Abstract
Brachytherapy used in local cervical cancer is still widely based on 2D standard dose planning with the prescription to point A, which is invisible on imaging and located at a high‐dose gradient. In this study, the geometric location error of point A was investigated. It is traditionally reconstructed in the treatment planning system after carefully digitizing the point marks that were previously drawn on the orthogonal radiographs into the system. Two Cartesian coordinates of point A were established and compared. One was built up based on the geometric definition of point A and would be taken as the true coordinate, while the other was built up through traditional clinical treatment procedures and named as the practical coordinate. The orthogonal film reconstruction technique was used and the location error between the practical and the true coordinate introduced from the variations of, first, the angle between the tandem and the simulator gantry rotation axis, and second, the interval between the tandem flange and the simulator isocenter, was analyzed. The location error of point A was higher if the tandem was rotated away from the gantry rotation axis or if the location of the tandem flange was set away from the isocenter. If a tandem with a 30° curvature was rotated away from the gantry rotation axis 10° in the anterior–posterior (AP) view, and there was an 8.7 cm interval between the flange and the isocenter, the location error of point A would be 3 mm without including other errors from simulator calibration, data input, patient setup, and movements. To reduce the location error of point A calculated for traditional reconstruction procedures, it is suggested to move the couch or patient to make the mid‐point of two points A near the isocenter and the tandem in the AP view parallel to the gantry rotation axis as much as possible.PACS number: 87.55.km
Highlights
1953, the definition of point A was modified as a point 2 cm superior to the external cervical os and 2 cm lateral to the cervical canal.[21]. The modified definition is still referenced in standard medical physics textbooks.[22]. In the Madison system, the reference point M was used instead of point A and was defined as “2 cm lateral to the center of the uterine canal and 2 cm cephalad from a line joining the center dwell position of the vaginal colpostat sources”.(23) Recently, the earliest definition of point A has been readopted with some adjustments by the American Brachytherapy Society (ABS)(24-26) and European Society for Therapeutic Radiation Oncology (ESTRO).(7,27)
The position of point A is still widely calculated based on 2D X-ray imaging[32] and is generally reconstructed through the point marks predrawn on the orthogonal radiographs.[33,34,35] as indicated by Bentel,(19) “ point A is defined in relation to important anatomic structures, these cannot be visualized on a radiograph.”
Before beginning BT treatment for cervical cancer, the patient is placed in a supine position on a movable homemade couch with her feet facing the gantry of a Toshiba DC50N simulator (Tokyo, Japan), and the orthogonal X-ray images are taken for film reconstruction
Summary
Brachytherapy (BT) has been widely used for decades in the adjuvant treatment of cervical carcinoma.[1,2,3,4] For the curative treatment of all stages, BT truly plays an essential role, giving the patient a needed boost dose.[3,4,5,6,7] Through delivering a substantially high dose to the tumor in the central pelvis, while sparing the nearby organs at risk due to the rapid dose falloff,(8) BT leads to an improvement in the patient survival rate with a decrease in the patient recurrence rate.[9,10,11,12] there is no doubt that the curative potential of radiation therapy in the management of cervical cancer was proved to be greatly enhanced by the use of intracavitary BT.[9,13,14,15]Throughout decades of clinical practice and as a result of the abundant experience accumulated by radiation oncologists, delivery of a certain dose to point A is still a commonly used prescription for cervical cancer BT.[16,17,18] Besides delivering the desired dose to point A, the associated isodose curves are preferred to be a pear shape with the widest part near the cervix.[19]. 0.5 mm deviation in distance relative to a treatment distance of 20 mm in brachytherapy means a 5% variation in dose delivery”.(29) For low-dose-rate brachytherapy, Zhang et al[30] demonstrated that a 9 mm shift in point A can cause a 14% dose rate difference. Another ESTRO study demonstrated the high-dose gradient around point A, in that “the dose along an axis perpendicular to the intrauterine source at the level of point A decreases from approximately. The dependence of the error will be analyzed on the angle between the tandem and the simulator gantry rotation axis, and the interval between the tandem flange and the simulator isocenter
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