The dosimetric impact of air in vaginal vault brachytherapy.
Vaginal vault brachytherapy is an adjuvant treatment to reduce risk of local recurrence in endometrial cancer. Axial imaging has demonstrated the presence of air gaps between the surface of a cylindrical applicator and mucosal wall. The impact of these on dosimetry and applicability of the TG-43 formalism in the presence of air has been assessed.



The planning CT scans for a retrospective sample of 82 patients were examined for the presence of air gaps. These were quantified in terms of the radial and longitudinal aspect with reference to the applicator, frequency per patient, and location along the applicator. Monte Carlo calculations and ionization chamber measurements were made in a phantom to estimate the uncertainty of the TG-43 algorithm.



The overall incidence of air gaps was 91.4%. The most common radial size was between 2 and 3 mm (43.0%) that resulted in an average dose reduction of 14.8%. There is a strong correlation between radial dimension and TG-43 calculated dose reduction. Monte Carlo simulations and phantom measurements indicated that the inhomogeneity resulted in a maximum of 2.4% deviation from doses calculated using TG-43.



Air gaps are common. TG-43 formalism is not significantly compromised by the presence of air. However, the presence of the air does produce a displacement of the mucosal surface away from the applicator and this causes a significant dose reduction.
Cancer Centre, Mount Vernon Hospital, Middlesex, UK.
Áine
Á
Maxwell
Cancer Centre, Mount Vernon Hospital, Middlesex, UK.
Niluja
N
Thiruthaneeswaran
Cancer Centre, Mount Vernon Hospital, Middlesex, UK.
Gerry
G
Lowe
Cancer Centre, Mount Vernon Hospital, Middlesex, UK.
Rachel
R
Wills
Cancer Centre, Mount Vernon Hospital, Middlesex, UK.
Samadara
S
Sripali
Cancer Centre, Mount Vernon Hospital, Middlesex, UK. Electronic address: peterhoskin@nhs.net.
Peter J
PJ
Hoskin
D000388:Air; D000465:Algorithms; D001918:Brachytherapy / Q000379:methods*; D016889:Endometrial Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D009010:Monte Carlo Method; D009364:Neoplasm Recurrence, Local / Q000517:prevention & control; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D018714:Radiotherapy, Adjuvant; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed; D014621:Vagina
Brachytherapy; Endometrial cancer; TG-43; Vaginal vault