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Towards automated on-line adaptation of 2-Step IMRT plans: QUASIMODO phantom and prostate cancer cases.

Holubyev K, Bratengeier K, Gainey M, Polat B, Flentje M - Radiat Oncol (2013)

Bottom Line: The large interfractional deformations of the clinical target volume (CTV) still require introduction of safety margins which leads to undesirably high rectum toxicity.The CTV is expanded by 10 mm resulting in the PTV; the posterior margin is limited to 7 mm.The adapted plans show statistically significant improvement of the target coverage and of the rectum sparing compared to those plans in which only the isocenter is relocated.

View Article: PubMed Central - HTML - PubMed

Affiliation: Klinik und Poliklinik für Strahlentherapie, Universitätsklinikum Würzburg, Würzburg, Germany. holubyev_k@klinik.uni-wuerzburg.de.

ABSTRACT

Background: The standard clinical protocol of image-guided IMRT for prostate carcinoma introduces isocenter relocation to restore the conformity of the multi-leaf collimator (MLC) segments to the target as seen in the cone-beam CT on the day of treatment. The large interfractional deformations of the clinical target volume (CTV) still require introduction of safety margins which leads to undesirably high rectum toxicity. Here we present further results from the 2-Step IMRT method which generates adaptable prostate IMRT plans using Beam Eye View (BEV) and 3D information.

Methods: Intermediate/high-risk prostate carcinoma cases are treated using Simultaneous Integrated Boost at the Universitätsklinkum Würzburg (UKW). Based on the planning CT a CTV is defined as the prostate and the base of seminal vesicles. The CTV is expanded by 10 mm resulting in the PTV; the posterior margin is limited to 7 mm. The Boost is obtained by expanding the CTV by 5 mm, overlap with rectum is not allowed. Prescription doses to PTV and Boost are 60.1 and 74 Gy respectively given in 33 fractions.We analyse the geometry of the structures of interest (SOIs): PTV, Boost, and rectum, and generate 2-Step IMRT plans to deliver three fluence steps: conformal to the target SOIs (S0), sparing the rectum (S1), and narrow segments compensating the underdosage in the target SOIs due to the rectum sparing (S2). The width of S2 segments is calculated for every MLC leaf pair based on the target and rectum geometry in the corresponding CT layer to have best target coverage. The resulting segments are then fed into the DMPO optimizer of the Pinnacle treatment planning system for weight optimization and fine-tuning of the form, prior to final dose calculation using the collapsed cone algorithm.We adapt 2-Step IMRT plans to changed geometry whilst simultaneously preserving the number of initially planned Monitor Units (MU). The adaptation adds three further steps to the previous isocenter relocation: 1) 2-Step generation for the geometry of the day using the relocated isocenter, MU transfer from the planning geometry; 2) Adaptation of the widths of S2 segments to the geometry of the day; 3) Imitation of DMPO fine-tuning for the geometry of the day.

Results and conclusion: We have performed automated 2-Step IMRT adaptation for ten prostate adaptation cases. The adapted plans show statistically significant improvement of the target coverage and of the rectum sparing compared to those plans in which only the isocenter is relocated. The 2-Step IMRT method may become a core of the automated adaptive radiation therapy system at our department.

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The prostate case of the largest volumetric difference between target SOIs in CT1 (left) and CT2 (right): top – BEVs; middle, bottom – CT layers showing an effect of increased bladder and rectum filling on Boost and PTV contours.
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Figure 9: The prostate case of the largest volumetric difference between target SOIs in CT1 (left) and CT2 (right): top – BEVs; middle, bottom – CT layers showing an effect of increased bladder and rectum filling on Boost and PTV contours.

Mentions: The adaptation result for the case with the largest volumetric difference between target SOIs in CT1 and CT2 is shown in Figure 8, right. Due to increased bladder and rectum filling in CT2 (Figure 9, top), the prostate is pushed down and the seminal vesicles are compressed between the bladder and the rectum. Consequently, the upper part of the PTV which encloses the base of the seminal vesicles extends further up in CT2 compared to CT1, and the Boost shrinks from 89 to 70 cm3, by 25%, as compared to much more moderate difference of Boost volumes for other patients, of the order of several percent. Additionally, the rectum folds in some CT2 layers in (Figure 9, bottom). As a result, the plans optimized for native geometries require 697 and 593 MUs, with the difference of the order of 15%, as compared to the difference of the order of 2% for other four patients. As DVHs in Figure 8, right, and corresponding evaluation in Table 2 show, the result of the adaptation to such a different geometry is marginal. The coverage gets even worse: SD for the adapted plan increases against the relocated plan from 31.5 to 36.8 Gy; however, rectum sparing improves considerably.


Towards automated on-line adaptation of 2-Step IMRT plans: QUASIMODO phantom and prostate cancer cases.

Holubyev K, Bratengeier K, Gainey M, Polat B, Flentje M - Radiat Oncol (2013)

The prostate case of the largest volumetric difference between target SOIs in CT1 (left) and CT2 (right): top – BEVs; middle, bottom – CT layers showing an effect of increased bladder and rectum filling on Boost and PTV contours.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4225755&req=5

Figure 9: The prostate case of the largest volumetric difference between target SOIs in CT1 (left) and CT2 (right): top – BEVs; middle, bottom – CT layers showing an effect of increased bladder and rectum filling on Boost and PTV contours.
Mentions: The adaptation result for the case with the largest volumetric difference between target SOIs in CT1 and CT2 is shown in Figure 8, right. Due to increased bladder and rectum filling in CT2 (Figure 9, top), the prostate is pushed down and the seminal vesicles are compressed between the bladder and the rectum. Consequently, the upper part of the PTV which encloses the base of the seminal vesicles extends further up in CT2 compared to CT1, and the Boost shrinks from 89 to 70 cm3, by 25%, as compared to much more moderate difference of Boost volumes for other patients, of the order of several percent. Additionally, the rectum folds in some CT2 layers in (Figure 9, bottom). As a result, the plans optimized for native geometries require 697 and 593 MUs, with the difference of the order of 15%, as compared to the difference of the order of 2% for other four patients. As DVHs in Figure 8, right, and corresponding evaluation in Table 2 show, the result of the adaptation to such a different geometry is marginal. The coverage gets even worse: SD for the adapted plan increases against the relocated plan from 31.5 to 36.8 Gy; however, rectum sparing improves considerably.

Bottom Line: The large interfractional deformations of the clinical target volume (CTV) still require introduction of safety margins which leads to undesirably high rectum toxicity.The CTV is expanded by 10 mm resulting in the PTV; the posterior margin is limited to 7 mm.The adapted plans show statistically significant improvement of the target coverage and of the rectum sparing compared to those plans in which only the isocenter is relocated.

View Article: PubMed Central - HTML - PubMed

Affiliation: Klinik und Poliklinik für Strahlentherapie, Universitätsklinikum Würzburg, Würzburg, Germany. holubyev_k@klinik.uni-wuerzburg.de.

ABSTRACT

Background: The standard clinical protocol of image-guided IMRT for prostate carcinoma introduces isocenter relocation to restore the conformity of the multi-leaf collimator (MLC) segments to the target as seen in the cone-beam CT on the day of treatment. The large interfractional deformations of the clinical target volume (CTV) still require introduction of safety margins which leads to undesirably high rectum toxicity. Here we present further results from the 2-Step IMRT method which generates adaptable prostate IMRT plans using Beam Eye View (BEV) and 3D information.

Methods: Intermediate/high-risk prostate carcinoma cases are treated using Simultaneous Integrated Boost at the Universitätsklinkum Würzburg (UKW). Based on the planning CT a CTV is defined as the prostate and the base of seminal vesicles. The CTV is expanded by 10 mm resulting in the PTV; the posterior margin is limited to 7 mm. The Boost is obtained by expanding the CTV by 5 mm, overlap with rectum is not allowed. Prescription doses to PTV and Boost are 60.1 and 74 Gy respectively given in 33 fractions.We analyse the geometry of the structures of interest (SOIs): PTV, Boost, and rectum, and generate 2-Step IMRT plans to deliver three fluence steps: conformal to the target SOIs (S0), sparing the rectum (S1), and narrow segments compensating the underdosage in the target SOIs due to the rectum sparing (S2). The width of S2 segments is calculated for every MLC leaf pair based on the target and rectum geometry in the corresponding CT layer to have best target coverage. The resulting segments are then fed into the DMPO optimizer of the Pinnacle treatment planning system for weight optimization and fine-tuning of the form, prior to final dose calculation using the collapsed cone algorithm.We adapt 2-Step IMRT plans to changed geometry whilst simultaneously preserving the number of initially planned Monitor Units (MU). The adaptation adds three further steps to the previous isocenter relocation: 1) 2-Step generation for the geometry of the day using the relocated isocenter, MU transfer from the planning geometry; 2) Adaptation of the widths of S2 segments to the geometry of the day; 3) Imitation of DMPO fine-tuning for the geometry of the day.

Results and conclusion: We have performed automated 2-Step IMRT adaptation for ten prostate adaptation cases. The adapted plans show statistically significant improvement of the target coverage and of the rectum sparing compared to those plans in which only the isocenter is relocated. The 2-Step IMRT method may become a core of the automated adaptive radiation therapy system at our department.

Show MeSH
Related in: MedlinePlus