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8 The RTOG contouring recommendations state the femurs are to be contourned to the bottom of the ischial tuberosity. 8

9 This slide shows the hourglass configuration. It is only present in about half of the patients, but when it is seen, it represents a good landmark for the top of the urogenital diaphragm. The prostate apex should start the next slice up. It is hard to accurately contour the prostate because CT doesn t always discriminate the prostate from the surrounding muscle. Using the anterior rectal surface as a landmark for the posterior border of the prostate can lead to contouring errors too. The circular structures in the genitourinary diaphragm can easily be mistaken for prostate. 9

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11 There is fibromuscular stroma that is often hard to see on CT as being different from the prostate. This anterior fibromuscular stroma can have a volume of up to 30% of the prostate gland. It is most prominent in men with small glands. Contouring the anterior fibromuscular stroma as prostate can lead to overdosing the bladder. T2 MRI shows the stroma much better than CT. 11

12 Sometimes the prostate wraps around the anterior rectum, so that there is prostate tissue posterior to the most anterior part of the rectum. The femur contour needs to start at the top of the ball of the femur and extend to the bottom of the ischial tuberosity. It is to include the femoral neck and the trochanters. 12

13 Current RTOG contouring recommendations advise including the trochanters as part of the femur volume 13

14 It is difficult to always tell the top of the prostate from the bottom of the bladder. Sometimes looking at the sagittal images helps. If you have a T2 MRI, this also helps a lot. It is usually easier to tell the prostate from the bladder in men with smaller prostates. 14

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17 The femur contour starts at the top of the ball of the femur. 17

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28 On CT it sometimes becomes a judgment call as to what is seminal vesicle and what is prostate. This is much clearer on T2 MRI, but not everyone has an MRI scan. The MRI also makes it easier to tell prostate from bladder. Sometimes the sagittal images help. 28

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30 The rectum is to be contoured superiorly to the last slice in which the rectum has a round shape. 30

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34 The prostate is contoured a little too generously anteriorly and lateraly. 34

35 The prostate was over contoured laterally. When the muscle can clearly be seen as separate from the prostate, it should not be included. 35

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37 The entire seminal vesicle should be contoured. All the contours except the LN were turned off for this image. 37

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39 Stop contouring the external iliac LN at the top of the femoral heads. This is also a boney landmark for the inguinal ligament. 39

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42 In contouring the LN the recommended expansion is 7 mm, but carve out the muscle. 42

43 From the 7 mm expansion, carve out the portion that extends into the bowel, bladder, or muscle. 43

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47 General rule: start contouring the LN at the L5/S1 interspace 47

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52 The penile bulb is contoured. It starts at the bottom of the urogenital diaphragm. A common error is to extend the penile bulb through the urogenital diaphragm to match the apex of the prostate. The penile bulb is posterior to the urethra and has a round shape. It is much easier to see if there is contrast in the urethra. The penile bulb is bright on T2 MRI. 52

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58 It is a common mistake to only contour the femoral head. Current RTOG contouring guidelines advise including the femoral neck and the trochanters. Contour from the top of the femur ball to the bottom of the ischial tuberosity. 58

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67 The apex is over contoured by CT. 67

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70 There are a couple of problems here. The prostate was over-contoured at the apex and the base on CT. The fusion is off a little in the AP direction. Looking at the sagittal images often helps to spot errors not easily seen from axial images alone. 70

71 Ring structures- help to reduce high dose bleed off and improve overall conformality. This works well even with tools like the NTO. The high dose bleed off in the direction of the rectum and/or bladder can greatly increase the risk of serious side effects. Posterior Rectum Contours- These tend to be extremely helpful in carving out dose behind the rectum. This is important for two reasons 1). As the rectum fills the amount of it receiving significant levels of dose can also increase. 2) if there is a significant amount of contributing dose traversing the rectum en route to the PTV and the rectum fills with gas the resulting rebuild up of dose beyond the rectum and in the target can cause under-dosing in the posterior portion of the PTV. If we can limit the amount of posterior dose we can mitigate the risk of under-dosing diseased tissue. Other considerations:- For Differential dosing cases remember to use gradient margins in order to better control SIB prescriptions. -Is the bladder volume used for the CT reproducible on a daily basis on the machine? 71

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