Three-dimensional dosimetric considerations from different point A definitions in cervical cancer low-dose-rate brachytherapy.
OBJECTIVE
To investigate the dosimetric difference due to the different point A definitions in cervical cancer low-dose-rate (LDR) intracavitary brachytherapy.


METHODS
Twenty CT-based LDR brachytherapy plans of 11 cervical patients were retrospectively reviewed. Two plans with point As following the modified Manchester system which defines point A being 2 cm superior to the cervical os along the tandem and 2 cm lateral (Aos), and the American Brachytherapy Society (ABS) guideline definition in which the point A is 2 cm superior to the vaginal fornices instead of os (Aovoid) were generated. Using the same source strength, two plans prescribed the same dose to Aos and Aovoid. Dosimetric differences between plans including point A dose rate, treatment volume encompassed by the prescription isodose line (TV), and dose rate of 2 cc of the rectum and bladder to the prescription dose were measured.


RESULTS
On average Aovoid was 8.9 mm superior to Aos along the tandem direction with a standard deviation of 5.4 mm. With the same source strength and arrangement, Aos dose rate was 19% higher than Aovoid dose rate. The average TV(Aovoid) was 118.0 cc, which was 30% more than the average TV(Aos) of 93.0 cc. D2cc/D(Aprescribe) increased from 51% to 60% for rectum, and increased from 89% and 106% for bladder, if the prescription point changed from Aos to Aovoid.


CONCLUSIONS
Different point A definitions lead to significant dose differences. Careful consideration should be given when changing practice from one point A definition to another, to ensure dosimetric and clinical equivalency from the previous clinical experiences.
Department of Radiation Oncology, The Cancer Institute of New Jersey, New Brunswick, New Jersey, USA.
Miao
M
Zhang
Department of Radiation Oncology, The Cancer Institute of New Jersey, New Brunswick, New Jersey, USA.
Ting
T
Chen
Department of Radiation Oncology, The Cancer Institute of New Jersey, New Brunswick, New Jersey, USA.
Leonard H
LH
Kim
Department of Radiation Oncology, The Cancer Institute of New Jersey, New Brunswick, New Jersey, USA.
Carl
C
Nelson
Department of Radiation Oncology, The Cancer Institute of New Jersey, New Brunswick, New Jersey, USA.
Molly
M
Gabel
Department of Radiation Oncology, The Cancer Institute of New Jersey, New Brunswick, New Jersey, USA.
Venkat
V
Narra
Department of Radiation Oncology, The Cancer Institute of New Jersey, New Brunswick, New Jersey, USA.
Bruce
B
Haffty
Department of Radiation Oncology, The Cancer Institute of New Jersey, New Brunswick, New Jersey, USA.
Ning J
NJ
Yue
LDR brachytherapy; cervical cancer; point A