Angle. |

A 45 degree wedge is inserted into a field to modify the isodose curve. The toe section will allow (greater or lesser) intensity in part of the beam. | Greater. |

What type of corrections must be made to account for bone, tissue and muscle when doing calculation? | Heterogeneity Corrections. |

Palpable tumor; visible areas of known disease. | GTV. |

Treatment volume which allows for patient motion and set up uncertainties. | PTV. |

The area enclosed by the isodose surface selected. | Treated volume. |

Contains a margin for subclinical extensions of the disease. | CTV. |

The anatomical point A used when calculating dose for cervical and uterine treatments is located: | 2 cm superior and 2 cm lateral to the center of the cervical os. |

If setting a 10 x 10 field size using an isocentric technique, the field size on the patient’s skin would be? | Smaller. |

The most widely used radium substitutes is: | Cesium-137 (W/L, pg. 303). |

Which radium substitute would be best suited to temporary implants of the breast and tongue? | Iridium-192 (W/L, pg. 305). |

What is the half-life of radium-226? | 1,622 years (W/L, pg. 303). |

What is the half-life of cobolt-60? | 5.27 years (W/L, pg. 303). |

What is the half-life of cesium-137? | 30.0 years (W/L, pg. 303). |

What is the half-life of iridium-192? | 73.83 days (W/L, pg. 303). |

What is the half-life of iodine-125? | 59.4 days (W/L, pg. 303). |

What is the half-life of palladium-103? | 16.99 days (W/L, pg. 303). |

What is the half-life of gold-198? | 2.7 days (W/L, pg. 303). |

What is the half-life of radon-222? | 3.82 days (W/L, pg. 303 |

A patient is being treated at 115 SSD on a 100 cm SAD machine. The collimators have been set to 35 cm X 40 cm. What is the field size on the patientâs skin? | 35/100 = x/115; 40/100 = y/115; 100x = 4025; 100y = 4600; x = 40.25 y = 46 |

What is the purpose of bolus? | Fills in deficits to have a more homogenous dose distribution. Shifts dose lines and brings Dmax closer to the skin surface when skin sparing is not desirable (Mosby’s RT Study Guide, pg. 102). |

For non-isocentric treatments, _______ is the factor of choice to demonstrate central axis dose at a given depth. | %DD or PDD (W/L, pg. 509). |

When looking up the PDD or TMR for a given depth and field size, _______ should be used when there are blocks or MLC. | Effective Square (W/L, pg. 504). |

HDR isotopes deliver at a dose rate =_______cGy/min | 20 cGy/min. (Mosbyâs RT Study Guide, pg. 108). |

LDR isotopes deliver at a dose rate =_______cGy/min | 0.5 to 2.0 cGy/min. (Mosbyâs RT Study Guide, pg. 108). |

The wedge angle is determined by the tilt of the isodose lines at_______. | The 50% isodose line in low energy beams like Cobalt 60 or the isodose line at a depth of 10 cm for higher energy beams used in modern linear accelerators (Mosby’s RT Study Guide, pg. 101). |

What is the Dmax for a 1.25 MV beam? | 0.5 cm (RT Essentials, pg. 140). |

What is the Dmax for a 4 MV beam? | 1.0 cm (RT Essentials, pg. 140). |

What is the Dmax for a 6 MV beam? | 1.5 cm (RT Essentials, pg. 140). |

What is the Dmax for a 10 MV beam? | 2.5 cm (RT Essentials, pg. 140). |

What is the Dmax for a 18 MV beam? | 3.5 cm (RT Essentials, pg. 140). |

What is the Dmax for a 24 MV beam? | 4.0 cm (RT Essentials, pg. 140). |

Treatment planning systems often combine CT images with images from other modalities. What is this called? | Image fusion or image registration (W/L, pg. 542). |

What is the practical range in tissue for a 10 MeV electron beam? | Er =MeV/2; Er = 10 MeV/2; Er = 5 cm |

A dose of 200 cGy/fraction is to be delivered to a depth of 10 cm using an AP:PA treatment arrangement. The fields are weighted 3:2 AP:PA. What is the dose per field? | 200 cGy/5 = 40 cGy; AP field = 40 x 3 = 120 cGy; PA field = 40 x 2 = 80 cGy. |