jasonlaher.weebly.com · Web viewthrough the medium. The 100% isodose line covers 47.78% of the PTV volume and the 95% isodose line covers 58.27% of the PTV volume. - The hot spot

Pelvis Clinical Lab AssignmentBy: Jason Laher RT (T)

Prescription: 45 Gy in 25 Fractions to the PTV

Planning Directions: Place the isocenter in the center of the designated PTV (note: calculation point will be at isocenter). Create a PA field with a 0.5 cm margin around the PTV. Use the lowest beam energy available at your clinic. Apply the following changes (one at a time) as listed in each plan exercise below. After adjusting each plan, answer the provided questions. Tip: Copy and paste each plan after making the requested changes so you can compare all of them as needed.

All tasks completed. Note the PTV has been changed to the color wash khaki to provide a better visual of coverage.

Plan 1: Calculate the single PA beam. Describe the isodose distribution as it relates to PTV coverage. If a screen shot is helpful

to show this, you may include it. Where is the hot spot and what is it? What do you think creates the hot spot in this location?

Responses to Plan 1:-Using a 6MV energy beam the isodose distribution as it relates to PTV coverage can be described as decreasing in dose as the beam travels posterior to anterior. The isodose lines resemble a ladder which steps move farther apart the more anterior the beam penetrates (see screen shot 1). The 100% isodose line covers 47.53% of the PTV volume and the 95% isodose line covers 56.37% of the PTV volume. -The hot spot is located 1.5cm deep in tissue and 4.5cm lateral (patients left) from the central axis of the PA beam on CT slice 126 and is 77.6Gy. -The reason the hot spot is in this location is due to the beam energy and density of tissue in which the beam is passing through. The Dmax depth of a 6MV beam is 1.5cm so it is expected that if only one beam is used the point of maximum dose in homogenous tissue will be at the Dmax depth.

screen shot 1

Plan 2: Change the field to a higher energy and calculate the dose. Describe how the isodose distribution changed. Where is the hot spot and what is it? What do you think creates the hot spot in this location?

Responses to Plan 2:- After changing the energy from 6MV to 10MV the isodose distribution has changed by penetrating deeper in tissue. Due to the higher energy this deeper penetration is to be expected. The overall shape of the isodose lines look similar with the exception of depth and the “ladder lines” spaced further apart due to higher energies ability to penetrate more easily

through the medium. The 100% isodose line covers 47.78% of the PTV volume and the 95% isodose line covers 58.27% of the PTV volume. - The hot spot is located approximately 2cm deep in tissue and .7cm laterally (patients left) from the central axis of the PA beam on CT slice 130. The hot spot dose is 70.3Gy.- The reason the hot spot is in this location is due to the beam energy, density of tissue and curvature of the skin in which the beam is passing through. As seen in the attached screen shot 2 the hot spot is on the gluteal fold and because of the curved tissue in the gluteal fold the hot spot occurs at 2cm depth in tissue instead of the 10MV Dmax depth of 2.5cm.

screen shot 2

Plan 3: Insert a left lateral beam with a 0.5 cm margin around the PTV. Copy and oppose the left lateral field to create a right lateral field. Use the lowest beam energy available for all 3 fields. Calculate the dose and apply equal weighting to all 3 beams.

Describe the isodose distribution. Where is the hot spot and what is it? What do you think creates the hot spot in this location?

Responses to Plan 3:- Going back to the 6MV energy with all three beams being equally weighted the isodose distribution is more conformal to the PTV and the dose on the patients posterior tissue and gluteal fold has been spared with the trade-off of a good amount of dose being absorbed along the patients lateral beams points of entry starting at approximately 1.5cm deep in tissue and decreasing as they move medially (see screen shot 3). The 100% isodose line covers 50.93% of the PTV volume and the 95% isodose line covers 73.2% of the PTV volume. - The hot spot is located approximately where the PA and Rt Lat fields intersect in CT slice 113 and is 51.5Gy.- The hot spot is created in this location due to the depth in tissue in which the three beams intersect. As shown in screen shot 3 we can see from our isodose lines that regions of high dose build-up are found at the intersection of the PA beam with and both the Rt and Lt Lat beams. So, why is our region of highest dose located in the hot spot where the Rt Lat and PA beams intersect instead of where the Lt Lat and PA beams intersect? The answer is because the Rt Lat beam travels less distance in tissue (approximately 14.2 cm) before intersecting with the PA beam than the Lt Lat beam (approximately 16 cm). So, by traveling in less tissue there is less attenuation of the Rt Lat beam thereby causing the beam to possess a higher energy when it interacts with the PA beam versus when the Lt Lat beam interacts with the PA beam.

screen shot 3

Plan 4: Change the 2 lateral fields to a higher energy and calculate the dose. Describe the impact on the isodose distribution. Where is the hot spot and what is it? What do you think creates the hot spot in this location?

Responses to Plan 4:- The increase from 6MV lateral beams to 10MV lateral beams causes tissue dose sparing to the lateral areas of the patient that had high dose regions in Plan 3 because the higher energy has a deeper Dmax depth and penetrates deeper in tissue due to the higher energy. This tissue dose sparing in the patients lateral regions is seen when comparing screen shot 3 to screen shot 4.

The 100% isodose line covers 47.34% of the PTV volume and the 95% isodose line covers 70.13% of the PTV volume.- The hot spot is located approximately where the PA and Rt Lat fields intersect in CT slice 113 and is 50.9Gy.- The hot spot is created in this location by the same rationale presented in Plan 3.

screen shot 4

Plan 5: Increase the energy of the PA beam and calculate the dose. What change do you see? Where is the hot spot and what is it? What do you think creates the hot spot in this location?

Responses to Plan 5:

- The two main changes I notice are the regions of dose at the 110% isodose lines are greatly reduced and my 100% and 105% isodose lines are penetrating deeper into the PTV from posterior to anterior but I’m losing coverage inferior and superiorly. These changes can be seen when comparing screen shot 4 to screen shot 5. The 100% isodose line covers 46.02% of the PTV volume and the 95% isodose line covers 71.95% of the PTV volume.- The hot spot is located approximately where the PA and Rt Lat fields intersect in CT slice 115 and is 49.63Gy.- The hot spot is created in this location by the same rationale presented in Plan 3.

screen shot 5

Plan 6: Add the lowest angle wedge to the two lateral beams.

What direction did you place the wedge and why? How did it affect your isodose distribution? (To describe the wedge orientation you may

draw a picture, provide a screen shot, or describe it in relation to the patient. (e.g., Heel towards anterior of patient, heel towards head of patient..)

Where is the hot spot and what is it? What do you think creates the hot spot in this location?

Responses to Plan 6:- I placed the 15 degree wedges with the toes pointing anteriorly (up/towards the ceiling) see screen shot 6. The reason for my placement in this direction is to avoid having too much dose to the posterior portion of the patient. If not for the PA beam I would have placed my wedges with the toes pointed posteriorly to compensate for the tissue curvature in the patient. The heel of the wedge would compensate for the lack of tissue on the patient anteriorly compared to the patient’s posterior as these two regions relate to the patient’s lateral midline, but due to having a PA field this would only create regions of high dose similar to the ones in Plans 3 and 4. - The change caused the 95% isodose line to extend more anteriorly in the patient thereby creating more coverage of the PTV. The 100% isodose line covers 44.97% of the PTV volume and the 95% isodose line covers 81.07% of the PTV volume.- The hot spot is located more anteriorly than plan 5 but still where the PA and Rt Lat fields intersect in CT slice 114 and is 47.75Gy. The reason the hot spot is more anterior is due to the slope of the wedge attenuating more beam in the patient’s posterior portion of their body.- The hot spot is created in this location by the same rationale presented in Plan 3.

screen shot 6

Plan 7: Continue to add thicker wedges on both lateral beams and calculate for each wedge angle you try (when you replace a wedge on the left, replace it with the same wedge angle on the right). You may weight your fields to get a better dose distribution.

What final wedge angles and weighting did you use? How did each change affect the isodose distribution? Where is the hot spot and what is it? What do you think creates the hot spot in this location?

Responses to Plan 7:- The final wedge angles used are 30 degree wedges.

- The change caused the 95% isodose line to extend more anteriorly in the patient thereby creating more coverage of the PTV even though some coverage was lost super posteriorly see screen shot 7. The 100% isodose line covers 48.79% of the PTV volume and the 95% isodose line covers 88.399% of the PTV volume.- The hot spot is located approximately where the PA and Rt Lat fields intersect in CT slice 115 and is 47.2Gy.- The hot spot is created in this location by the same rationale presented in Plan 3.

screen shot 7

Plan 8: Copy and oppose the PA field to create an AP field and adjust the collimators to keep a 0.5 cm margin around the PTV. Keep the lateral field arrangement. Remove any wedges that

may have been used. Calculate the four fields and weight them equally. Adjust the weighting of the fields, determine which energy to use on each field, and, if wedges will be used, determine which angle is best. Evaluate your plan in every slice throughout your planning volume. Discuss your plan with your preceptor and adjust it based on their input. Normalize your final plan so that 95% of the PTV is receiving 100% of the dose.

What energy(ies) did you decide on and why? What is the final weighting of your plan? Did you use wedges? Why or why not? Where is the region of maximum dose (“hot spot”) and what is it? What do you think caused the hot spot in this location? What is the purpose of normalizing plans? What impact did you see after normalization? Why? Embed a screen cap of your final plan’s isodose distributions in the axial, sagittal and

coronal views. Show the PTV and any OAR’s. Include a final DVH. Be sure to include clear labels on each image. If you were treating this patient to 45 Gy, use the table below to list typical organs at

risk, critical planning objectives, and the achieved outcome. Please provide a reference for your planning objectives.

Organ at Risk (OAR) Desired Planning Objective Planning Objective OutcomeRectum V50 50%1 V46.3 50% - PASSBladder V65 50%1 V46.7 50% - PASSFemurs (Femural heads) V50 5%2 V45.3 5% - PASSBowel space (small bowel) Max Dose 52Gy2 47.4Gy - PASS

1. Quantec (https://en.wikibooks.org/wiki/Radiation_Oncology/Toxicity/RTOG)

2. RTOG (https://en.wikibooks.org/wiki/Radiation_Oncology/Toxicity/QUANTEC)

Responses to Plan 8:- I decided on the 10MV energy due to the higher energy being able to penetrate deeper in the skin and provide sharper edges to cover my PTV with minimal “high dose” outside of the PTV.- The final weighting of my plan has the PA at 26% the AP at 24% and both laterals at 25%. - I did not use wedges due to the uniformity of the isodose lines as they related to the PTV coverage being greater without them. - The hot spot is at 48.2Gy and has shifted to the intersecting area of the left lateral and AP beam on CT slice 114. -The reason for this shift from the previous location is due to the patient decreasing in thickness anteriorly and the addition of the AP beam. -The purpose of normalizing plans is to achieve coverage of a clinical area with the desire dose as directed by the physician.

-After normalizing my plan to 94.6% I saw my max dose increase to 50.7Gy from 48.24Gy but my coverage greatly increased. The reason my plan essentially “got hotter” is because when we normalize to a percentage we’re telling the TPS to bring 100% of the prescribed dose to the isodose line % we’re normalizing to. So if we normalize the a plan from 100% to 95% then we’re increasing dose in all areas receiving dose by 5%. The opposite effect occurs when we normalize to above 100%, our plans “cool down”.- My final plan layout is seen in screen shot 8.- My final labeled DVH is seen in screen shot DVH and includes the assigned PTV coverage on display.

screen shot 8

screen shot DVH