How would, or how does, the patient position (chin extended) affect your beam arrangement?

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1 1 Megan Sullivan Clinical Practicum II Parotid Lab July 29, 2016 PLAN 1: IPSILATERAL WEDGE PAIR TECHNIQUE The ipsilateral wedge pair technique consisted of an anterior oblique field at 45 degrees and a posterior oblique field at 315 degrees. This arrangement created a hinge angle of 90 degrees. As hinge angles decrease, the superficial dose between these angles increases as this is the area of beam overlap. 1,2 In order to spread out this high dose and deposit dose deeper, wedges can be used with the heels facing one another. The wedge size is dependent on the number of degrees and in this case, 45 degree wedges were appropriate given the 90 degree hinge angle. The isocenter was placed in the center of the PTV and a block margin of 0.7cm around the PTV was placed on both fields. Weighting of the two fields was adjusted based on the location of the hotspots as well as taking into consideration the low dose into critical structures such as the spinal cord, brainstem, and oral cavity. To incorporate a lower neck field, the isocenter was shifted inferiorly in order to generate a half beam block technique. The block for the lower neck field was drawn to match the parotid field superiorly, follow along the ipsilateral clavicle inferiorly, block the spinal cord, esophagus, and larynx medially and provide a few centimeters of margin out laterally. This field was calculated at a depth of 3.0 cm to a dose of 1.8 /day to a total of How would, or how does, the patient position (chin extended) affect your beam arrangement? Having an extended chin offers the benefit of avoiding the patient s eyes with exit dose from an ipsilateral wedged pair beam arrangement. 1,3 When the chin is extended, the position of the eyes drops inferiorly thus falling outside of the beam path. Fortunately, this patient s chin was slightly extended so the beam arrangement with angles 15 degrees off in either direction of a straight lateral was sufficient. Additionally, the extended chin is well out of the way of the AP lower neck field. If you were not able to get adequate coverage on the parotid using the wedged pair technique, what were your constraints? The part of the PTV lacking coverage was the superficial area. Relatively speaking, parotids are a superficial structure so the 1 cm margin that was added to generate the PTV resulted in a target volume outside of the body. Dose needs buildup before it can be deposited in tissue so it is unreasonable to expect this part of the PTV to receive coverage. If the physician is concerned about the lack of superficial coverage, bolus could be an option. However, bolus placement on this area of the body is difficult and often times inaccurate because of the proximity of the ear canal and overall surface irregularity. Unless skin involvement is of concern, it is generally not

2 2 necessary to bolus this type of treatment. To assess PTV coverage in a realistic sense, a PTV_opti structure was created and cropped 0.5 cm away from the surface. The dose to this PTV structure is recorded for all three plans. The same goes for the GTV. A GTV_opti structure was created by cropping 0.5 cm away from the surface. The images on the axial slices, however, show the true PTV. Organ/Volume of Interest Desired objective(s) Achieved objective(s) PTV V60 = 95% V60 = 71.5% PTV_OPTI V60 = 80% GTV V60 = 100% V60 = 92% GTV_OPTI V60 = 96% LOWER NECK NODES V50.4 = 100% 95% volume receiving 43 SPINAL CORD Max 50 Max = BRAIN STEM LARYNX EYE COCHLEA PAROTID MANDIBLE Max < 54 V59 < 1-10 cc Mean 44 V50 27% Max Mean <35 Max <54 Mean < 35 Mean < 20 (1 gland) Max 70 (if not possible, then V75 < 1cc) Max = V59 = 0 cc Mean = 1.58 V50 = 0% Max = LEFT Mean = 0.98 Max = 1.5 LEFT Mean = 0.92 Max = 1.4 Mean = 25.8 Mean = 1.29 Mean = 5.9 Max = 64.5

3 3 Axial slice of ipsilateral wedge pair technique. GTV is contoured in light green PTV is contoured in magenta Isodose lines shown: o 6000 c o 5700 c o 4500 c o 2400 c Max hotspot = 6473 c

4 4 Axial slice of lower neck field. Lower neck nodes are contoured in yellow Isodose lines shown: o 5400 c o 4788 c o 3780 c o 2016 c Calc point placed at 3.0 cm depth Max hotspot = 5352 c

5 5 DVH shown in absolute dose of ipsilateral wedge pair and lower neck node field technique. Field data of ipsilateral wedge pair and lower neck node fields.

6 6 PLAN 2: IPSILATERAL MIXED ENERGY PHOTON/ELECTRON TECHNIQUE The ipsilateral mixed energy photon/electron technique consisted of a straight lateral at 90 degrees for the photon and an en face electron field with a gantry angle of 67 degrees and SSD of 100 cm. The isocenter was placed at the center of the PTV. Both fields were created with a 0.7 cm block margin around the PTV. The photon field used an energy of 6 MV and contributed to treating the deeper portion of the parotid gland. The electron field used an energy of 16 MeV and offered coverage superficially. The weighting of the two beams started out with a 1:4 photon to electron ratio as per recommendation of Khan. 4 Ultimately this plan reflects about a 2:3 photon to electron beam weighting. How does this plan compare to your wedged pair plan? Overall, this plan offers better coverage to both the PTV and GTV when compared to the wedged pair plan. This is due to the superficial coverage that was obtained from the electrons. This plan provided a homogeneous dose distribution, but at the price of a hotter plan. When evaluating organs at risk, the structures that receive less dose than the wedged pair plan are the brain stem and the mandible (specifically low dose on the mandible). The structures that receive more dose are both cochleae and the spared parotid. According to Washington and Leaver 1, the opposite salivary glands typically receive 3000 c or less with the mixed energy photon/electron technique. This all makes sense considering the beam angles of each plan. The oblique angles are each exiting into the brain stem and mandible and the straight lateral angle is exiting into both cochleae and right parotid. Were there any dose constraints not met? As previously mentioned with the wedge pair plan, the target coverage objective was not met due to the superficial position of the contours. However, the opti structures are meeting the dose objectives. As far as normal structures are concerned, all dose constraints were met.

7 7 Organ/Volume of Interest Desired objective(s) Achieved objective(s) PTV V60 = 95% V60 = 82% PTV_OPTI V60 = 96% GTV V60 = 100% V60 = 98% GTV_OPTI V60 = 100% SPINAL CORD BRAIN STEM LARYNX EYE COCHLEA PAROTID MANDIBLE Max 50 Max < 54 V59 < 1-10 cc Mean 44 V50 27% Max Mean <35 Max <54 Mean < 35 Mean < 20 (1 gland) Max 70 (if not possible, then V75 < 1cc) Max = 37.2 Max = 26.8 V59 = 0 cc Mean = 6.6 V50 = 0.6% Max = 5.5 LEFT Mean = 1.4 Max = 2.0 LEFT Mean = 1.3 Max = 2.0 Mean = 33.8 Mean = 8.3 Mean = 14.9 Max = 66

8 8 Axial slice of ipsilateral mixed energy photon/electron technique. GTV is contoured in light green PTV is contoured in magenta Isodose lines shown: o 6000 c o 5700 c o 4500 c o 2400 c Max hotspot = 7293 c

9 9 DVH shown in absolute dose of ipsilateral mixed energy photon/electron technique. Field data of ipsilateral mixed energy photon/electron fields.

10 Beam s eye view of electron block with a 0.7 cm margin around the PTV. 10

11 11 PLAN 3: IMRT (VMAT) The use of an IMRT plan offers the ability to create a conformal plan while simultaneously reducing the dose to surrounding normal structures. As previously mentioned, the original PTV and GTV volumes are too superficial to realistically deposit dose. Therefore PTV_opti and GTV_opti structures were created for the optimizer. The only normal structure that overlaps with this target volume was the mandible so and spare structure for the mandible was created in efforts to abstain from giving the optimizer conflicting instructions. The isocenter was placed in the center of the PTV. The energy used was 6 MV. Two different IMRT plans were created, one VMAT and one true IMRT. In the end, the VMAT plan was chosen for evaluation for this project. What beam arrangements did you try? For the VMAT plan, two partial arcs were used: 335 degrees to 179 degrees and the counter arc. These beams were used because the 335 angle followed parallel with the angle of the PTV and the 179 angle was just beyond the exit of the 335 angle and therefore offered degrees beyond the overlap. For true IMRT, a 5 beam arrangement was used with two different sets of angles. The first set was: 30, 60, 90, 120, 150. The second set was: 10, 50, 90, 130, 170. The second set offered a superior plan to the first set, but ultimately the VMAT plan was the best option. Why did you decide on your final one? The VMAT plan was chosen for a few reasons. First, the IMRT plan was hotter and had the 100% isodose line breaking up whereas the VMAT was more uniform and less hot. Another reason involved the assessment of the normal structure dose. Majority of the normal structures were spared more on the VMAT plan, namely, right parotid, brain stem, left cochlea, larynx, and spinal cord. And finally, the 20% isodose line was spraying out more on the IMRT plan whereas this was rounded off in the VMAT plan.

12 12 Organ/Volume of Interest Desired objective(s) Achieved objective(s) PTV V60 = 95% V60 = 75% PTV_OPTI V60 = 96% GTV V60 = 100% V60 = 96.5% GTV_OPTI V60 = 100% SPINAL CORD BRAIN STEM LARYNX EYE COCHLEA PAROTID MANDIBLE Max 50 Max < 54 V59 < 1-10 cc Mean 44 V50 27% Max Mean <35 Max <54 Mean < 35 Mean < 20 (1 gland) Max 70 (if not possible, then V75 < 1cc) Max = 14.8 Max = 14.2 V59 = 0 cc Mean = 1.4 V50 = 0 % Max = 11.7 LEFT Mean = 7.7 Max = 1.2 LEFT Mean = 6.2 Max = 0.95 Mean = 14 Mean = 2.5 Mean = 5.75 Max = 63

13 13 Axial slice of ipsilateral mixed energy photon/electron technique. GTV is contoured in light green PTV is contoured in magenta Isodose lines shown: o 6000 c o 5700 c o 4500 c o 2400 c Max hotspot = 6386 c

14 14 DVH shown in absolute dose of VMAT technique. Field data of VMAT fields.

15 15 References: 1. Washington CM. Principles And Practice of Radiation Therapy. 3rd ed. St. Louis, Mo.: Mosby Elsevier; Khan FM. Classical Radiation Therapy. In: The Physics of Radiation Therapy. 4th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2010: Bentel GC. Treatment planning Head and Neck Region. In: Radiation Therapy Planning. 2nd ed. New York, NY: McGraw-Hill; 1996: Khan FM. Gerbi BJ. Radiation Therapy Using High-Energy Electron Beams. 3 rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012: 388.