Date Published: February 28, 2019
Publisher: Public Library of Science
Author(s): Maria Antico, Peter Prinsen, Francesco Cellini, Alice Fracassi, Alfonso A. Isola, David Cobben, Davide Fontanarosa, Gayle E. Woloschak.
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.
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.
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.
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.
One of the clinically most wide-spread treatment techniques in external beam radiation therapy (EBRT) for prostate cancer is intensity-modulated radiation therapy (IMRT), where the intensities of the treatment beams are optimized with an inverse planning method based on specific treatment objectives. Standard IMRT treatments, with techniques like step-and-shoot or sliding windows leaf sequencing algorithms, can be rather long (20 minutes or more). Therefore, it is important to ensure that the position of the treatment volume, which may move during treatment, is properly monitored in order to detect possible misdosage.
In this proof of principle study, we compared an IMRT adaptive planning method with the standard IMRT planning method and with an isocenter shift correction approach. Although the concept of library of plans has already been introduced in the literature, the adaptive planning method proposed here is based on the selection of beams during each treatment fraction for intra-fraction adaptation, as opposed to the selection of the treatment of the day implemented in previous works. The method produces inherently real-time adaptation, because no dose re-calculation or deformation is required at treatment, only a change of (pre-calculated) beams. Our hypothesis was that our method would be effective in a body region with a high degree of homogeneity in physical density: the pelvis for prostate EBRT. In such regions, in fact, small changes in tissue configurations will likely result in small changes (in terms of beam shapes and output values) in the treatment plans corresponding to the different anatomical configurations. This was indeed confirmed by the results we obtained, showing that beams can reasonably be combined from different plans because they have similar intensity matrices and similar MU values. This result is not trivial because, using inverse planning, the transition between beam shapes and MU values is not necessarily smooth when tissue configurations change.
The real-time adaptive planning method introduced in this proof-of-principle work for prostate cancer EBRT ensures, under the simulated treatment conditions, correct dose coverage for the treatment volume and significant toxicity reduction to the OARs. The adaptive planning method could be applied to standard fractionation treatments although in our study it proved most efficient when hypofrationation regimens are implemented. Further studies will be performed to test more extensively the proposed adaptive planning method and to evaluate it at a full clinical level.