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For this assignment, I chose to do a patient with a primary tumor in the pelvic region with lymph node involvement. This is a case that I am working on from start to finish. The patient is a 56 year old female presenting with stage IIIB adenocarcinoma of the rectum. The patient is going to be treated with pre-operative chemo-radiation.

1. The patient presented for her CT simulation on 2/15/18. The patient was simulated in a prone, head first position on a bellyboard. The bellyboard is a common immobilization device for patients with rectal cancer. Its purpose is to allow the small bowel to be better displaced out of the radiation field. The negative to simulating a patient prone is it can be more difficult to reproduce day to day. The patient’s arms were placed towards her head to keep them away from the pelvic area. A knee sponge was placed under the patient’s feet for comfort. The patient also received oral and IV contrast during the scan to assist the physician in locating the small bowel, large bowel, and rectum.

Figure 1. Patient during Simulation

Patient lying in the prone position.

2. The radiation oncologist has prescribed a dose of 45 Gy to the pelvis with a boost of 5.4 Gy to the primary tumor and presacral space. It is a common practice to treat patients with a T3 tumor to 45 Gy in 25 fractions along with chemo, before surgery.1 Using a boost to treat to 50.4 Gy in 28 fractions preoperatively improves local control and reduces toxicity compared to treating post operatively.1 The dose of 45 Gy can typically be to a large field which is including the tumor and regional lymph nodes.1 This dose and fractionation is enough to effectively treat the tumor and nodes before surgery, while remaining low enough to reduce toxicity, especially to the small bowel. The standard 1.8 Gy per fraction allows normal tissues to repair.

3. After importing this patient’s planning CT into Eclipse, I registered the CT with an MRI per physicians request to aid him in drawing the target volumes. Next, I began by

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contouring the organs at risk. I contoured both femoral heads and the bladder. The physician contoured the small and large bowel as well as the rectum since it’s the target volume. I also contoured the bellyboard so that it would be accounted for in the calculation as well as overriding the density of a wire placed and sim and the contrast.\

Figure 2. Target and Critical Structures

Sagittal view of the PTV4500 in red, PTV5040 in orange, large bowel in yellow, small bowel in brown, and bladder in blue.

Figure 3. Target and Critical Structures

Coronal view of the PTV4500 in red, PTV 5040 in orange, and femoral heads in purple and teal.

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Figure 4. Target and Critical Structures.

Axial view of the PTV4500 in red, PTV5040 in orange, large bowel in yellow, and small bowel in brown.

Table 1. Planning objectives

Priority Critical Structure

2 Small bowel/large bowel not in CTV: No more than 150cc is to exceed 35 Gy No more than 70cc is to exceed 40 Gy No more than 35cc is to exceed 45 Gy No point doses above 54 Gy (ideal) No more than 15 cc exceeds 54 Gy (absolute max)

2 Femoral Heads: No more than 50% is to exceed 30 Gy No more than 40% is to exceed 40 Gy None to exceed 50 Gy

3 Bladder Mean dose < 40 Gy

Above is the actual planning objectives I was given by the physician. The above objectives are based off of QUANTEC.2

If these objectives aren’t met, the patient is at increased risk of Grade 3+ toxicity, which can include symptoms such as severe diarrhea, mucus or blood discharge, fistula or perforation, dysuria, bladder spasms, and pathological fracture of the femoral heads.3

4. From the physician’s consult notes, I know that there is lymph node involvement specifically in the peri-rectal and pre-sacral nodes. Luckily, the physician also drew the affected nodes on the CT.

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Figure 5. Involved Nodes in the Axial View

Here, the patient has three involved perisacral nodes.

Figure 6. Example of Pre-sacral Nodes4

The pre-sacral nodes lie anterior of the sacrum.4

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Figure 7. Involved Node in the Axial View

Here is the involved peri-rectal node.

Figure 8. Example of Peri-Rectal Node5

Here is an example of a peri-rectal node in an axial view.5

5. The anatomical region to be treated for a patient with rectal cancer depends on the extent of their disease. With the use of CT simulation, the field borders can be more accurate than the “old school” style of field borders. However, a lot of the field borders today are similar to the old cookie cutter method. Superiorly, the treatment field can extend to L5/S1, inferiorly to below the obturator foramen, and laterally to 2 cm beyond the pelvic brim.6 Anteriorly, to the anterior portion of the femoral heads and posteriorly to the

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anterior portion of the sacrum, to include the pre-sacral nodes.6 The actual treatment borders for this patient are very similar.

Figure 9. BEV of the Treatment Area.

Here you can see that the PTV in red extends a couple cm laterally to the pelvic brim, and inferiorly to the obturator foramen.

Figure 10. Lateral BEV of the Treatment Area

Here you can see the superior border extending up to near the L5/S1 border, anteriorly to the front of the femoral head, and includes the anterior portion of the sacrum.

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6. As stated previously, the physician prescribed a dose of 45 Gy in 25 fractions to the tumor and lymph nodes and a boost of 5.4 Gy to the tumor and pre-sacral space. The patient was positioned prone, so I knew a three field technique would be ideal. The treatment is conformal. The reason a three field technique works so well for rectum treatments is that the PTV sits posterior and typically only extends to midline. Since the patient is prone, the bowel falls anteriorly, so two lateral beams and a PA beam can get coverage while avoiding the bowel. First, I set my iso at the center of the PTV4500. I also checked to make sure I could use that same iso for the boost volume. Next, I created my three beams. I made a L Lat, R Lat, and PA. Next, I decided to use 15 MV for my laterals because they would be traveling through a lot of tissue, and would not be weighted too highly. I knew they would not be weighted too highly from experience, but also because the PTV sits posteriorly and my PA beam was clearly going to be my most effective beam, meaning it would be weighted highly. I also used 15 MV on my PA beam because it reduced the hotspot, due to the fact it was more effective thus allowing me to normalize to a higher isodose line. Since the three beams meet posteriorly, where my highly weighted PA beams also enters, there ended up being a high hotspot. To solve this, I placed 60 degree dynamic wedges on my lateral fields. The wedges took the dose from the posterior area and pushed it to midline. The 60 wedges were appropriate because the laterals were weighted low. My final weighting was 59 percent on the PA and 20.5 percent on the laterals. For field size, I used a uniform 0.5 cm margin for my MLC’s around the PTV for all three fields. I ended up tweaking a few MLC’s to block the bowel a bit more in the end. My collimator was at 0 for my PA field and 90 for my laterals, to allow me to use the dynamic wedge. I then normalized so that 100 percent of the dose covers 95 percent of the target. That normalized me to the 100.2 percent.

To plan for the boost, I copy and pasted my original plan, changed the dose prescription to 540 cGy in 3 fractions and normalized it to the PTV 5040. The only thing I changed was my weighting to reduce the hotspot a bit. My final weighting was 53 percent on the PA and 23.5 percent on the laterals. I normalized so that 100 percent of the dose covers 95 percent of the target, thus normalizing me to the 99.93 isodose line. I created a plan summary to evaluate my total plan and I met the goals. Retrospectively, I maybe would have tried at least 10 MV on the PA versus 15 MV to see if there is significantly less bowel and bladder exit dose. It would have come at the expense of a higher hotspot posteriorly but it may have been a better plan.

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Figure 11. A three field technique for a prone rectum.

Here you can see that most of the bowel is out of the field.

Figure 12. A left lateral beams eye view.

A uniform 0.5 cm margin around my PTV4500. I moved my MLC’s in slightly to block more small and large bowel.

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7. As seen in Table 1, my OAR’s were the bowel, femoral heads, and bladder. For small bowel, my max point was 5233 cGy in the plan sum. For large bowel, my max point was 5182 cGy. This was due to the fact that the PTV overlapped into the bowel. I met this constraint as it was 5400 cGy. The other constraints were met as well. For Femoral heads, my max point was 4876 cGy for the left and 4797 cGy for the right. The objective was to not exceed 50 Gy. I also was well under my volume based objectives. Only around 5 percent of both femoral heads were receiving 3000 cGy. For bladder, my mean dose was 3292 cGy, and the objective was to have mean dose stay below 4000 cGy. As far as PTV coverage, the goal was for 100 percent of the dose to reach 95 percent of the target. I normalized in this manner so it was met.

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References

1. Chao KSC. Colon and Rectum. In: Chao KSC, Perez CA, Brady LW, eds. Radiation Oncology Management Decisions. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2011:443-454.

2. Radiation Oncology/Toxicity/QUANTEC. Wikibooks, https://en.wikibooks.org/wiki/Radiation_Oncology/Toxicity/QUANTEC. Last edited 23 Sep. 2015. Accessed 20 Feb. 2018.

3. Radiation Oncology/Toxicity Grading/RTOG. Wikibooks. https://en.wikibooks.org/wiki/Radiation_Oncology/Toxicity_grading/RTOG. Last edited 11 Aug. 2016. Accessed 20 Feb. 2018.

4. Wiegel T. Managing Pelvic Lymph Nodes. University Hospital Ulm. https://www.slideshare.net/ESOSLIDES/35-a-wiegel. Accessed 20 Feb. 2018.

5. Kim SH, Yang DH, Lee JS et al. Natural Course of Untreated Perirectal Lymph Node. Intestinal Research. https://synapse.koreamed.org/DOIx.php?id=10.5217/ir.2015.13.2.175&vmode=PUBREADER. Published 27 Apr. 2015. Accessed 20 Feb. 2018.

6. Bussman-Yeakel L. Digestive System Tumors. In: Washington CM, Leaver D, eds. Principles and Practice of Radiation Therapy. 4th ed. St. Louis, MO: Mosby-Elsevier; 2016:705-737.