Ellie HawkClinical Practicum II: Parotid LabAugust 6, 2015

Plan information:Right parotid gland is the GTV and displayed in red. The GTV is prescribed to receive 60Gy.Adding a 1cm margin to the GTV created the PTV. My clinical site typically plans to a PTV eval so I did that for this lab. To create the PTV eval I cropped the PTV from the body by 0.5 cm. This allows for dose build up. The PTV is displayed in dark blue and is to be covered by the 95% isodose line that is yellow. The 100%, 95%, 75% and 40% isodose lines are displayed on the axial images below. 100% is represented by a green, 95% by yellow, 75% by magenta, and 40% by orange. The patient’s head is in a neutral position, without chin extension or flexion.

Plan 1: Wedged Pair with Neck field

Wedged Pair Head PlanPlan 1 consisted of a wedged pair photon plan to the head to treat the parotid gland, and a

single AP neck field to treat nodes. To create the wedge pair, a RAO beam was used at 345º and a RPO beam at 190º, weighted 50/50. The collimators were turned 90º to accommodate wedge orientation from lateral to medial, with the heel positioned laterally and toe medial on the patient. Each of the beams utilized 6X energy, a 10cm x 7 cm field size, and 0.5cm of bolus.

Plan 1: Wedged Pair Plan

Beam Name

Beam Angle

(º)

Beam Energy

Field Size (cm)

Wedge(º)

(heel to toe)

Couch Angle

(º)

Colli-mator Angle

(º)

Beam Weight

(%)

BolusY/N (cm)

RAO Head Field 1

345 6X 10x7 30 degrees lateral to medial

n/a 90for wedge orientation

50 Yes0.5

RPO Head Field 2

190 6X 10x7 30 degrees lateral to medial

n/a 90for wedge orientation

50 Yes0.5

AP Neck Field 1

0 18X 9x6.5 n/a n/a 90 for MLC orientation

100 No

Below is an axial image at the reference point for the RAO and RPO fields. The isodose distribution is shown, where pink represents the 105%, green represents 100%, yellow represents 95%, pink represents 75%, and orange represents 40%. As displayed, the parotid gland (GTV) is

shown in red and is being fully covered by the 100% line, therefore meeting the criteria to receive 60Gy. On this slice the PTV (GTV +1cm margin, and cropped 0.5cm from skin surface) is being fully covered by the 95% isodose line as requested and is shown in blue. Image 2 shows how in some areas the 95% isodose line fails to cover on the medially deep PTV. This is due to heterogeneity corrections, presence of air and bone, along with the depth of the lobe medially.

Plan one created hotspot of 111%, which can be seen below. In efforts to reduce this hotspot 30º wedges were used with heels lateral and toes positioned medially. These wedges were not able to completely eliminate the hotspot, but were able to reduce its value and size.

Neck FieldFor the single AP neck field, an 18X beam was used. This beam did not use bolus, a gantry or

couch rotation. The collimator was turned 90º for MLC orientation, and consistency with the head wedged pair fields. The neck field size was 9 cm x 6.5 cm and this field possessed a 100% field weighting. The 100%, 95%, 75%, and 40% distribution can be seen at the location of the neck reference point below.

The maximum dose location for the neck plan is shown below. It has a value of 105% hot and falls in an ideal spot within the neck nodes that we are treating.

Plan 1: Dose DistributionsDue to the location of the parotid gland, there were many structures nearby that were at risk

of receiving a high dose. These structures included the spinal cord, brainstem, brain, both optic nerves, esophagus, optic chiasm, mandible, both cochleae, both lenses, both eyes, both lungs, the oral cavity, and neck nodes. The dose tolerances, dose received from the wedged pair treatment, and if their pass/fail status are listed in the table below. This plan had 2 structures that did not meet dose tolerance requirements, the right lens and right lung.

Plan 1: Wedge Pair- Head and Neck Plan Sum

Critical Structure

Dose Tolerance (Gy)

Dose Received (Gy)

Pass/Fail

Spinal Cord Dmax < 45 13.2 PASSBrainstem Dmax < 54 8.5 PASSBrain Dmax < 60 51.5 PASSOptic Nerve Dmax < 45 (R) 2.1 (L) 0.9 (R&L) PASSEsophagus Mean dose < 45 0.7 PASSOptic Chiasm Dmax < 45 0.9 PASSCochlea Mean dose < 30 (R) 7.4 (L) 0.5 (R&L) PASSMandible Dmax < 70 62.7 PASSLens Dmax < 7 (R) 10.9 (L) 0.6 (R) FAIL

(L) PASSEyes Dmax < 50 (R) 19.5 (L) 1.4 (R&L) PASSLungs Dmax < 45 (R) 61.4 (L) 1.1 (R) FAIL

(L) PASSOral Cavity Mean dose< 45 2 PASSNeck Nodes Received: Dmax:63.6 Mean:57.9

Plan 1 DVH: Plan Sum of wedge pair fields and AP neck field

Plan 2: Electron and Photon Plan

For plan 2, electrons and photons were to be used to deliver 60 Gy to the GTV and have the 95% isodose line cover the PTV. A 16 E electron beam was used from an RAO angle of 290º. 16 E was chosen due to its dmax location, and the coverage needed. Due to the GTV size and location a cone size of 15 cm x 15 cm was used with a field size of 17 cm x 17 cm. The photon was a right lateral beam at 270º that had beam energy of 18X to reach the deeper lobe of the parotid gland GTV. The field size was 8cm x 8.6cm. Both beams did not possess any wedges, couch rotations, or collimator rotations. Bolus of 0.5cm was used in efforts to increase dose to the GTV that was near the skin surface on both beams.

Plan 2: Electron & Photon Plan

Beam Name

Beam Angle

(º)

Beam Energy

Field Size (cm)

Wedge(º)

(heel to toe)

Couch Angle

(º)

Collimator Angle

(º)

Beam Weight

(%)

BolusY/N (cm)

RAO Electron Beam

290 16 E Cone: 15x15Field: 17x17

n/a n/a n/a 100 Yes0.5

Right Lateral Photon Beam

270 18X 8x8.6 n/a n/a n/a 100 Yes 0.5

Below you can see the an axial slice displaying the 100%, 95%, 75%, and 40% isodose distribution along with the plans hotspot. This plan’s hotspot has an undesirable value of 127%, but is ideally located on the edge of the GTV. A large 105% isodose line nearly mirrors the 100% isodose line, covering all the GTV and majority of the PTV. The GTV receives the required 60Gy, while the PTV loses the desired 95% isodose line on many slices on deep lobe coverage.

Plan 2: Dose Distributions

Plan 2: 16 E- Electron & 18 X Photon

Critical Structure

Dose Tolerance (Gy)

Dose Received Pass/Fail

Spinal Cord Dmax < 45 26 PASSBrainstem Dmax < 54 16.6 PASSBrain Dmax < 60 70.5 FAILOptic Nerve Dmax < 45 (R) 12.9 (L) 3.9 (R&L) PASSEsophagus Mean dose < 45 0.08 PASSOptic Chiasm Dmax < 45 5.4 PASSCochlea Mean dose < 30 (R) 43.3 (L) 6.4 (R) FAIL (L) PASS Mandible Dmax < 70 74.3 FAILLens Dmax < 6 (R) 2.3 (L) 1.4 (R&L) PASSEyes Dmax < 45 (R) 5.8 (L) 2.2 (R&L) PASSLungs Dmax < 45 (R&L) 0.3 (R&L) PASSOral Cavity Mean dose < 45 10.5 PASSNeck Nodes Received: Dmax: 49.1 Mean: 6.4

Plan 2 DVH: Plan Sum of 16E electron and 18X photon plan

Plan 3: IMRT RapidArc

Plan 3 consisted of planning a treatment to the parotid GTV with an IMRT technique, which I chose RapidArc. RapidArc delivered a plan that provided me with the best coverage out of the wedged pair and the electron/photon plans. The RapidArc plan was planned with 2 half-arc beams, creating a 114% hot spot. The first beam was a clockwise (CW) beam that traveled from 181º to 0º, while beam 2 was counterclockwise (CCW) traveling 0º to 181º. Both beams were 6X energy and did not use bolus or wedges. The CW beam’s field size was 12cm x 9cm, and the CCW beam’s field size was 10.5cm x 10.8cm. The CW beam had a collimator rotation of 65º, while the CCW beam’s collimator rotation was 295º. The collimator rotations help to smear out the interleaf radiation leakage that could reach the patient. By choosing differing collimator angles the leaves do not line up, therefore create a better blocking method.

Plan 3: IMRT RapidArc Plan(2) Half Arcs

Beam Name

Beam Angle (º)

Beam Energy

Field Size (cm)

Wedge(º)

(heel to toe)

Couch

Angle(º)

Collimator Angle

(º)

Beam Weight

(%)

BolusY/N (cm)

Clockwise Arc 1

181 to 0

6X 12x9 n/a n/a 65 .882 No

Counter-ClockwiseArc 2

0 to 181

6X 10.5x10.8

n/a n/a 295 .947 No

Below is an axial view of the 100%, 95%, 75%, and 40% isodose distribution along with the hotspot of the plan. The hotspot has a value of 114%. As shown the desired GTV coverage of 60Gy has been achieved, along with 95% to the PTV. The RapidArc treatment provided a conformal treatment plan, meeting prescription and dose requirements.

The conformal nature of the plan helped to spare surrounding structures by keeping dose tight around the GTV target. All the nearby structures were below their dose tolerances, making them passable and acceptable for treatment. Due to the hotspot value, dose distribution, and dose distribution to surrounding structures the RapidArc treatment proves to be the best technique to treatment of parotid gland patients.

Plan 3 DVH: IMRT RapidArc

Plan 3: IMRT Rapid Arc (2) Half Arcs

Critical Structure

Dose Tolerance (Gy)

Dose Received Pass/Fail

Spinal Cord Dmax < 45 23.2 PASSBrainstem Dmax < 54 22.2 PASSBrain Dmax < 60 37.4 PASSOptic Nerve Dmax < 45 (R) 3.1 (L) 2.5 (R&L) PASSEsophagus Mean dose < 45 0.08 PASSOptic Chiasm Dmax < 45 1.8 PASSCochlea Mean dose < 30 (R) 21.9 (L) 7.6 (R&L) PASSMandible Dmax < 70 65.6 PASSLens Dmax < 6 (R) 4.8 (L) 4.2 PASSEyes Dmax < 45 (R) 15.2 (L) 8.5 PASSLungs Dmax < 45 (R&L) 0.3 (R&L) PASSOral Cavity Mean dose < 45 14.1 PASSNeck Nodes Received: Dmax:63.6 Mean:57.9

Parotid Lab Questions

Plan 1: Wedge Pair with AP Neck Field

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

The patient’s head position can affect the dose to structures near the parotid gland. For example if the chin is extended, the occipital lobe of the brain will receive more dose along with the cerebellum since it will be lowered to be in line with the parotid gland. In contrast if the chin is lowered, tucked, or flexed than more facial structures will be involved. For example the eyes, lenses, and oral cavity will now be in line with the parotid therefore reducing options of beam angles due to the dose tolerances of these organs at risk.

2) If you were not able to get adequate coverage on the parotid using the wedged pair technique, what were your constraints?

While planning the wedged pair plan for the parotid gland, I was able to accomplish full 60Gy to the GTV as prescribed. I struggled to have the PTV receive full 95% isodose coverage or 57Gy. Some of the issues I ran into were due to heterogeneities within the head. The beam angles I chose had to bypass tissue, air, and bone; therefore reducing dose to the PTV. Dose tolerances were also a big concern while planning, especially to right sided structures. The right eye and lens consistently were within the beam’s path therefore causing them to have a high dose. I was never able to get the right lens low enough to pass the 7Gy dose tolerance. The lowest I was able to get the right lens dose was to 10.9Gy. The right lung is also receiving an intolerable dose of 61.4Gy, while its tolerance is 45Gy. This is due to the AP neck field having exit does directly into the lung.

Plan 2: Electron with Photon

1) How does this plan compare to your wedged pair plan?

Like the wedged pair plan, the GTV is receiving the prescribed 60Gy. The PTV has spots of failed coverage similar to the wedge pair plan, due to heterogeneities within the beams path, and a medially deep target volume. The plan does not compare to others with the extreme hot spot with a value of 127%. This hot spot is significantly higher than the wedged pair plan, and would not be ideal for treatment. Also many structures did not meet dose constraints.

2) Were there any dose constraints not met?

The wedged pair plan had 2 structures, right lens and lung, that did not meet dose constraints. The electron plan with photons had 3 structures that did not meet dose criteria. The brain, mandible, and right cochlea all received dose higher than their tolerance allows. The brain needs to have a dmax below 60Gy and received 70.5Gy, mandible a dmax below 70Gy and received 74.3Gy, and cochleae have a mean dose below 30Gy while the right cochlea received 43.3Gy in this plan. Some of these structures had doses that exceeded even the prescription dose, which is due to the size and intensity of the 127% hot spot.

Plan 3: IMRT RapidArc

1) What beam arrangements did you try?

Originally I tried RapidArc, full arcs around the head. This was giving unaffected (left) side structures unnecessary dose; therefore I reduced my full arcs to half arcs to the right side.

2) Why did you decide on your final one?

I decided upon half arcs to reduce dose to the unaffected side. I chose to have the treatment complete two half arcs. One arc was performed clockwise while the other was counterclockwise.

Conclusion:

Plan 3 IMRT RapidArc with 2 half arcs deemed to be the best treatment for parotid patients. Even though plan 1 with the wedge pair had a lower hot spot of 111% verses RapidArc’s 114% hot spot, the RapidArc plan excelled in other areas. The RapidArc plan was able to provide full 60Gy coverage to the GTV, while also providing 95% coverage to all of the PTV. This was not accomplished in plan 1 or 2. While being able to accomplish GTV and PTV coverage, the plan created a conformal dose distribution that did not put high dose into surrounding organs at risk. All of the surrounding organs to the parotid gland passed their dose tolerances, unlike in plan 1 and 2. Also, no bolus was needed for the RapidArc plan which is beneficial to the staff and patient. Sometimes patients and doctors do not like bolus since it can cause skin reactions. Also, eliminating bolus reduces the chance of therapists forgetting to place the bolus or incorrectly placing the bolus for daily treatments.