Practice Problems Week 6 2019

Practice Problems Week 6 1. Ionization chamber readings are adjusted for temperature and pressure at the time of calibration. If room temperature is 29º C and the pressure is 768 mmHg, what is the temp-pressure correction factor? 2. Calculate the gap for two adjacent fields if the doctor wants to match the fields at 5cm depth given that field 1 is 20 x 20 @80cm SSD and field 2 is 30 x 30 @ 100cm SSD. 3. Calculate the geometric penumbra for a 60Co source at 5cm depth, 80cm SSD where the source is 2cm and the source to collimator distance is 45cm. 4. Calculate the timer error for a 60Co unit given that the reading for a 2 exposure minute was 4.289 and the cumulative reading for 4 readings taken for 0.5 minute each was 4.356. 5. Calculate the temp-pressure correction factor for a temperature of 69ºF and 735mmHg. 6. The optical distance indicator reads 160cm when the distance is actually 162cm on a linac. The resulting error for a patient receiving an extended distance treatment is: a. Underdose of 1.3% b. Underdose of 2.5% c. Underdose of 3.8% d. Overdose of 1.3% e. Overdose of 2.5% 7. A 10 x 10cm supraclavicular field is treated at 80cm SSD on a 60Co unit to a depth of 3cm at 300cGy per fraction. What is the dose per fraction to the cord if it is located at a depth of 7 cm? (output = 200cGy/min at depth of 0.5cm). Use the table below. a. 233 b. 267 c. 300 d. 338 e. 386

Depth (cm) 0.5 3 5 7

PDD 100 90 80 70

8. The off-axis ratio is the dose at any point in a plane perpendicular to the central axis to that of the dose on the central axis in that plane. a. True

b. False

9. A patient is treated using two opposing AP/PA fields. The target center must receive a total dose of 300cGy. The two fields are weighted AP:PA= 2:1 at the target center. The target dose delivered through: (more than one answer) a. AP port is 100cGy b. AP port is 200cGy c. AP port is 300cGy d. PA port is 100cGy 10. A patient was previously treated to a whole pelvis field and is now returned for a treatment to the left pelvic area. The treatment field is set-up the same as previously treated (with the same CAX). The right side of the field is closed with the independent jaw to the CAX. Calculate the number of mu to deliver 200cGy per fraction to a depth of 7 cm at off axis point of 5cm. TMR = .913 Sc = 1.022 Sp = 1.000 SAD Factor = 1.035 OAR = 1.025 (5cm)