Presenter: M. Fitzek, K. Shahnazi, J. Simmons, A. Thornton Presenter's Affiliation: Midwest Proton Radiotherapy Institute, Bloomington, IN, USA Type of Session: Scientific
Options for the treatment of clinically localized prostate cancer include radical prostatectomy and radiation therapy, and the latter can be delivered via brachytherapy, external beam photon therapy, or proton therapy.
Protons differ from photons in their physical properties, as the latter lack mass and charge.
Charged particle beam therapy with protons allows the delivery of higher doses of conformal radiation due to the dose characteristics of proton beam radiotherapy, including finite range and steep dose fall off beyond the Bragg peak.
Most external-beam radiotherapy treatments are planned primarily on the basis of three-dimensional (3D) kilovoltage computed tomography (kVCT) scans.
Images are typically reconstructed from measurements of the photon transmission through the patient (Hoheisel, 2006).
Metallic implants (e.g., hip prostheses and dental implants) can severely reduce the transmission of photons, leading to a saturation of the CT image in the region of the implant and to streaking artifacts that may extend far from the implant (Jakel, 2007).
These artifacts can result in errors in the dose distribution calculated by the treatment planning system.
Specifically, for photon beam radiotherapy, the planning system converts CT images from Hounsfield units (HU) into electron densities.
Patients with unilateral hip replacements are not currently treated with protons secondary to the inability to deliver a lateral field through the hip prosthesis.
In photon beam therapy, streak artifacts are of less concern because they produce relatively small errors in predicted dose, as the therapeutic photon dose is attenuated exponentially with depth in the patient (Newhauser, 2008)
In contrast, streak artifacts are of greater concern in proton therapy because they translate into proton range errors, which then may result in severe underdosing of the target volume (Wei,2006) or overdosing of normal tissue distal to the target.
Therefore, if protons are to be used in patients with a unilateral hip prostheses, beam orientations should be selected to avoid passing through or near to the metal implant to the extent possible (Jakel, 2007), which is the clinical problem addressed in this presentation.
Materials and Methods
This work represents a small, single institution experience at the Midwest Proton Radiotherapy Institute in Bloomington, Indiana.
Five patients with unilateral hip replacements were treated at the Midwest Proton Radiotherapy Institute from 2008–2009.
3 patients were clinical T1c and 2 were clinical T2a
3 patients had Gleason 6 and 2 had Gleason 7 disease.
Pre-treatment PSAs ranged from 3.5-14 ng/mL.
Patients were treated with a combination of a lateral proton field through the natural hip and one or two anterior oblique fields.
The prescription doses to the prostate were 70.2 CGE (cobalt Gray equivalents), 75.6 CGE (1 patient), and 79.2 CGE (3 patients).
The target was identified as the prostate and seminal vesicles, with the anterior rectal wall and bladder contoured as organs at risk (OARs).
CTV1 was defined as the prostate (GTV) and seminal vesicles plus a 1.1 cm margin. CTV2 was defined as the prostate plus a 0.5 cm margin.
A PTV of 2 mm was added to CTV1 and CTV2.
CTV1 was prescribed 50.4 CGE, and CTV2 received between 19.8 and 28.8 CGE (1 x 19.8 CGE, 1 x 25.2 CGE and 3 x 28.8 CGE).
Dose constraints were as follows:
30% or less of the anterior rectal wall could receive 70.2 CGE.
10% or less of the anterior rectal wall could receive 77.4 CGE.
20% or less of the bladder could receive 60 CGE.
In one patient with a small bladder, a plan with 30% of the bladder receiving > 60 GyE was accepted.
The seminal vesicles were treated in all patients to 50.4 GyE.
The median prostate gland size on planning CT was 76 ml (range 23 - 81 ml).
A standardized bladder-filling regimen was used with a single prostate marker insertion, and a rectal balloon was used and filled with 60 cc of fluid in all patients.
A single prostate fiducial marker was implanted for image guidance.
In this small series of patients, the combination of a lateral proton field through the natural hip and one or two anterior oblique fields was successfully planned and executed in patients with low-risk and intermediate-risk organ-confined prostate cancer.
Setup and daily treatment was reproducible using anterior oblique fields, and good reproducibility of the plans was noted on repeat CT scans in each patient.
Treatment was well tolerated and comparable to the conventional technique with lateral fields.
The median follow-up period was 9 months.
All patients had evidence of falling PSA at the median follow-up of 9 months.
It is feasible to treat prostate cancer with protons in the presence of unilateral hip replacement by modifying the treatment approach, substituting the lateral field from the side of the replaced hip with 2 anterior oblique fields.
The treatment and setup technique employed in this study may be used to optimize the treatment of non-hip replacement prostate cancer patients.
Historically, patients with unilateral hip prostheses have not been treated with protons. This is due to concerns regarding the perceived difficulty with delivering a lateral field through the hip prosthesis because of streak artifact and proton range errors, which then may result in severe underdosing of the target volume or overdosing of normal tissue distal to the target.
This study employs an approach using a lateral proton field through the natural hip and one or two anterior oblique fields in patients with a single hip prosthesis.
Based on this small series of patients, treatment using this alternative beam arrangement may render it possible to safely treat organ-confined prostate cancer patients using protons in patients with a single hip prosthesis without concern regarding target underdosing and normal tissue overdose.
The data presented here appear interesting and promising, as they establish the possibility of safely and reproducibly treating this previously proton-ineligible group of patients. However, the small sample size and short follow-up time presented preclude making statements regarding effectiveness and toxicity in the definitive setting.
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