Seth R. Duffy

Parotid Lab

Plan Ia: Wedge Pair

6MV Beams (2 Beams)

Gantry Angles 340o and 250o

Collimator Rotation of 90os for both beams

Two 45o Wedges (GA 340 Wedge OUT) (GA 250 Wedge IN)

GA 340 Field Size: X(cm) 8.4: [ X1] 4.3 and [X2] 4.1

Y(cm) 6.4: [Y1] 2.9 and [Y2] 3.5

GA 240 Field Size: X(cm) 8.5: [X1] 4.3 and [X2] 4.2

Y(cm) 6.8: [Y1] 3.6 and [Y2] 3.2

The planning for this treatment required 1.0cm of bolus around the PTV. This was necessary because the PTV was expanded 1cm from the parotid gland, thus abutting the most superficial portion of the skin surface. Bolus, combined with 6MV (1.5cm dmax) allowed for adequate skin coverage while maintaining prescribed PTV coverage.

The beams were roughly weighted equally. (As is the case with most wedge pairs.)

Isocenter Placement: X = -5.50, Y = 1.00, Z = 3.00

Question 1) Use the optimal wedge angle equation: Θ = 90o - Φ/2

Question 1a) Having the chin extended would make this planning process slightly easier. I only say this because I was not given a chance to plan on a patient who had their chin extended. Also, looking forward to planning options outside of just wedged pairs, the chin extended would remove it from the field should you want to treat the nodal groups within the neck as well as placing the eyes/lenses up and away from the parotid treatment fields.

This would affect the beam arrangement in a way that would allow the dosimetrist to create beams that were not limited in angle choice due to exiting through the front of the chin, entering into the chin / mouth, both of which create a difficult planning scenario where portions of the field are either treating unnecessary tissues or the angles must be altered.

Question 1b) I was able to get adequate coverage of the GTV and PTV. This was due in part to the planning choice of adding bolus and sticking with a lower energy beam. The constraints are found below.

Seth R. Duffy CP II Parotid Lab

Wedge Pair (1a) & Anterior Neck (1b) : Parotid LabCRITICAL STRUCTURE Recommended Tolerance Achieved Dose (Gy) Met or ExceededOrbital (RT) <45Gy 2.00Gy MetOrbital (LT) <45Gy 3.96Gy MetOptic Nerve (RT) 5% Vol < 54Gy 1.31Gy MetOptic Nerve (LT) 5% Vol < 54Gy 2.61Gy MetLens (RT) 8Gy 1.64Gy MetLens (LT) 8Gy 1.63Gy MetOptic Chiasm 50Gy 1.43Gy MetBrain 47Gy 43.41Gy METBrain Stem <54Gy 18.6 Gy Met

Spinal Cord <45Gy 27.27Gy Met

Middle Ear<30Gy (Acute) <55Gy (Chronic)

26.17Gy (RT) & 2.64Gy (LT) Met

Uninvolved Parotid 50% Vol < 30Gy 17.0Gy MetGlottic Larynx 65% Vol < 50Gy 7.61Gy Met

Mandible <70Gy 67Gy EXCEEDED

Only one structure nearly exceeded the constraints I set before beginning this clinical lab. Although, I should not understate the importance of the OR that nearly exceeded the limit, as it was the brain. If I were to continue using this planning technique, a wedge pair and concise setup to y=1cm, it would be necessary to continue maneuvering the BEV to find a path that avoided the spinal column posteriorly, the lenses anteriorly, as well as traversing past/under the brain. This would be more easily accomplished if the patient had their chin extended I believe. As the plan is now, it is crucial that the y-plane never exceed 0.3cm of cephalad shift because the 75%+ isodose line comes very close to the inferior portion of the right-most temporal lobe.

Dose color wash view is an important tool in any dosimetrist's arsenal. It is crucial to understand how the wedge pair being used is depositing dose along lower isodose levels. Monitoring only higher level isodose levels can be deceiving and appear much cleaner than what distribution is actually occurring. In the case above, our distribution is good and does not jeopardize the brainstem, spinal column, or optic structures. We can see however, that the lower isodose levels are passing through, very close to, the right middle ear.

Lower Anterior Neck Field 1b: [Teal Isodose Line Represents 40%]

(These Contours Are Used In ALL Plans)

Lower Anterior Neck Field Info:

6MV Static Beam

11o Gantry Rotation & 90o Collimator Rotation

Field X (cm) 10.5: [X1] 10.5 [X2] 0

Field Y (cm) 10.5: [Y1] 2.5 [Y2] 8.0

Isocenter Placement X = -3.00, Y = 2.00, Z = -1.50

Plan 2: Ipsilateral Photon/Electron (Mixed Beam)

2 Different Beams: Both Linked To 1cm Bolus Applied To Right Outer Cheek

One 12E Electron Beam:

Gantry Angle 270o

Field X(cm) = 14 : [X1] 7cm , [X2] 7cm

Field Y(cm) = 14 : [Y1] 7cm , [Y2] 7cm

Isocenter Placement X = -7.00, Y = 1.3, Z = 3.00

The Electron Field also utilized a custom block cutout - which would eventually be made out of the alloy Cerobend (Cutout from Eclipse TPS seen

below).

One 6 MeV Photon Beam:

The electron beam was chosen because of the depth-dose curves (maximum range) electrons in water/tissue as well as the rule of thumb that the electrons deliver

the 80% isodose at depth approximately E/2.8cm and the 90% isodose at E/3.2cm-90% (E=energy) [Khan Physics, 272]. Practical range of electrons is calculated as E/2 with an energy loss of 2 MeV per cm of tissue or water.

One 6MV Photon Beam: Also chosen because of the low dmax and the fact that our PTV lies so close to the surface. This beam energy will work synergistically with the

electron beam to deliver precise doses in the desirable region.6 MeV Portal and Beam data:

Gantry Angle 270o

Field X(cm) = 7.1 : [X1] 3.5cm , [X2] 3.6cm

Field Y(cm) = 7.5 : [Y1] 4.0cm , [Y2] 3.5cm

Isocenter Placement X = -5.53 , Y = 1.30, Z = 3.00

Photon / Electron Fields - Parotid Lab Plan II

CRITICAL STRUCTURE Recommended Tolerance Achieved Dose Met or ExceededOrbital (RT) <45Gy .55Gy METOrbital (LT) <45Gy .8Gy METOptic Nerve (RT) 5% Vol < 54Gy .93Gy METOptic Nerve (LT) 5% Vol < 54Gy 1.04Gy METLens (RT) 8Gy .31Gy METLens (LT) 8Gy .47Gy METOptic Chiasm 50Gy 0Gy METBrain 47Gy 45.1Gy METBrain Stem <54Gy 27.6Gy METSpinal Cord <45Gy 32.3Gy MET

Middle Ear<30Gy (Acute) <55Gy (Chronic) 38Gy RT - 19.6Gy LT MET

Uninvolved Parotid 50% Vol < 30Gy 20Gy METGlottic Larynx 65% Vol < 50Gy 0Gy MET

Mandible <70Gy 68Gy MET

Mandible was also evaluated within this plan because the dose color-wash indicated (to me) that it was receiving a higher dose than previously noted - in the wedge pair plan. The dose to this structure could more than likely be improved through the following: Chin extension with slight collimator rotation to continue covering the PTV while directing dose away from the mandible. Also, the block could potentially be drawn closer to the PTV - with caution as to not drastically diminish the PTV coverage.

From this dose color wash, in the transverse plane, we can see that with the use of 1cm bolus and low energy beam synergy, we can create a dose distribution conducive to treating the roughly 0.8cm deep parotid gland while also covering the superficial (and deep) portions of the PTV. It is important to always monitor the lower dose region(s) to fully understand the overall impact of any given plan on the patients anatomy.

Plan II Questions:

a) This plan, the photon and electron mix, was slightly better than the initial wedge pair plan in terms of deposited dose among the contoured anatomical structures within the head and neck region. This is due in part to the location of the PTV, GTV, and the ability to effectively treat this region with electrons as opposed to multiple photon beams. Noteworthy decreases in dose are noted within the optic structures (seen below in the DVH labeled optics).

It should be noted that the dose to the brainstem is slightly higher with the electron - photon mix. This is in part due to the angle at which the beams are placed. The beams are both placed at 270 degrees because, when using electrons, almost any beam to be completely honest, it is beneficial to be impinging upon a flat surface. 270 degrees offers not only direct entry into PTV and GTV but also a semi-flat surface. Just to reiterate, this could potentially be improved through chin extension established during simulation.

b) No dose constraints were directly violated within this treatment plan. I should note that upon review of the plan I have begun to wonder if a chin extension would not be the best option here. I strongly believe that some of the critical structures near the parotid PTV are going to receive dose regardless but that mitigation of dose along these proximal ORs could decrease any uneasy feelings I have about this plan. I digress. I believe this plan is acceptable with room for improvement, if the institution has the time to re-simulate the patient and if the oncologist believes the potential benefits are even significant.

[Optics DVH for Mixed Energies Treatment Plan]

Dual (2x) Partial Arcs: Parotid Gland Plan III:

CRITICAL STRUCTURE Recommended Tolerance Achieved Dose (Gy) Met or ExceededOrbital (RT) <45Gy 1.78Gy METOrbital (LT) <45Gy 1.85Gy METOptic Nerve (RT) 5% Vol < 54Gy 1.3Gy METOptic Nerve (LT) 5% Vol < 54Gy 1.4Gy METLens (RT) 8Gy 0.85Gy METLens (LT) 8Gy 0.76Gy METOptic Chiasm 50Gy 0.74Gy METBrain 47Gy 27.8Gy METBrain Stem <54Gy 15.3Gy METSpinal Cord <45Gy 17.03Gy MET

Middle Ear<30Gy (Acute) <55Gy (Chronic)

5.35Gy(LT) 20.04Gy(RT) MET

Uninvolved Parotid 50% Vol < 30Gy 7.2Gy METGlottic Larynx 65% Vol < 50Gy .67Gy MET

METMandible <70Gy 63.07Gy MET

Two partial arcs were utilized within this plan.

The arcs ran in opposite directions, one clockwise the other counterclockwise, both running a total of 181 degrees, (zero) 0 to (one-hundred eighty-one) 181 degrees and then reversed.

Collimator rotation of 30o and 330o for the clockwise and counterclockwise arcs, respectively.

No normalization was utilized and the arcs were optimized using Varian Eclipse VMAT Optimization functions.

Constraints, upper and lower limits, were placed on both the PTV as well as the GTV to ensure required coverage was obtained.

Automatic NTO was turned on, as I found that this worked better than the few attempts I made at utilizing my own NTO values.

Isocenter placement at: X: -5.50, Y: 1.30, Z: 3.00

Bolus of 0.3cm was linked to both arcs.

No fluence editing was performed on this plan.

Plan III Questions:

a) The beam arrangement chosen for the final step of this parotid lab was two partial arcs, VMAT.

b) This beam arrangement was chosen because of my learning experiences with the previous two plans. I began to understand that: a) this tumor was somewhat superficial and would require bolus on most planning techniques (0.3cm), b) the tumor was oblong and round in shape presenting an opportunity to use an arc, and c) the GTV/PTV was in a location where an IMRT technique would provide a much steeper dose gradient / fall-off that would allow us (the dosimetrist) to effectively treat the parotid (GTV) as well as the delineated PTV without excessive dose spilling into other areas.

I believed that through the use of multiple angles I would be able to deliver a conformal dose to the parotid while lowering the doses to ORs within a proximal distance. I was able to achieve this, as seen in the dose color wash, spreadsheet, and DVH above. Just as the other plans, I still believe I would have been able to reduce doses to the mandible and potentially even the oral cavity (if only by a small amount) if the simulation had been performed with the patient's chin extended.

Although this plan does have an 8.2 percent hotspot, it lies directly inside the inferior border of the PTV and does not jeopardize any critical structures.