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Session Item

Monday
May 09
14:15 - 15:30
Room D1
ESTRO-EANO: Twists and turns of brain irradiation
Giuseppe Minniti, Italy;
Maximilian Niyazi, Germany
Advances in radiation technology and imaging allow for highly accurate radiation treatments enabling the use of reirradiation as an increasing option for several cancers, including thoracic, head and neck cancer, brain and spine tumors; however, retreatments remain challenging because prior radiation dose is often close to the tolerance limit of surrounding normal structures. Published data suggest the efficacy and safety of a repeated course of radiotherapy, although prospective validation of this strategy is still pending. The session will provide an overview on current reirradiation strategies and issues. Technical aspects and clinical outcomes of reirradiation, radiological changes and toxicity at follow-up will be addressed in four different presentations.
Joint Symposium
Clinical
11:50 - 12:00
development of a dynamic-shielding intensity modulated endorectal brachytherapy applicator
OC-0112

Abstract

development of a dynamic-shielding intensity modulated endorectal brachytherapy applicator
Authors:

Alana Thibodeau-Antonacci1, Té Vuong2, Hamed Bekerat3, LiHeng Liang2, Shirin Abbasinejad Enger1

1McGill University, Medical Physics Unit, Montreal, Canada; 2Jewish General Hospital, Department of Radiation Oncology, Montreal, Canada; 3Jewish General Hospital, Department of Radiation Onclology, Montreal, Canada

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Purpose or Objective

Surgical resection is the primary curative treatment for rectal adenocarcinoma, preceded by neoadjuvant chemoradiotherapy. External beam radiation therapy is the best-studied form of neoadjuvant radiotherapy but is associated with significant toxicities. An alternative is high dose rate (HDR) brachytherapy (BT), with initial results indicating fewer toxicities and similar perioperative outcomes. However, radiation sources used in BT conventionally provide rotationally-symmetric dose distributions, which deliver a high dose to the tumor, but often with poor target conformity. This results in dose spillage to the organs at risk (OAR). By incorporating dynamically-rotating metallic shields, intensity modulated BT (IMBT) opens the possibility to deliver more conformal dose distributions by directing the radiation towards the tumor and away from OAR. The goal of this study was to develop an MRI compatible dynamic-shield IMBT applicator for the treatment of rectal cancer and compare the results with static shield HDR-BT. 

Material and Methods

Two single-grooved tungsten dynamically-rotating shields with diameters 15 and 18 mm were designed. Currently, in our clinic, static shield HDR-BT is performed by using a flexible cylindrical intracavitary mold applicator (length 28 cm, diameter 2 cm) (Elekta Brachytherapy, Veenendaal, The Netherlands). The central lumen fits an 8 mm diameter tungsten rod for OAR shielding. Computer models of the static shield and IMBT applicators were imported to an in-house Monte Carlo based treatment planning system called RapidBrachyMCTPS and superimposed onto the patient geometry to retrospectively compare static shield HDR-BT with IMBT using a generic Ir-192 source for 2 patients. The prescribed dose was 3 fractions of 10 Gy. Treatment plans were optimized for both modalities using the fast mixed-integer method. The distance between dwell positions was 5 mm and the shield rotation was limited to 15° increments. Dose was scored by simulating 108 radioactive decays using a 1 mm3 voxel grid. Measurements with Gafchromic film were performed in solid-water for the 18 mm IMBT shield to demonstrate the safety of the new system.

Results

Preliminary results show that the dose to the clinical target volume (CTV)  and OAR is similar between static shield HDR-BT and IMBT with the 15 mm shield, which indicates that this method can effectively be used in cases where static shielding is not possible. Additionally, the CTV D90 is significantly increased when using the 18 mm shield compared to static shield HDR-BT (9.87 ± 0.11 Gy vs 9.99 ± 0.04 Gy, P = 0.02) and the contralateral rectal wall D2cc is significantly decreased (7.19 ± 0.95 Gy vs 5.29 ± 1.14 Gy, P < 0.01).  Radiochromic film measurements in solid water with the 18 mm shield showed that the dose decreased by 90.07% ± 3.80% on the shielded side.








Conclusion

An applicator enabling dynamic-shield IMBT for rectal cancer was developed with the possibility to increase the dose to the tumor while more effectively shielding OAR.