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. 2003 Autumn;4(4):321-33.
doi: 10.1120/jacmp.v4i4.2503.

Utilization of custom electron bolus in head and neck radiotherapy

R J Kudchadker  1 , J A Antolak, W H Morrison, P F Wong, K R Hogstrom
Affiliations

Utilization of custom electron bolus in head and neck radiotherapy

R J Kudchadker et al. J Appl Clin Med Phys. 2003 Autumn.

Abstract

Conventional methods of treating superficial head and neck tumors, such as the wedge pair technique or the use of multiple electron fields of varying energies, can result in excellent tumor control. However, in some cases, these techniques irradiate healthy tissue unnecessarily and/or create hot and cold spots in junction regions, particularly in patients with complex surface contour modification or varying planning target volume (PTV) thickness. The objective of this work is to demonstrate how bolus electron conformal therapy can be used for these patients. Two patients treated using this technique are presented. The first patient was diagnosed with malignant fibrous histiocytoma involving the right ear concha and was treated with 12-MeV electrons. The second patient was diagnosed with acinic cell carcinoma of the left parotid gland and was treated with 20-MeV electrons after having undergone a complete parotidectomy. Each patient's bolus was designed using bolus design tools implemented in an in-house treatment-planning system (TPS). The bolus was fabricated using a computer-controlled milling machine. As part of the quality assurance process to ensure proper fabrication and placement of the bolus, the patients underwent a second computed tomography (CT) scan with the bolus in place. Using that data, the final dose distribution was computed using the Philips Pinnacle(3) TPS (Philips Medical Systems, Andover, MA). Results showed that the 90% isodose surface conformed well to the PTV and that the dose to critical structures such as cord, brain, and lung was well below tolerance limits. Both patients showed no evidence of disease six months post-radiotherapy. In conclusion, electron bolus conformal therapy is a viable option for treating head and neck tumors, particularly patients having a variable thickness PTV or surface anatomy with surgical defects.

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Figures

Figure 1
Figure 1. (Color) View of Patient 1 immobilization. A thermal plastic patient immobilization system (Aquaplast, Wycoff, NJ) is used to immobilize the patient's head and neck.
Figure 2
Figure 2. (Color) Transverse CT slices of Patient 1 showing the location, depth, and shape of the parotid PTV at various levels.
Figure 3
Figure 3. (Color) Transverse and sagittal isodose distribution (Gy) for initial bolus design plan for Patient 1. A dose of 60 Gy was prescribed to the 90% of the given dose using 12 MeV electrons. The PTV is indicated with the yellow contour.
Figure 4
Figure 4. (Color) (a) Custom electron bolus proximal surface used to treat Patient 1. The proximal side is designed to modulate the penetration of the electron beam to match the PTV. (b) Custom electron bolus distal surface used to treat Patient 1. The distal side is designed to match the patient skin surface.
Figure 5
Figure 5. (Color) The custom 3D electron bolus in treatment position for Patient 1.
Figure 6
Figure 6. (Color) Comparison of isodose distribution with bolus in place QA plan and bolus design plan for Patient 1 (Note: Prescription 60 Gy at 90% i.e. 90% 2D plan equals 60 Gy 3D plan).
Figure 7
Figure 7. Dose volume histograms for the customized electron bolus plan for Patient 1. The histograms show that the electron bolus treatment plan covers the target volume well while minimizing dose to the critical organs.
Figure 8
Figure 8. (Color) View of Patient 2 immobilization. A thermal plastic patient immobilization system (Aquaplast, Wycoff, NJ) is used to immobilize the patient's head and neck.
Figure 9
Figure 9. (Color) Transverse CT slices of Patient 2 showing the surface anatomical defect and the location, depth, and shape of the parotid PTV at various levels.
Figure 10
Figure 10. (Color) Transverse and sagittal isodose distribution (%) for initial bolus design plan for Patient 2. The maximum depth of the PTV is such that 20 MeV electrons were required for the bolus electron conformal treatment plan. The PTV is indicated with the green contour.
Figure 11
Figure 11. (Color) (a) Custom electron bolus distal surface used to treat Patient 2. The distal side is designed to match the patient surface. (b) Custom electron bolus proximal surface used to treat Patient 2. The proximal side is designed to modulate the penetration of the electron beam to match the PTV.
Figure 12
Figure 12. (Color) Isodose distribution with bolus in place QA plan for Patient 2.
Figure 13
Figure 13. Dose‐volume histograms for the PTV, spinal cord, left lung, and brain for the custom bolus treatment plan (black lines) and a two‐field plan with a 20 MeV electrons superiorly and 12‐MeV electrons inferiorly (gray lines).
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References

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