Treatment Planning Comparison for Esophageal Cancer

IntroductionVolumetric modulated arc therapy (VMAT) is commonly used at my clinical site when

planning esophageal cases. A traditional beam arrangement for this technique includes two full gantry arcs with opposing collimator angles. Although VMAT is known for creating very conformal plans while sparing normal tissues, an intensity-modulated radiation therapy (IMRT) technique known as the “SupaFireFly” will be discussed as an alternative planning solution.1 Case DescriptionVMAT Planning Technique

The VMAT plan used to treat this patient consisted of two arcs, one clockwise with gantry angles consisting of 182-178° and one counterclockwise with the gantry angles of 179-181°. The arcs had opposing collimator angles to limit leaf leakage of 5° and 355°. The isocenter was placed in the center of the planning target volume (PTV) volume as defined by the radiation oncologist. A beam energy of 10 MV was selected for planning and the prescription was set to 180 cGy per fraction to give a total dose of 5040 cGy to the PTV volume. During optimization, critical structures such as the lungs, heart, cord, and liver were added by the medical dosimetrist to meet normal tissue constraints. In addition, 4 ring structures were used to increase plan conformity.

According to the guidelines of my clinical site, a boarding pass was implemented with the planning objectives for target coverage goals and normal tissue constraints. Upon plan evaluation, the VMAT plan successfully met all normal tissue constraints, and exceeded the target coverage goal of 95% with 98% of the PTV volume receiving the prescription dose. Overall, the isodose lines appear very homogeneous with limited 105% throughout the plan (Figure 1). SupaFirefly Planning Technique

The SupaFirefly IMRT technique consisted of 7 beams with the following gantry angles: 60, 80, 120, 140, 160, 180, and 200°. The same isocenter, prescription, and beam energies were maintained from the VMAT plan; however the optimizer objectives were changed to follow the SupaFirefly protocol (Figure 2). Following optimization, the plan was normalized to achieve the same target coverage as the VMAT plan for comparison purposes.

The SupaFirefly plan did have a significant amount of 105% isodose lines after normalizing to achieve the same target coverage (Figure 3). Therefore a hotspot contour was added and the plan was optimized once more to create a clinically acceptable plan. This significantly reduced the amount of 105% within the PTV volume, giving a comparable plan to the VMAT technique (Figure 4). Plan Evaluation

Overall when assessing boarding pass parameters, both plans met all planning objectives (Figure 5). The SupaFirefly technique did create a hotter plan, with a global max dose of 54.3 Gy versus 53.7 Gy with the VMAT technique. However, there was a noticeable reduction of dose to critical structures in the SupaFirefly plan, with the exception of the cord (Figure 6). This may be

due to the avoidance of critical structures through limited beam angles in the SupaFirefly technique. By avoiding beam entrances through structures such as the liver, dose could be spared from this organ as opposed to the VMAT technique. In addition, using a VMAT technique has the potential to spread more low-dose to surrounding tissues, which is evident in the 1000 cGy (purple) isodose line when comparing plans (Figure 7). Conclusion

Both planning techniques produced clinically acceptable plans for the treatment of this patient’s esophageal cancer. While the VMAT plan might have fewer hot spots and take less time to treat, the SupaFireFly technique has an improved ability to spare dose to critical structures in the surrounding area. In addition, VMAT planning is generally more complex for treatment planning whereas the SupaFirefly protocol offered a treatable solution within a limited amount of planning time.

In general, I think the simplicity of the Supafirefly technique makes this a very lucrative option to choose assuming the patient can manage a lengthier treatment time. The premise of the beam arrangement used in the SupaFirefly protocol was very efficient in sparing dose to critical structures. By evaluating the results of the SupaFirefly technique, some concerns were raised regarding current practices of VMAT techniques using full arcs. Perhaps greater attention to beam entrances and limiting unnecessary treatment of normal tissue structures could help lessen the gap between these two planning techniques.

References

1. Palmer M. Advances in Treatment Planning and Technologies for Esophagus Cancer. AAMD. Accessed August 27, 2018.

Figures

Figure 1. Isodose distribution for VMAT planning technique (5040cGy line in green)

Figure 2. Optimizer objectives used in SupaFirefly technique

Figure 3. Hot spot indicating 105% of the prescription dose (teal contour)

Figure 4. Final isodose distribution using SupaFireFly technique (5040cGy line in green)

Organs at Risk

Dose (Gy) Limit Dose (Gy or Volume (%)VMAT plan

Dose (Gy or Volume (%)SupaFirefly plan

Spinal Cord <46 Max point dose < 0.1 cc

24.8 28.9

Lungs-CTV ≤15 Mean dose 6.2 5.4Lungs-CTV V20 ≤25% 9.2% 9.0%Lungs-CTV V5 ≤65% 31.4% 28.5%Lungs-CTV V10 <40% 24.4% 18.5%Lungs-CTV V15 <30% 15% 13.2%Heart (V45 equivalent)

V36 ≤35% 10% 7.2%

Heart (V30 equivalent)

V25 ≤50% 21.6% 11.1%

Heart <65 Max point dose <0.1 cc

52.0 52.7

Liver ≤21 Mean dose 17.3 9.3%Liver V30 ≤30% 8.5 10.3

Figure 5. Boarding pass comparison between VMAT plan and Supafirefly technique

Figure 6. Dose-volume histogram (DVH) comparing VMAT (dashed line) to SupaFirefly (solid line) technique

Figure 7. Low-dose isodose line (1000 cGy in purple) shown in plan comparison