Improving plan quality and consistency by standardization of dose constraints in prostate cancer patients treated with CyberKnife - PubMed (original) (raw)

Improving plan quality and consistency by standardization of dose constraints in prostate cancer patients treated with CyberKnife

Martina Descovich et al. J Appl Clin Med Phys. 2013.

Abstract

Treatment plans for prostate cancer patients undergoing stereotactic body radiation therapy (SBRT) are often challenging due to the proximity of organs at risk. Today, there are no objective criteria to determine whether an optimal treatment plan has been achieved, and physicians rely on their personal experience to evaluate the plan's quality. In this study, we propose a method for determining rectal and bladder dose constraints achievable for a given patient's anatomy. We expect that this method will improve the overall plan quality and consistency, and facilitate comparison of clinical outcomes across different institutions. The 3D proximity of the organs at risk to the target is quantified by means of the expansion-intersection volume (EIV), which is defined as the intersection volume between the target and the organ at risk expanded by 5 mm. We determine a relationship between EIV and relevant dosimetric parameters, such as the volume of bladder and rectum receiving 75% of the prescription dose (V75%). This relationship can be used to establish institution-specific criteria to guide the treatment planning and evaluation process. A database of 25 prostate patients treated with CyberKnife SBRT is used to validate this approach. There is a linear correlation between EIV and V75% of bladder and rectum, confirming that the dose delivered to rectum and bladder increases with increasing extension and proximity of these organs to the target. This information can be used during the planning stage to facilitate the plan optimization process, and to standardize plan quality and consistency. We have developed a method for determining customized dose constraints for prostate patients treated with robotic SBRT. Although the results are technology specific and based on the experience of a single institution, we expect that the application of this method by other institutions will result in improved standardization of clinical practice.

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Figures

Figure 1

Figure 1

Anatomy of prostate patients. Examples of favorable (left) and unfavorable (right) patient anatomy for a prostate SBRT treatment. Linear dimensions of bladder‐PTV and rectum‐PTV interface are displayed on the same scale.

Figure 2

Figure 2

Correlation between V75% and EIV of bladder and rectum. V75% of bladder plus rectum (total V75%) plotted as a function of EIV of bladder plus rectum (total EIV). The open circles represent data points that are above the dose constraints: total V75% <5 cm3. The open square represents the data point that is above the empirical threshold line.

Figure 3

Figure 3

Correlation between V75% and EIV of bladder and rectum — re‐optimized plans. V75% of bladder plus rectum (total V75%) plotted as a function of EIV of bladder plus rectum (total EIV). Correlation line and prediction intervals are displayed. The open circles represent data for the re‐optimized treatment plans.

Figure 4

Figure 4

Correlation between V75% and EIV of bladder and rectum — validation data. V75% of bladder plus rectum (total V75%) plotted as a function of EIV of bladder plus rectum (total EIV). The open squares represent the validation data.

Figure 5

Figure 5

Geometrical disposition of prostate and organs at risk.

Figure 6

Figure 6

Correlation between V75% and EIV of bladder and rectum — conformity index. Total V75% as a function of total EIV for plans with low nCI (black triangles) and high nCI (open squares). The two correlation lines are also displayed.

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