RADT 5105 Final Exam Questions And
Answers 2025 Update
Define Isocenter -
correct answer ✅"equal center" - intersection of axis of rotation of
collimator, gantry, and couch
How does an isocentric treatmnet unit differ from a non isocentric
unit -
correct answer ✅ISOCENTRIC UNIT - treatment machine where
axis of rotation of collimator, gantry and couch intersects at
isocenter
NON ISOCENTRIC UNIT - axis of rotation of collimator, gantry and
couch doesnt necessarily intersect in this machine (treatment unit
head may swivel, gantry may move up and down)
On isocentric unit, what direction can couch move? -
correct answer ✅vertical, lateral, and longitudinal
What is the purpose of the field light? The cross hairs? -
correct answer ✅both visually define central axis of the field
what is unique about the central axis of a radiation beam? -
correct answer ✅there is no divergence at the central axis?
,RADT 5105 Final Exam Questions And
Answers 2025 Update
What is SSD stand for? -
correct answer ✅Source Skin Distance (or FSD/TSD focus/target)
-distance between souroce of radiation and patient's skin, usually
measured along central axis
-in SSD set up distance between source and skin kept constant for
all beams
-NON-ISOCENTRIC TREATMENT
What does SAD stand for? -
correct answer ✅Source Axis Distance
-distance between source of radiation and isocenter constant
-center of target volume placed at units isocenter
-isocenter is a fixed value for each treatment unit
-remains constant indepddent of patient variables
ISOCENTRIC TREATMENT
What is standard SAD for most LINACs? Cobalt? -
correct answer ✅100 cm for LINAC, 80 cm for Cobalt
Define separation in reference to radiation treatment planning -
correct answer ✅separation is measure of thickness of patient
along central axis or at specificed point within indicated volume
, RADT 5105 Final Exam Questions And
Answers 2025 Update
If patient has AP separation of 25 cm and isocenter is placed at mid-
sep, what is table height and AP SSD? -
correct answer ✅Table height = distance between top surface of
couch and isocenter in patient = 25 cm/2 = 12.5 cm
AP SSD = 100cm - 12.5 cm = 87.5 cm
If same patient has lateral seperation of 32 cm and isocenter is
placed at midline, what is table height? What is LAT SSD? -
correct answer ✅Table height still 12.5 cm
LAT SSD = 100 - (32/2) = 84 cm
You set a patient TH to 13 cm. the AP SSD reads 85 cm. What is
patient separation? What is PA SSD read? -
correct answer ✅100 cm - 85 cm = 15 cm
15cm from skin to isocenter + 13 cm from isocenter to table top =
28 cm patient separation
PA SSD = 100 - 13 = 87 cm
Patient is lying supine. you set AP SSD of 92.5 cm. what is depth of
isocenter in patient? If you move isocenter posteriorly 3 cm, what
should AP SSD read? Which way will you move the couch to make
Answers 2025 Update
Define Isocenter -
correct answer ✅"equal center" - intersection of axis of rotation of
collimator, gantry, and couch
How does an isocentric treatmnet unit differ from a non isocentric
unit -
correct answer ✅ISOCENTRIC UNIT - treatment machine where
axis of rotation of collimator, gantry and couch intersects at
isocenter
NON ISOCENTRIC UNIT - axis of rotation of collimator, gantry and
couch doesnt necessarily intersect in this machine (treatment unit
head may swivel, gantry may move up and down)
On isocentric unit, what direction can couch move? -
correct answer ✅vertical, lateral, and longitudinal
What is the purpose of the field light? The cross hairs? -
correct answer ✅both visually define central axis of the field
what is unique about the central axis of a radiation beam? -
correct answer ✅there is no divergence at the central axis?
,RADT 5105 Final Exam Questions And
Answers 2025 Update
What is SSD stand for? -
correct answer ✅Source Skin Distance (or FSD/TSD focus/target)
-distance between souroce of radiation and patient's skin, usually
measured along central axis
-in SSD set up distance between source and skin kept constant for
all beams
-NON-ISOCENTRIC TREATMENT
What does SAD stand for? -
correct answer ✅Source Axis Distance
-distance between source of radiation and isocenter constant
-center of target volume placed at units isocenter
-isocenter is a fixed value for each treatment unit
-remains constant indepddent of patient variables
ISOCENTRIC TREATMENT
What is standard SAD for most LINACs? Cobalt? -
correct answer ✅100 cm for LINAC, 80 cm for Cobalt
Define separation in reference to radiation treatment planning -
correct answer ✅separation is measure of thickness of patient
along central axis or at specificed point within indicated volume
, RADT 5105 Final Exam Questions And
Answers 2025 Update
If patient has AP separation of 25 cm and isocenter is placed at mid-
sep, what is table height and AP SSD? -
correct answer ✅Table height = distance between top surface of
couch and isocenter in patient = 25 cm/2 = 12.5 cm
AP SSD = 100cm - 12.5 cm = 87.5 cm
If same patient has lateral seperation of 32 cm and isocenter is
placed at midline, what is table height? What is LAT SSD? -
correct answer ✅Table height still 12.5 cm
LAT SSD = 100 - (32/2) = 84 cm
You set a patient TH to 13 cm. the AP SSD reads 85 cm. What is
patient separation? What is PA SSD read? -
correct answer ✅100 cm - 85 cm = 15 cm
15cm from skin to isocenter + 13 cm from isocenter to table top =
28 cm patient separation
PA SSD = 100 - 13 = 87 cm
Patient is lying supine. you set AP SSD of 92.5 cm. what is depth of
isocenter in patient? If you move isocenter posteriorly 3 cm, what
should AP SSD read? Which way will you move the couch to make
