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Conformal orbit sparing radiation therapy: a treatment option for advanced skin cancer of the parotid and ear region

View Article: PubMed Central - PubMed

ABSTRACT

Introduction: New surgical methods have enabled resection of previously in‐operable tumours in the region of the parotid gland and ear. This has translated to deeper target volumes being treated with adjuvant radiotherapy. Due to the limitations of existing conformal techniques, alternative planning approaches are required to cover the target volume with appropriate sparing of adjacent critical structures. Although intensity modulated radiation therapy (IMRT) may be able to achieve these goals compared with the existing conformal method, a new orbital sparing radiation therapy (OSRaT) technique was evaluated as an alternative conformal planning process. The study objective was to evaluate the dosimetry of three planning methods: pre‐existing conformal, IMRT and OSRaT techniques.

Methods: Ten patients were planned retrospectively using the existing three‐dimensional conformal radiotherapy (3DCRT), IMRT and OSRaT techniques. Dosimetry was analysed using the homogeneity index (HI), conformity index (CI), the volume of planning target volumes (PTV) under and over treated by the 95% isodose and dose to critical structures.

Results: OSRaT achieved superior 95% coverage of the high‐dose PTV while delivering HI similar to IMRT for intermediate and high‐dose PTVs. The CI for the high‐dose PTV was comparable between the three techniques, however IMRT was statistically better for the low‐ and intermediate dose PTVs. All three techniques showed adequate orbital sparing, however OSRaT and IMRT achieved this with less under dosing of the PTVs.

Conclusion: For the treatment of patients with advanced skin cancer of the parotid and ear, both IMRT and the OSRaT techniques are viable options.

No MeSH data available.


(A) OSRaT – orbital sparing contralateral post‐oblique field and (B) beams eye view.
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jmrs161-fig-0002: (A) OSRaT – orbital sparing contralateral post‐oblique field and (B) beams eye view.

Mentions: This new technique comprised 5–6 mono‐isocentric static 6 MV photon fields to encompass the PTV and avoid the use of an electron–photon junction. Like the previously described 3DCRT technique, the OSRaT technique comprised a 6‐MV anterior oblique photon field (motorised wedge thick to posterior), a 6‐MV posterior oblique photon field (motorised wedge thick to anterior) and a lowly weighted 6 MV lateral photon field. The superior border of the ipsilateral posterior oblique field was positioned 0.75–1 cm inferior to lens by the radiation therapist to reduce dose to the lens and create a ‘Pseudo’ junction. In contrast to the 3DCRT technique where an electron field was utilised, a 6‐MV photon posterior or contralateral posterior oblique field covering the superior portion of the PTV was used and junctioned with the ipsilateral posterior oblique field as shown in Figure 2. This allowed the reduction in optical and brain dose while increasing PTV coverage.


Conformal orbit sparing radiation therapy: a treatment option for advanced skin cancer of the parotid and ear region
(A) OSRaT – orbital sparing contralateral post‐oblique field and (B) beams eye view.
© Copyright Policy - creativeCommonsBy-nc-nd
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5016620&req=5

jmrs161-fig-0002: (A) OSRaT – orbital sparing contralateral post‐oblique field and (B) beams eye view.
Mentions: This new technique comprised 5–6 mono‐isocentric static 6 MV photon fields to encompass the PTV and avoid the use of an electron–photon junction. Like the previously described 3DCRT technique, the OSRaT technique comprised a 6‐MV anterior oblique photon field (motorised wedge thick to posterior), a 6‐MV posterior oblique photon field (motorised wedge thick to anterior) and a lowly weighted 6 MV lateral photon field. The superior border of the ipsilateral posterior oblique field was positioned 0.75–1 cm inferior to lens by the radiation therapist to reduce dose to the lens and create a ‘Pseudo’ junction. In contrast to the 3DCRT technique where an electron field was utilised, a 6‐MV photon posterior or contralateral posterior oblique field covering the superior portion of the PTV was used and junctioned with the ipsilateral posterior oblique field as shown in Figure 2. This allowed the reduction in optical and brain dose while increasing PTV coverage.

View Article: PubMed Central - PubMed

ABSTRACT

Introduction: New surgical methods have enabled resection of previously in‐operable tumours in the region of the parotid gland and ear. This has translated to deeper target volumes being treated with adjuvant radiotherapy. Due to the limitations of existing conformal techniques, alternative planning approaches are required to cover the target volume with appropriate sparing of adjacent critical structures. Although intensity modulated radiation therapy (IMRT) may be able to achieve these goals compared with the existing conformal method, a new orbital sparing radiation therapy (OSRaT) technique was evaluated as an alternative conformal planning process. The study objective was to evaluate the dosimetry of three planning methods: pre‐existing conformal, IMRT and OSRaT techniques.

Methods: Ten patients were planned retrospectively using the existing three‐dimensional conformal radiotherapy (3DCRT), IMRT and OSRaT techniques. Dosimetry was analysed using the homogeneity index (HI), conformity index (CI), the volume of planning target volumes (PTV) under and over treated by the 95% isodose and dose to critical structures.

Results: OSRaT achieved superior 95% coverage of the high‐dose PTV while delivering HI similar to IMRT for intermediate and high‐dose PTVs. The CI for the high‐dose PTV was comparable between the three techniques, however IMRT was statistically better for the low‐ and intermediate dose PTVs. All three techniques showed adequate orbital sparing, however OSRaT and IMRT achieved this with less under dosing of the PTVs.

Conclusion: For the treatment of patients with advanced skin cancer of the parotid and ear, both IMRT and the OSRaT techniques are viable options.

No MeSH data available.