Real-time adaptive planning method for radiotherapy treatment delivery for prostate cancer patients, based on a library of plans accounting for possible anatomy configuration changes

Abstract

Background and purpose: In prostate cancer treatment with external beam radiation therapy (EBRT), prostate motion and internal changes in tissue distribution can lead to a decrease in plan quality. In most currently used planning methods, the uncertainties due to prostate motion are compensated by irradiating a larger treatment volume. However, this could cause underdosage of the treatment volume and overdosage of the organs at risk (OARs). To reduce this problem, in this proof of principle study we developed and evaluated a novel adaptive planning method. The strategy proposed corrects the dose delivered by each beam according to the actual position of the target in order to produce a final dose distribution dosimetrically as similar as possible to the prescribed one.

Material and methods: Our adaptive planning method was tested on a phantom case and on a clinical case. For the first, a pilot study was performed on an in-silico pelvic phantom. A "library" of intensity modulated RT (IMRT) plans corresponding to possible positions of the prostate during a treatment fraction was generated at planning stage. Then a 3D random walk model was used to simulate possible displacements of the prostate during the treatment fraction. At treatment stage, at the end of each beam, based on the current position of the target, the beam from the library of plans, which could reproduce the best approximation of the prescribed dose distribution, was selected and delivered. In the clinical case, the same approach was used on two prostate cancer patients: for the first a tissue deformation was simulated in-silico and for the second a cone beam CT (CBCT) taken during the treatment was used to simulate an intra-fraction change. Then, dosimetric comparisons with the standard treatment plan and, for the second patient, also with an isocenter shift correction, were performed.

Results: For the phantom case, the plan generated using the adaptive planning method was able to meet all the dosimetric requirements and to correct for a misdosage of 13% of the dose prescription on the prostate. For the first clinical case, the standard planning method caused underdosage of the seminal vesicles, respectively by 5% and 4% of the prescribed dose, when the position changes for the target were correctly taken into account. The proposed adaptive planning method corrected any possible missed target coverage, reducing at the same time the dose on the OARs. For the second clinical case, both with the standard planning strategy and with the isocenter shift correction target coverage was significantly worsened (in particular uniformity) and some organs exceeded some toxicity objectives. While with our approach, the most uniform coverage for the target was produced and systematically the lowest toxicity values for the organs at risk were achieved.

Conclusions: In our proof of principle study, the adaptive planning method performed better than the standard planning and the isocenter shift methods for prostate EBRT. It improved the coverage of the treatment volumes and lowered the dose to the OARs. This planning method is particularly promising for hypofractionated IMRT treatments in which a higher precision and control on dose deposition are needed. Further studies will be performed to test more extensively the proposed adaptive planning method and to evaluate it at a full clinical level.

Fig 1. Workflow chart for the tests implemented.

Fig 2. The DVHs for the plans generated for the phantom case.

Top: PTV planning method with NO intra-fraction motion (continuous line); PTV planning method with intra-fraction motion (dashed line). Bottom: adaptive planning method (continuous line) and PTV planning method with intra-fraction motion (dashed line). The prostate shift at the end of the fraction is 1.25 cm. The arrows indicate the DVHs for the prostate (CTV). The DVHs are shown for all the organs involved: the diamonds correspond to the objectives imposed on each of the organs. The doses refer to the total treatment prescriptions.

Fig 3. Isodose lines for two pelvic slices.

In the top and the bottom figure a, b, c and d correspond respectively to the PTV planning method without/with intra-fraction motion and the adaptive planning method with/without MU rescaling (both with intra-fraction motion). In the top figure in a and b the PTV is shown as a grey shaded area surrounding the prostate. The prostate is visible only in the slice shown in the top figure.

Fig 4. The DVHs for the plans optimized using pure fluence (top figure) and DMPO (bottom figure).

PTV planning method (dashed line); PTV planning method with rigid isocenter shifts (dashed dot line); adaptive planning method (continuous line). The prostate shift at the end of the fraction is about 0.5 cm. The arrows indicate the DVHs for the prostate (CTV), bladder and rectum. The DVHs are shown for all the organs involved: the diamonds correspond to the objectives imposed on each of the organs and the stars correspond to the objectives not satisfied by the PTV planning method. The doses refer to the total treatment prescriptions.