RADT 5105 Final Exam Quizzes With Complete
Questions And Correct Answers
Define Isocenter - 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 - 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? - ANSWER vertical, lateral, and
longitudinal
What is the purpose of the field light? The cross hairs? - ANSWER both visually define
central axis of the field
what is unique about the central axis of a radiation beam? - ANSWER there is no
divergence at the central axis?
What is SSD stand for? - 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? - 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? - ANSWER 100 cm for LINAC, 80 cm for
Cobalt
Define separation in reference to radiation treatment planning - ANSWER separation is
measure of thickness of patient along central axis or at specificed point within indicated
volume
If patient has AP separation of 25 cm and isocenter is placed at mid-sep, what is table
height and AP SSD? - 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? - 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? - 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 this move? - ANSWER 100 - 92.5 = 7.5 cm = depth of isocenter
new isocenter is 10.5, so AP SSD = 92.5 - 3 = 89.5 cm
Questions And Correct Answers
Define Isocenter - 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 - 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? - ANSWER vertical, lateral, and
longitudinal
What is the purpose of the field light? The cross hairs? - ANSWER both visually define
central axis of the field
what is unique about the central axis of a radiation beam? - ANSWER there is no
divergence at the central axis?
What is SSD stand for? - 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? - 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? - ANSWER 100 cm for LINAC, 80 cm for
Cobalt
Define separation in reference to radiation treatment planning - ANSWER separation is
measure of thickness of patient along central axis or at specificed point within indicated
volume
If patient has AP separation of 25 cm and isocenter is placed at mid-sep, what is table
height and AP SSD? - 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? - 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? - 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 this move? - ANSWER 100 - 92.5 = 7.5 cm = depth of isocenter
new isocenter is 10.5, so AP SSD = 92.5 - 3 = 89.5 cm
