Harmonization of proton treatment planning for head and neck cancer using pencil beam scanning: first report of the IPACS collaboration group

Robustness evaluation of pencil beam scanning proton therapy treatment planning: A systematic review

Compared to conventional radiotherapy using X-rays, proton therapy, in principle, allows better conformity of the dose distribution to target volumes, at the cost of greater sensitivity to physical, anatomical, and positioning uncertainties. Robust planning, both in terms of plan optimization and evaluation, has gained high visibility in publications on the subject and is part of clinical practice in many centers. However, there is currently no consensus on the methods and parameters to be used for robust optimization or robustness evaluation. We propose to overcome this deficiency by following the modified Delphi consensus method. This method first requires a systematic review of the literature. We performed this review using the PubMed and Web Of Science databases, via two different experts. Potential conflicts were resolved by a third expert. We then explored the different methods before focusing on clinical studies that evaluate robustness on a significant number of patients. Many robustness assessment methods are proposed in the literature. Some are more successful than others and their implementation varies between centers. Moreover, they are not all statistically or mathematically equivalent. The most sophisticated and rigorous methods have seen more limited application due to the difficulty of their implementation and their lack of widespread availability.

Proton therapy monitoring: spatiotemporal emission reconstruction with prompt gamma timing and implementation with PET detectors

Objective. In this study we introduce spatiotemporal emission reconstruction prompt gamma timing (SER-PGT), a new method to directly reconstruct the prompt photon emission in the space and time domains inside the patient in proton therapy.Approach. SER-PGT is based on the numerical optimisation of a multidimensional likelihood function, followed by a post-processing of the results. The current approach relies on a specific implementation of the maximum-likelihood expectation maximisation algorithm. The robustness of the method is guaranteed by the complete absence of any information about the target composition in the algorithm.Main results. Accurate Monte Carlo simulations indicate a range resolution of about 0.5 cm (standard deviation) when considering 10⁷primary protons impinging on an homogeneous phantom. Preliminary results on an anthropomorphic phantom are also reported.Significance. By showing the feasibility for the reconstruction of the primary particle range using PET detectors, this study provides significant basis for the development of an hybrid in-beam PET and prompt photon device.

Study of relationship between dose, LET and the risk of brain necrosis after proton therapy for skull base tumors

PURPOSE

We investigated the relationship between RBE-weighted dose (DRBE) calculated with constant (cRBE) and variable RBE (vRBE), dose-averaged linear energy transfer (LETd) and the risk of radiographic changes in skull base patients treated with protons.

METHODS

Clinical treatment plans of 45 patients were recalculated with Monte Carlo tool FRED. Radiographic changes (i.e. edema and/or necrosis) were identified by MRI. Dosimetric parameters for cRBE and vRBE were computed. Biological margin extension and voxel-based analysis were employed looking for association of DRBE(vRBE) and LETd with brain edema and/or necrosis.

RESULTS

When using vRBE, Dmax in the brain was above the highest dose limits for 38% of patients, while such limit was never exceeded assuming cRBE. Similar values of Dmax were observed in necrotic regions, brain and temporal lobes. Most of the brain necrosis was in proximity to the PTV. The voxel-based analysis did not show evidence of an association with high LETd values.

CONCLUSIONS

When looking at standard dosimetric parameters, the higher dose associated with vRBE seems to be responsible for an enhanced risk of radiographic changes. However, as revealed by a voxel-based analysis, the large inter-patient variability hinders the identification of a clear effect for high LETd.

Proton pencil-beam scanning radiotherapy in the treatment of nasopharyngeal cancer: dosimetric parameters and 2-year results

OBJECTIVES

 Patients with nasopharyngeal cancer are candidates for proton radiotherapy due to large and comprehensive target volumes, and the necessity for sparing of healthy tissues. The aim of this work is to evaluate treatment outcome and toxicity profile of patients treated with proton pencil-beam scanning radiotherapy.

MATERIALS AND METHODS

 Between Jan 2013 and June 2018, 40 patients were treated for nasopharyngeal cancer (NPC) with IMPT (proton radiotherapy with modulated intensity). Median age was 47 years and the majority of patients had locally advanced tumors (stage 2-8 patients. (20%); stage 3-18 patients (45%); stage 4A-10 patients. (25%); stage 4B-4 patients. (10%). Median of total dose was 74 GyE (70-76 GyE) in 37 fractions (35-38). Bilateral neck irradiation was used in all cases. Concomitant chemotherapy was applied in 34 cases. (85%). Median follow-up time was 24 (1.5-62) months.

RESULTS

Two-year overall survival (OS), disease-free survival (DFS), and local control (LC) were 80%, 75%, and 84%, respectively. Acute toxicity was generally mild despite large target volumes and concurrent application of chemotherapy with skin toxicity and dysphagia reported as the most frequent acute side effects. The insertion of a percutaneous endoscopic gastrectomy (PEG) was necessary in four cases (10%). Serious late toxicity (G > 3. RTOG) was observed in two patients (5%) (dysphagia and brain necrosis).

CONCLUSION

 IMPT for nasopharyngeal cancer patients is feasible with mild acute toxicity. Treatment outcomes are promising despite the high percentage of advanced disease in this group.

Combined proton-photon treatments - A new approach to proton therapy without a gantry

PURPOSE

Although the number of proton therapy centres is growing worldwide, proton therapy is still a limited resource. The primary reasons are gantry size and cost. Therefore, we investigate the potential of a new design for proton therapy, which may facilitate proton treatments in conventional bunkers and allow the widespread use of protons.

MATERIALS AND METHODS

The treatment room consists of a standard Linac for IMRT, a motorized couch for treatments in lying position, and a horizontal proton beamline equipped with pencil beam scanning. As proton beams are limited to a coronal plane, treatment plans may be suboptimal for many tumour sites. However, high-quality plans may be realized by combining protons and photons. Treatment planning is performed by simultaneously optimizing IMRT and IMPT plans based on their cumulative physical dose. We demonstrate this concept for three head&neck cancer cases.

RESULTS

Optimal combinations use photons to improve dose conformity while protons reduce the integral dose to normal tissues. In fact, combined treatments improve on single-modality IMRT and fixed beamline IMPT plans for quality-of-life-limiting OARs and retain most of the integral dose reduction in the healthy tissues of the pure IMPT plans. The lower doses that can be obtained with multi-modality treatments reduce the risk for side effects compared to single-modality IMRT plans.

CONCLUSION

Combined proton-photon treatments may play a role in developing a new solution for proton therapy without a gantry. Optimal combinations improve on IMRT plans and reduce the risk of side effects while making protons available to more patients.