| Περίληψη: | In the treatment of prostate cancer with external beam radiation therapy (EBRT), internal changes in tissue distribution that can occur during the course of radiotherapy treatment can lead to inaccurate dose delivery, that is, underdosage of the treatment volume and overdosage of the organs at risk. This results in decrease in plan quality. Adaptive radiation therapy (ART) is a technique that intends to improve the therapeutic ratio by monitoring and identifying patient-specific treatment variations and incorporating them to modify the treatment plan, reducing the negative effects of these variations.
The purpose of this study is to assess the volumetric and dosimetric changes in the rectum during the course of radiotherapy using cone-beam computed tomography (CBCT) images that were acquired prior to treatment delivery and compare these changes with the corresponding ones of the original plan. Then, an adaptive planning method will be proposed which aims to mitigate these variations and result in an improved outcome as close as possible to that of the original plan.
This retrospective study included 17 prostate cancer patients who underwent hypofractionated volumetric modulated arc therapy (VMAT) treatment. The patients were treated with a total dose of 65 Gy over 26 fractions (2.5 Gy/fraction). CBCT images that were acquired prior to the delivery of every fraction, were used for delineation of the rectum and calculation of the rectal volume. Then, with the application of rigid image registration, calculations of rectal volume that was included in the high-dose region of the PTV (Vin), the Dice coefficient (DSC), the mean (Dmean) and maximum (Dmax) dose to the rectum, as well as the volume of the rectum that receives at least 66 Gy (V66Gy) were performed. Afterwards, a library of plans was created for 5 selected patients where the V66Gy was exceeding the acceptance criteria for rectum constraints (V66Gy>5%), and for 1 additional patient where the rectal volume was significantly larger in the pCT compared to the CBCTs. The plans were created using the Monaco Treatment Planning System (TPS) (Elekta, Crawley, UK). The aforementioned calculations were performed on the adaptive plans and the differences between non-adaptive and adaptive treatment were assessed.
In the non-adaptive treatment, rectal volume varied significantly over the course of the treatment for all patients and the Vin was considerably greater in the CBCTs compared to the pCT for most of the cases. Also, the actual Dmean and Dmax received by the rectum as well as the V66Gy were higher compared to the original plan in most patients. Specifically, V66Gy was exceeding 5% in 7 patients. The adaptive treatment compensated for anatomical changes and improved volume changes, Vin, and DSC significantly (p<0.05), as well as the respective dispersions. Compared with non-adaptive treatment, this strategy resulted in a significant (p<0.05) decrease of Dmean, Dmax, and V66Gy. Additionally, for the patient where the rectal volume was significantly larger in the pCT compared to the CBCTs, DSC was significantly improved.
In conclusion, an adaptive strategy using plan selection performed better than the non-adaptive treatment for prostate cancer radiotherapy. The dose to the rectum was decreased significantly when the adaptive treatment was applied. This is important for hypofractionated treatments in which a higher control and accuracy on dose deposition is required.
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