Spot scanning proton therapy: single field simultaneous integrated boost
Reporter: Gita Suneja, MD
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: May 14, 2011
Presenter: X Ronald Zhu, PhD Presenter's Affiliation: The University of Texas MD Anderson Cancer Center
Spot scanning proton therapy (SSPT) is the fast, dynamic scanning of a proton pencil beam over a target volume to provide better conformal coverage. Many individualized static dose applications are superimposed, creating a total volume conforming very well to the target.
In treatment planning of SSPT, the weight of each spot is optimized using an inverse planning process. This means once dose to the target volume and dose constraints to normal structures are set, a computer program generates fields that satisfy dose tolerances.
In multi-field optimization (MFO) or intensity-modulated proton therapy (IMPT), all spots from all fields are optimized simultaneously.
In single field optimization (SFO), each field is optimized individually to deliver the prescribed dose to the target volume while respecting tolerance of normal structures. Either single field uniform dose (SFUD) or multi-field dose can be delivered. Multiple fields are normally used for treatment.
At present, use of MFO-IMPT is limited due to uncertainty and concerns that optimization may not be robust. SFO is generally considered to be more robust but less flexible than MFO.
The purpose of this work was to devise a single field integrated boost (SFIB) technique for SSPT. This would afford the flexibility of MFO with the robustness of SFO.
Materials and Methods
SFIB plans were designed to deliver multiple dose levels to multiple contoured targets using SFO for each field.
In the Eclipse planning system, SFO with simultaneous spot optimization was used.
Dose levels and dose constraints were determined by the treating physicians as per routine clinical protocol.
The treatment plans of three patients were presented.
The first is an 11 year old boy with a brain tumor treated with 54 CGE to the GTV and 50 CGE to the CTV using a 3-field technique. The treatment was planned using SFIB technique and a sequential (two plan) technique. The advantages of the SFIB were more conformal dose distribution with better DVH characteristics and more favorable quality assurance testing.
The second case is a 55 year old male with stage III prostate cancer. He was treated with 78 CGE to the CTV1 (prostate and proximal seminal vesicle) and 60 CGE to CTV2 (distal seminal vesicle) using a 2-field technique. The challenge in this case was the anatomical shape of the seminal vesicles. The dose distribution was similar with SFIB and sequential technique, however the SFIB plan was more conformal.
The third case is a 39 year old female with an L1-L2 ependymoma treated post-operatively with one posterior field at three dose levels: 54 CGE, 50 CGE, and 45 CGE. The normal structure of concern in this case was skin, and dose constraints were met.
The SFIB technique for SSPT was successfully implemented in this cohort of patients.
The advantages of SFIB over conventional sequential delivery include uninterrupted treatment delivery, as well as more efficient treatment planning and quality assurance checks.
The development of a sequential boost technique using SFIB instead of IMPT allows for more patients to received highly conformal proton therapy in the absence of advanced treatment planning technology.
This accomplishment also removes the obstacle of inferior robustness of IMPT plans while maintaining the flexibility afforded by IMPT.
As seen in the cases presented, patients with complex anatomy can be treated with very conformal plans with SFIB technique.
This work also highlights the presence of a "learning curve" when developing a new treatment technique. Collaboration between institutions with more and less proton experience will improve treatment planning and patient outcomes.
As various proton therapy techniques are developed, evaluation of both the dosimetric parameters and the biologic effectiveness will be of critical importance.
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