Evaluation of the Advanced Collapsed-cone Engine for High Dose Rate Ir-192 GYN and Scalp Brachytherapy

• Author / Creator

  Cawston-Grant, Brie C

• Standard treatment planning in brachytherapy (BT) uses the well-accepted TG-43 dose calculation formalism, which does not account for tissue or material heterogeneities. However, recent developments in BT have led to commercial treatment planning systems (TPSs) using model based dose calculation algorithms (MBDCAs), and their introduction requires critical evaluation before clinical implementation. MDBCAs consider tissue and medical accessory composition, and therefore they have the potential to improve dose calculation accuracy. The work presented in this thesis investigates the performance of the Advanced Collapsed-cone Engine (ACE) in Oncentra® Brachy v4.5 (Elekta, Stockholm, Sweden) for two types of high-dose-rate brachytherapy treatments that have not yet been investigated by others: gynecological treatments using a multi-channel vaginal cylinder (MCVC) applicator, and treatments of non-melanoma skin cancers (NMSC) of the scalp. The evaluation of ACE was performed by comparing ACE dose calculations to radiochromic film measurements in clinically relevant phantoms. For the MCVC experiments, the TG-43 formalism was used to prescribe 500 cGy to the surface of the applicator. Film measurements were made at the applicator surface in a water tank. When the central channel of the applicator was used, the film measurements showed a dose increase of (11 ± 8)% (k=2) above two outer grooves on the applicator surface. This increase in dose was confirmed with the high accuracy mode ACE calculations (hACE), but was not confirmed with the standard accuracy mode ACE calculations (sACE) at the applicator surface. Additionally, a baseline dose variation of (10 ± 4)% (k=2) of the mean dose was measured azimuthally around the applicator surface. This variation was not confirmed with either sACE or hACE. 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. A tissue equivalent slab phantom was designed to model variable heterogeneities that are present in scalp BT treatments: air gaps between the mold and skin, skin thickness, and skull thickness. ACE dose calculations were assessed for six variations of this phantom. Radiochromic film measurements were performed at four different depths within the phantoms, and were compared to the ACE calculations, which used computed tomography images of the phantoms. The TG-43 and hACE calculations were found to overestimate the dose below the skull layer by an average of (8 ± 2)% and (9 ± 3)%, respectively. This underestimation of attenuation through the skull most likely results from the ACE algorithm’s use, in non-water material, of photon scatter spectra generated using Monte Carlo simulations in water, rather than spectra generated within the material itself. Overall, this work has identified attributes of the ACE algorithm that should be considered if it is used in similar clinical situations.

• Subjects / Keywords

  • Collapsed-cone
  • Gynecological cancer
  • High dose rate
  • Dose calculation
  • Scalp cancer
  • Brachytherapy

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3PV6BK54

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.