An innovative method to acquire the location of point A for cervical cancer treatment by HDR brachytherapy.

Liyun Chang,Tsair-Fwu Lee,Pang-Yu Chen,Shyh-An Yeh,Sheng-Yow Ho

doi:10.1120/jacmp.v17i6.6355

Abstract

Brachytherapy of local cervical cancer is generally accomplished through film‐based treatment planning with the prescription directed to point A, which is invisible on images and is located at a high‐dose gradient area. Through a standard reconstruction method by digitizing film points, the location error for point A would be 3 mm with a condition of 30° curvature tandem, which is 10° away from the gantry rotation axis of a simulator, and has an 8.7 cm interval between the flange and the isocenter. To reduce the location error of the reconstructed point A, this paper proposes a method and demonstrates its accuracy. The Cartesian coordinates of point A were derived by acquiring the locations of the cervical os (tandem flange) and a dummy seed located in the tandem above the flange. To verify this analytical method, ball marks in a commercial “Isocentric Beam Checker” were selected to simulate the two points A, the os, and the dummies. The Checker was placed on the simulator couch with its center ball coincident with the simulator isocenter and its rotation axis perpendicular to the gantry rotation axis. With different combinations of the Checker and couch rotation angles, the orthogonal films were shot and all coordinates of the selected points were reconstructed through the treatment planning system and compared with that calculated through the analytical method. The position uncertainty and the deviation prediction of point A were also evaluated. With a good choice of the reference dummy point, the position deviations of point A obtained through this analytical method were found to be generally within 1 mm, with the standard uncertainty less than 0.5 mm. In summary, this new method is a practical and accurate tool for clinical usage to acquire the accurate location of point A for the treatment of cervical cancer patient.PACS number(s): 87.55.km

Highlights

The definition of point A in 1953 was modified as a point 2 cm superior to the external cervical os and 2 cm lateral to the cervical canal.(22) This modified definition is still referenced in standard medical physics textbooks.(23) Lately, the earliest definition of point A was readopted with some adjustments by the American Brachytherapy Society (ABS)(5,24,25) and European Society for Therapeutic Radiation Oncology (ESTRO).(8,26)
Point A is defined in relation to important anatomic structures, but cannot be visualized on a radiograph.(20) Its location cannot be exactly determined through a radiograph mainly due to its unknown magnification on film
It is generally reconstructed in the treatment planning system after carefully digitizing the point marks that were previously drawn on the orthogonal radiographs into the system

Summary

Introduction

435 Chang et al.: Method to acquire location of point A therapy for cervical cancer has been demonstrated to be greatly enhanced by the treatment of intracavitary BT.(9-12) By 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,(13) BT leads to an improvement in the patient survival rate with a decrease in the recurrence rate.(9,14-16) Throughout the abundant clinical experience accumulated by radiation oncologists, delivery of a certain dose to point A is still a commonly used prescription for cervical cancer BT.(17-19) Traditionally, the treatment planning is performed through the reconstructed dummy seed positions within the applicators and the prescribed point doses from two orthogonal film images,(5) in which the isodose lines passing through point A form a pear shape encompassing the intended boost treatment volume.(20)Historically, several definitions have been used to define the location of point A in terms of its location along the direction of the tandem (intrauterine applicators). An analytical method to calculate the coordinates of point A is proposed through the use of the reconstructed position of the tandem flange and one reference point on the tandem.

Results

Conclusion