Dosimetric Comparison Between High Dose Rate Brachytherapy Boost and Volumetric Arc Therapy Boost in Locally Advanced Cancer cervix

Abstract

Concurrent chemoradiotherapy is considered the standard treatment for the locally advanced cancer cervix (LACC).Radiotherapy is usually administered by a three-dimensional conformal external beam(3DCRT EBRT) approach to whole pelvis to a minimum dose of 45Gy, followed by a brachytherapy ( BT)boost to give additional dose to the gross tumor within the cervix and parametria. High dose rate (HDR) brachytherapy is commonly administered with intracavitory applicator. HDR BT allows delivery of a high radiation dose to the tumor site with rapid fall off so protect normal tissue. Besides, less target motion compared to EBRT. However, several drawbacks exist including invasive technique, pain, requiring spinal or general anesthesia and operative risks such as uterine perforation, infection, and bleeding. Due to the above risks, we tried to assess the possibility of using high technique  EBRT to replace the BT boost  in patients who are either medically unfit for or refuse a brachytherapy boost. we tried to achieve a similar dose distribution with comparable or improved normal tissue sparing to that seen in previously treated HDR BT plans at our institution. Dosimetric comparison between high technique of external beam radiotherapy volumetric arc therapy (VMAT) and high technique Computer topography (CT) guided HDR BT.Ten patients were selected with LACC, representing typical clinical situations according to initial tumor extension and response after EBRT. A boost was given by intracavitary CT guided HDR BT. High risk clinical target volume (HR CTV), bladder, rectum, sigmoid and small bowel were delineated.

Planning was done using Sagi planning system and was manually optimized with respect to organ dose limits. A VMAT planning was created using the variance planning system and a margin of 5mm were added to the CTV to create the target planning target volume (PTV).The inversely planned VMAT was challenged to deliver the highest possible doses to PTVs while respecting D2cc limits from BT, assuming the same fractionation (7 Gy in 3 fractions). When VMAT was limited to D2cc from BT, the D90 for the PTV in VMAT boost was lower than received by the HR CTV in the BT boost (6.3Gy vs. 6.9Gy, p value 0.037). Mean volume of the PTV in VMAT was higher than that of HR CTV in the BT (89 cm³vs 41.7cm³).The dose to the organ at risks (OARs) was comparable.D2cc was higher in VMAT for bladder, sigmoid and rectum (5.6Gy, 0.51, 5.5Gy vs. 4.3Gy, 0.33, 3.9Gy) while D2cc for the small bowel in BT was higher compared to the VMAT (4.1Gy vs. 1.94Gy    ).The VMAT had comparable target coverage and potential for improved sparing of most normal tissues compared to brachytherapy boost. It is an option that exists for patients who refuse BT or can’t tolerate it or in case of non availability of BT or non working machine. However, this is a dosimetric comparison that needs larger number of patients and further application to study the drawbacks that might exist for the VMAT use.