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KMID : 1164220160280020179
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Journal of Korean Society for Radiotherapeutic Technology
2016 Volume.28 No. 2 p.179 ~ p.186
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The evaluation of the feasibility about prostate SBRT by analyzing interfraction errors of internal organs
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Hong Soon-Gi
Son Sang-Jun
Mun Jun-Ki
Kim Bo-Gyum
Lee Je-Hee
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Abstract
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Purpose : To figure out if the treatment plan for rectum, bladder and prostate that have a lot of interfraction errors satisfies dosimetric limits without adaptive plan by analyzing MR image.
Materials and Methods : This study was based on 5 prostate cancer patients who had IMRT(total dose: 70Gy) Using ViewRay MRIdian System(ViewRay, ViewRay Inc., Cleveland, OH, USA) The treatment plans were made on the same CT images to compare with the plan quality according to adaptive plan, and the Eclipse(Ver 10.0.42, Varian, USA) was used. After registrate the 5 treatment MR images to the CT images for treatment plan to analyze the interfraction changes of organ, we measured the dose volume histogram and the changes of the absolute volume for each organ by appling the first treatment plan to each image. Over 5 fractions, the total dose for PTV was V36.25 Gy ? 95%. To confirm that the prescription dose satisfies the SBRT dose limit for prostate, we measured V100%, V95%, V90% for CTV and V100%, V90%, V80% V50% of rectum and bladder.
Results : All dose average value of CTV, rectum and bladder satisfied dose limit, but there was a case that exceeded dose limit more than one after analyzing the each image of treatment. After measuring the changes of absolute volume comparing the MR image of the first treatment plan with the one of the interfraction treatment, the difference values were maximum 1.72 times at rectum and maximum 2.0 times at bladder. In case of rectum, the expected values were planned under the dose limit, on average, V100%=0.32%, V90%=3.33%, V80%=7.71 %, V50%=23.55% in the first treatment plan. In case of rectum, the average of absolute volume in first plan was 117.9 cc. However, the average of really treated volume was 79.2 cc. In case of CTV, the 100% prescription dose area didn’t satisfy even though the margin for PTV was 5 mm because of the variation of rectal and bladder volume.
Conclusion : There was no case that the value from average of five fractions is over the dosimetric limits. However, dosimetric errors of rectum and bladder in each fraction was significant. Therefore, the precise delivery is needed in case of prostate SBRT. The real-time tracking and adaptive plan is necessary to meet the precision delivery.
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KEYWORD
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Prostate SBRT , Interfraction error , Rectum variation
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