Quality assurance evaluation of spot scanning beam proton therapy with an anthropomorphic prostate phantom

Objective:

The purpose of this study was to evaluate spot scanning proton therapy with an anthropomorphic prostate phantom at the Proton Therapy Center of The University of Texas MD Anderson Cancer Center at Houston, TX (PTCH).

Methods:

An anthropomorphic prostate phantom from the Radiological Physics Center (RPC), The University of Texas MD Anderson Cancer Center, Houston, TX, was used, which contained thermoluminescent dosemeters and GAFCHROMIC^® EBT2 film (ISP Technologies, Wayne, NJ). The phantom was irradiated by the Hitachi synchrotron (Hitachi America, Ltd, Tarrytown, NY), and the results were compared between the treatment planning system (TPS) and RPC measurements.

Results:

RPC results show that the right/left, inferior/superior and posterior/anterior aspects of the coronal/sagittal and EBT2 film measurements were within ±7%/±4 mm of the TPS. The RPC thermoluminescent dosemeter measurements of the prostate and femoral heads were within 3% of the TPS.

Conclusion:

The RPC prostate phantom is a useful mechanism to evaluate spot scanning beam proton therapy within certain confidence levels.

Advances in knowledge:

The RPC anthropomorphic prostate phantom could be used to establish quality assurance of spot scanning proton beam for patients with prostate cancer.During the past decade, the use of proton beams in the treatment of cancer has increased. Passive scattering has been the most common technique for delivery of proton beams. Passive scattering uses range modulation wheels with a combination of field shaping apertures and compensators to deliver a uniform dose distribution to the target [1–4]. Until recently, the proton scanning technology has only been available at one facility, namely the Paul Scherrer Institute in Switzerland [5]. The spot-scanning beam at the Paul Scherrer Institute moves only along the longitudinal axis and is combined with a moving couch. The scanning proton beams confine the dose distribution to the target volume by depositing the dose using pencil beams of different energies to deliver the dose without using any scattering or range-modulating devices [6]. This is considered an improvement with respect to passively scattered proton beams because there are no apparatuses in the beam path to produce contamination by neutrons [7].The principle of scanning beam is simple and is based on the fact that protons, being charged particles, are subject to Lorentz forces. That is, when subjected to an electric field, protons are accelerated, and when subjected to a magnetic field, protons are deflected. In depth, the Bragg peaks are stacked by altering the proton energy. Through this combination of scanning and energy variation, the Bragg peak can be effectively placed anywhere within the target in three dimensions. Dose uniformity is then achieved by a treatment planning optimisation of the individual fluences of each pencil beam [8]. The Radiological Physics Center (RPC), The University of Texas MD Anderson Cancer Center, Houston, TX, has several anthropomorphic phantoms, which are used as part of the credentialing services for participation in the National Cancer Institute (NCI, at the National Institutes of Health, Bethesda, MD) sponsored clinical trials. The pelvis phantom was originally designed to be of mailable quality assurance to test intensity-modulated radiation therapy (IMRT) procedures. With this in mind, the design and materials were chosen to simulate the pelvis region of a patient with the anatomy present to create restrictions for treatment planning and delivery in typical IMRT cases. The relative stopping power of each material is used to construct the phantom to verify the tissue equivalence of the materials used. In the treatment of prostate cancer, the Proton Therapy Center at Houston, TX (PTCH), uses two opposing beams. Previously, RPC used a prostate phantom for passive scattering proton beam and found that by using an appropriate value of relative stopping power, the prostate phantom could be used to evaluate proton passive beam therapy. The evaluation of spot scanning is a new challenge. Therefore, the purpose of this study was to evaluate spot scanning for prostate therapy using the RPC phantom.