Dosimetric Effects of Air Pockets Around High–Dose Rate Brachytherapy Vaginal Cylinders

Richardson, Susan; Palaniswaamy, Geethpriya; Grigsby, Perry W.

doi:10.1016/j.ijrobp.2009.11.004

Cited by 46 publications

(47 citation statements)

References 15 publications

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“…Even a small gap of 1–2 mms can result in a significant dose reduction to the mucosa. As noticed by our and other institutions,3, 4 it is not uncommon to notice from patient CT scans the presence of air gaps at the apex of or along a solid applicator, as long as the vaginal cavity is not in a perfect cylindrical shape after applicator insertion.…”

Section: Introductionsupporting

confidence: 62%

Dosimetric assessment of an air‐filled balloon applicator in HDR vaginal cuff brachytherapy using the Monte Carlo method

Jiang

Badkul

Pokhrel

2018

J Applied Clin Med Phys

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PurposeAs an alternative to cylindrical applicators, air‐inflated balloon applicators have been introduced into high‐dose‐rate (HDR) vaginal cuff brachytherapy to achieve sufficient dose to the vagina mucosa as well as to spare organs at risk, mainly the rectum and bladder. Commercial treatment planning systems which employ formulae in the AAPM Task Group No. 43 (TG 43) report do not take into account tissue inhomogeneity. Consequently, the low‐density air in a balloon applicator induces different doses delivered to the mucosa from planned by these planning systems. In this study, we investigated the dosimetric effects of the air in a balloon applicator using the Monte Carlo (MC) method.MethodsThe thirteen‐catheter Capri™ applicator by Varian™ for vaginal cuff brachytherapy was modeled together with the Ir‐192 radioactive source for the microSelectron™ Digital (HDR‐V3) afterloader by Elekta™ using the MCNP MC code. The validity of charged particle equilibrium (CPE) with an air balloon present was evaluated by comparing the kerma and the absorbed dose at various distances from the applicator surface. By comparing MC results with and without air cavity present, dosimetric effects of the air cavity were studied. Clinical patient cases with optimized multiple Ir‐192 source dwell positions were also explored. Four treatment plans by the Oncentra Brachy™ treatment planning system were re‐calculated with MCNP.Results CPE fails in the vicinity of the air‐water interface. One millimeter beyond the air‐water boundary the kerma and the absorbed dose are equal (0.2% difference), regardless of air cavity dimensions or iridium source locations in the balloon. The air cavity results in dose increase, due to less photon absorption in the air than in water or solid materials. The extent of the increase depends on the diameter of the air balloon. The average increment is 3.8%, 4.5% and 5.3% for 3.0, 3.5, and 4.0 cm applicators, respectively. In patient cases, the dose to the mucosa is also increased with the air cavity present. The point dose difference between Oncentra Brachy and MC at 5 mm prescription depth is 8% at most and 5% on average.ConclusionsExcept in the vicinity of the air‐mucosa interface, the dosimetric difference is not significant enough to mandate tissue inhomogeneity correction in HDR treatment planning.

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Section: Introductionsupporting

confidence: 62%

Dosimetric assessment of an air‐filled balloon applicator in HDR vaginal cuff brachytherapy using the Monte Carlo method

Jiang

Badkul

Pokhrel

2018

J Applied Clin Med Phys

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“…This resulted in a mean average of 0.86% of the vaginal surface being displaced from the surface of the cylinder. Richardson et al 6 also reported that 20 of 25 patients (80%) had $1 air pocket present in the upper vagina in at least 1 of 6 treatment fractions. The average total pocket volume was 0.34 cm 3 (range, 0.01-1.32 cm 3 ), and the average distance that the mucosa was displaced was 3.7 mm (range, 1.3-8.0 mm) with 88.8% of the vaginal mucosa displaced away from the cylinder surface by $2 mm.…”

Section: Discussionmentioning

confidence: 94%

“…2 The dosimetric effect of air gaps around the vaginal cylinder has not been well described. Richardson et al 6 reported a 27% (range, 9-58%) dose reduction to the vaginal mucosa at the air pocket. However, the entire vaginal mucosal dose was not reported.…”

Section: Discussionmentioning

confidence: 98%

“…The location of air gaps might have been different at each treatment, further diluting their overall effect. 6 Thirdly, the dose required to eradicate microscopic metastases is unknown. It is unclear whether an even lower dose would be equally effective for low-or intermediate-risk patients.…”

Section: Discussionmentioning

confidence: 99%

“…Richardson et al 6 reported that 20 of 25 patients (80%) had at least 1 air pocket in the upper vagina. In another study, Humphrey et al 7 demonstrated .2 mm air gaps in 11/103 patients, while repositioning or use of a larger cylinder reduced air gaps in 7/103 patients.…”

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confidence: 99%

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The impact of air pockets around the vaginal cylinder on vaginal vault brachytherapy

Onal

Güler²,

Dolek³

2015

BJR

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Objective: To evaluate the incidence, size and predisposing factors for air pockets around the vaginal cylinder and their dosimetric effect on the vaginal mucosa. Methods: We investigated 174 patients with endometrial carcinoma treated with external radiotherapy (RT) and brachytherapy (BRT) (101 patients, 58%) or BRT alone (73 patients, 42%). The quantity, volume and dosimetric impact of the air pockets surrounding the vaginal cylinder were quantified. The proportions of patients with or without air pockets during application were stratified according to menopausal status, treatment modality and interval between surgery and RT. Results: Air pockets around the vaginal cylinder were seen in 75 patients (43%), while 99 patients (57%) had no air pockets. Only 11 patients (6.3%) received less than the prescribed dose (average 93.9% of prescribed dose; range, 79.0-99.2%). Air pockets were significantly fewer in pre-menopausal patients or in patients treated with the combination of external RT and BRT than in postmenopausal patients or patients treated with BRT alone. A significant correlation existed between the mucosal displacement of the air gap and the ratio of the measured dose at the surface of the air gap and prescribed dose (Pearson r 5 20.775; p , 0.001). Conclusion: Air pockets were still a frequent problem during vaginal vault BRT, especially in post-menopausal patients or in patients treated with BRT alone, which may potentially cause dose reductions at the vaginal mucosa. Advances in knowledge: Air pockets around the vaginal cylinder remain a significant problem, which may potentially cause dose reduction in the target volume.The primary treatment of choice in localized endometrial cancer is surgery. Adjuvant radiotherapy (RT) is recommended in intermediate-and high-risk patients in order to diminish disease recurrence. RT can be in the form of external RT (ERT) with vaginal vault brachytherapy (BRT) or BRT only, depending on the risk factors and stage of disease.The purpose of vaginal vault BRT is to eradicate a microscopic tumour at the lymphatics located in the vaginal vault. It was demonstrated that .90% of lymphatics lie within 2-3 mm from the surface of stretched mucosa.1 For this reason, in order to deliver adequate doses to the submucosal lymphatics, the vaginal cylinder must be in direct contact with the vaginal surface, as recommended by the American Brachytherapy Society (ABS).2 The Group Europeén de Curiethérapie and the European Society for Radiotherapy & Oncology (GEC-ESTRO) guidance 3 is to prescribe vaginal BRT to 5 mm from the applicator surface with a 2-mm tolerance. The most commonly used applicator for vaginal vault high-dose-rate BRT is a segmented cylinder. 4 However, during application, air gaps may be observed, which may potentially cause underdosage of the vaginal mucosa.Cameron et al 5 found that 18 of 25 patients (72%) had air gaps .2 mm in the cranial part of the vagina, with the median number of air pockets per patient being 1 (range, 0-5). Richardson et al 6 reported that 20 o...

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Experimental verification of Advanced Collapsed‐cone Engine for use with a multichannel vaginal cylinder applicator

Cawston-Grant

Morrison

Menon

et al. 2017

J Applied Clin Med Phys

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Model‐based dose calculation algorithms have recently been incorporated into brachytherapy treatment planning systems, and their introduction requires critical evaluation before clinical implementation. Here, we present an experimental evaluation of Oncentra® Brachy Advanced Collapsed‐cone Engine (ACE) for a multichannel vaginal cylinder (MCVC) applicator using radiochromic film. A uniform dose of 500 cGy was specified to the surface of the MCVC using the TG‐43 dose formalism under two conditions: (a) with only the central channel loaded or (b) only the peripheral channels loaded. Film measurements were made at the applicator surface and compared to the doses calculated using TG‐43, standard accuracy ACE (sACE), and high accuracy ACE (hACE). When the central channel of the applicator was used, the film measurements showed a dose increase of (11 ± 8)% (k = 2) above the two outer grooves on the applicator surface. This increase in dose was confirmed with the hACE calculations, but was not confirmed with the sACE calculations at the applicator surface. When the peripheral channels were used, a periodic azimuthal variation in measured dose was observed around the applicator. The sACE and hACE calculations confirmed this variation and agreed within 1% of each other at the applicator surface. Additionally for the film measurements with the central channel used, a baseline dose variation of (10 ± 4)% (k = 2) of the mean dose was observed azimuthally around the applicator surface, which can be explained by offset source positioning in the central channel.

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Dosimetric Effects of Air Pockets Around High–Dose Rate Brachytherapy Vaginal Cylinders

Cited by 46 publications

References 15 publications

Dosimetric assessment of an air‐filled balloon applicator in HDR vaginal cuff brachytherapy using the Monte Carlo method

Dosimetric assessment of an air‐filled balloon applicator in HDR vaginal cuff brachytherapy using the Monte Carlo method

The impact of air pockets around the vaginal cylinder on vaginal vault brachytherapy

Experimental verification of Advanced Collapsed‐cone Engine for use with a multichannel vaginal cylinder applicator

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