Radical radiation therapy for carcinoma of the prostate in patients with a single hip prosthesis: a technique analysis using dose-volume histograms

Bilateral metallic hip implants: Are avoidance sectors necessary for pelvic VMAT treatments?

PURPOSE

Metallic hip implants (MHI) are common in elderly patients. For pelvic cancers radiotherapy, conventional approaches consist of MHI avoidance during treatment planning, which leads, especially in case of bilateral MHI, to a decreased quality or increased complexity of the treatment plan. The aim of this study is to investigate the necessity of using avoidance sectors (AvSe) using a 2-arcs coplanar pelvic volumetric modulated arc-therapy (VMAT) planning.

METHODS

We evaluated: (1) The dose calculation error of a static 6MV open beam traversing a MHI; (2) The magnitude of an error's decrease within the planning target volume (PTV) for a 360° VMAT treatment without AvSe as compared to the static open beam; (3) The dosimetric influence of MHI misalignment generated by patient's repositioning rolls during image-guided radiotherapy (IGRT).

RESULTS

(1) In the static 6MV beam configuration, for distances between 0.5cm and 6cm from the MHI, the median (maximum, number of points) dose calculation error was -1.55% (-2.5%, 11); (2) Compared to the static open beam, in the 360° VMAT treatment without AvSe a simulated error was decreased by a factor of 4.4/2.4 (median/minimum); (3) MHI anterior-posterior misalignment exceeding 0.6cm, resulted in error at PTV surface of >2%.

CONCLUSIONS

A standard 2 coplanar arcs 360° VMAT treatment, with dedicated artifact reduction algorithms applied, decreased the error of static beam traversing MHI, in patients presenting a bilateral MHI and might be used to treat the pelvic region without MHI avoidance.

Dosimetry Effects Caused by Unilateral and Bilateral Hip Prostheses: A Monte Carlo Case Study in Megavoltage Photon Radiotherapy for Computed Tomography Data without Metal Artifacts

Background:

Hip prostheses (HPs) are routinely used in hip augmentation to replace painful or dysfunctional hip joints. However, high-density and high-atomic-number (Z) inserts may cause dose perturbations in the target volume and interface regions.

Aim:

To evaluate the dosimetric influence of various HPs during megavoltage conformal radiotherapy (RT) of the prostate using Monte Carlo (MC) simulations.

Materials and Methods:

BEAMnrc and DOSXYZnrc MC user-codes were respectively used to simulate the linac head and to calculate 3D absorbed dose distributions in a computed tomography (CT)-based phantom. A novel technique was used to synthetically introduce HPs into the raw patient CT dataset. The prosthesis materials evaluated were stainless steel (SS316L), titanium (Ti6Al4V), and ultra-high-molecular-weight polyethylene (UHMWPE). Four, five, and six conformal photon fields of 6–20 MV were used.

Results:

The absorbed dose within and beyond metallic prostheses dropped significantly due to beam attenuation. For bilateral HPs, the target dose reduction ranged up to 23% and 17% for SS316L and Ti6Al4V, respectively. For unilateral HP, the respective dose reductions were 19% and 12%. Dose enhancement was always <1% for UHMWPE. The 6-field plan produced the best target coverage. Up to 38% dose increase was found at the bone–SS316L proximal interface.

Conclusions:

The novel technique used enabled the complete exclusion of metal artifacts in the CT dataset. High-energy plans with more oblique beams can help minimize dose attenuation through HPs. Shadowing and interface effects are density dependent and greatest for SS316L, while UHMWPE poses negligible dose perturbation.

[Dosimetric influence of hip prosthesis during radiotherapeutic treatement]

As the population become aged, many patients with hip prosthesis are treated for a pelvic cancer. The recommended ballistic must avoid to pass in the prosthesis, but sometimes it is inevitable. So it is essential to quantify with accuracy the dose modifications linked to the presence of metallic implant. The aim of this study is to analyze by Monte Carlo method these modifications in simple and complex models (anthropomorphic phantom) which take into account the geometry and the composition of the prosthesis and its coatings. Then, this methodology was used to study the behaviour of a treatment planning system in theses extreme conditions.

A case study of radiotherapy planning for a bilateral metal hip prosthesis prostate cancer patient

The purpose of this report is to communicate the observed advantage of intensity-modulated radiotherapy (IMRT) in a patient with bilateral metallic hip prostheses. In this patient with early-stage low-risk disease, a dose of 74 Gy was planned in two phases--an initial 50 Gy to the prostate and seminal vesicles and an additional 24 Gy to the prostate alone. Each coplanar beam avoided the prosthesis in the beam's eye view. Using the same target expansions for each phase, IMRT and 3D-conformal radiotherapy (CRT) plans were compared for target coverage and inhomogeneity as well as dose to the bladder and rectum. The results of the analysis demonstrated that IMRT provided superior target coverage with reduced dose to normal tissues for both individual phases of the treatment plan as well as for the composite treatment plan. The dose to the rectum was significantly reduced with the IMRT technique, with a composite V 80 of 35% for the IMRT plan versus 70% for 3D-CRT plan. Similarly, the dose to the bladder was significantly reduced with a V 80 of 9% versus 20%. Overall, various dosimetric parameters revealed the corresponding 3D-CRT plan would not have been acceptable. The results indicate significant success with IMRT in a clinical scenario where there were no curative alternatives for local treatment other than external beam radiotherapy. Therefore, definitive external beam radiation of prostate cancer patients with bilateral prosthesis is made feasible with IMRT. The work described herein may also have applicability to other groups of patients, such as those with gynecological or other pelvic malignancies.

Dosimetric considerations for patients with HIP prostheses undergoing pelvic irradiation. Report of the AAPM Radiation Therapy Committee Task Group 63

This document is the report of a task group of the Radiation Therapy Committee of the AAPM and has been prepared primarily to advise hospital physicists involved in external beam treatment of patients with pelvic malignancies who have high atomic number (Z) hip prostheses. The purpose of the report is to make the radiation oncology community aware of the problems arising from the presence of these devices in the radiation beam, to quantify the dose perturbations they cause, and, finally, to provide recommendations for treatment planning and delivery. Some of the data and recommendations are also applicable to patients having implanted high-Z prosthetic devices such as pins, humeral head replacements. The scientific understanding and methodology of clinical dosimetry for these situations is still incomplete. This report is intended to reflect the current state of scientific understanding and technical methodology in clinical dosimetry for radiation oncology patients with high-Z hip prostheses.