DA Martin BAppSc; G Hruby BHB, MBChB; MK Whitaker BAppSc (Hon), MAppSc; KY-M Foo BSc, MBBS (Hon).
Radiation Oncology—Technical Article
Constrained-beam inverse planning for intensity-modulated radiation therapy of prostate cancer patients with bilateral hip prostheses
Article first published online: 9 OCT 2012
© 2012 The Authors. Journal of Medical Imaging and Radiation Oncology © 2012 The Royal Australian and New Zealand College of Radiologists
Journal of Medical Imaging and Radiation Oncology
Volume 56, Issue 6, pages 703–707, December 2012
How to Cite
Martin, D. A., Hruby, G., Whitaker, M. K. and Foo, K. Y.-M. (2012), Constrained-beam inverse planning for intensity-modulated radiation therapy of prostate cancer patients with bilateral hip prostheses. Journal of Medical Imaging and Radiation Oncology, 56: 703–707. doi: 10.1111/j.1754-9485.2012.02456.x
Conflict of interest: None.
- Issue published online: 5 DEC 2012
- Article first published online: 9 OCT 2012
- Manuscript Accepted: 3 JUN 2012
- Manuscript Received: 15 FEB 2012
- hip prosthesis;
- intensity-modulated radiation therapy;
- prostate cancer;
Hip prostheses present a technical challenge in the planning of curative external beam radiation treatment for patients with prostate cancer. Bilateral prostheses compel planners to compromise between target coverage and avoidance of beam entry through the prostheses. Inverse planning systems given objectives to avoid dose to prostheses are overly restricted from allowing exit dose to them. We report a novel inverse planning technique for intensity-modulated radiation therapy of patients with prostate cancer and bilateral hip prostheses, by constraining beam characteristics rather than dose in the inverse planning process.
Prostate cancer affects an older male population, with age-standardised incidence increasing sharply from less than 400 per 100 000 population aged younger than 60 years to approximately 1000 per 100 000 population aged 65 years or more. This older population is furthermore affected by other age-related co-morbidities such as osteoarthritis, for which a common treatment is hip replacement. The number of hip replacements performed per year has increased from 210 per 100 000 males in 1998–1999 to 265 in 2006–2007. It is likely that there will be increasing numbers of patients with bilateral hip prostheses who are also seeking treatment for localised prostate cancer.
The treatment options for these patients are the same as that of the general prostate cancer population. Depending on risk, options include: radical prostatectomy; external beam radiation therapy; interstitial brachytherapy (with or without external beam radiation); and active surveillance. In the post-prostatectomy setting, either adjuvantly or where biochemical failure has occurred, curative (salvage) radiation therapy may also be recommended.
Where patients with hip prostheses choose curative-intent treatment with external beam radiation, two major technical difficulties can arise. The first, more pertinent to the clinician, is that CT artefact from the high-density, high atomic number prosthesis, or bilateral prostheses, can obscure anatomical features and impair accurate delineation of target volumes as well as organs at risk. A number of approaches have been suggested to overcome this, including the use of co-registered or fused MRI. Due to scheduling difficulties, MRI was not used in this case study.
The second difficulty predominantly affects the radiation therapist and physicist, and arises from the inability to accurately determine the electron density of the prostheses from the planning CT scan, and for planning systems to accurately model dose in, near and beyond the prosthesis for each megavoltage photon beam.[6, 7]
The American Association of Physicists in Medicine: Radiation Therapy Committee Task Group 63 (AAPM TG 63) recommends a beam arrangement that avoids the prostheses in the first instance, including a protocol for dealing with CT streak artefacts and verification imaging prior to treatment to ensure that the prosthesis does not shadow any part of the planning target volume (PTV). In the event that the beam arrangement includes dose through the prosthesis, AAPM TG 63 recommends consideration of the dose to organs at risk, the approximate magnitude of the dose perturbation due to the prosthesis and the ability of the treatment planning system to calculate this accurately; it also includes a protocol of calculation and dose verification should the type of prosthesis be known.
In this report, we outline our modified departmental approach of allowing beams to exit, but not enter through, the prostheses in patients with bilateral hip replacements.
A 69-year-old man with a history of a left total hip replacement underwent radical prostatectomy in July 2007 for a Gleason score of 4 + 3 adenocarcinoma involving 20% of the gland. The posterior margin of resection was positive at the mid-gland corresponding to a region of extracapsular extension. He was not referred for consideration of adjuvant radiation at this time. In 2008, his prostate-specific antigen (PSA) was undetectable, and he underwent a right total hip replacement. The PSA rose from 0.02 in January 2009 to 0.11 in September 2010. After multidisciplinary discussion, he was recommended, and consented to, a course of salvage radiation treatment to the prostatic fossa.
The challenge in the treatment planning of this case was to deliver salvage radiation therapy to a dose of 66 Gy in 33 fractions, avoiding the hip prostheses as per AAPM TG 63, and respecting the rectal dose–volume constraints and target coverage as per the Australia/New Zealand consensus guidelines. These guidelines state that rectal dose should not exceed either 40 Gy to 60% of rectal volume or 60 Gy to 40% of rectal volume (contoured from sigmoid flexure to 1.5 cm inferior to the clinical target volume (CTV)). Target coverage in these guidelines is defined more completely in the reference document, but in brief, CTV borders are: 6 mm distal to anastamosis (inferior); posterior pubic symphysis/posterior bladder wall (anterior); seminal vesicle bed (superior); anterior rectal wall/mesorectal fascia (posterior). PTV expansion is 1 cm.
The patient attended the simulation appointment with an empty bowel and comfortably filled bladder. He had opened his bowels that morning and, 45 minutes before the scan, had emptied his bladder and consumed 600 mL of water. He was positioned supine with a Civco knee rest and ankle-fixation (Civco Medical Solutions, Orange County, CA, USA). CT data acquisition was performed using a Toshiba Aquilion LB CT scanner (Toshiba Medical Systems Corporation, Tochigi, Japan) and reconstruction grid size was 3 mm.
In the planning phase, we followed AAPM TG 63 recommendations to turn off heterogeneity correction in regions affected by CT artefact. To do this, we created a structure encompassing the affected regions and applied a Hounsfield density value of 0. We then selected beam angles that avoided entry through prostheses. An intensity-modulated radiation therapy (IMRT) plan was generated using the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA, USA). Inverse planning objectives are listed in Table 1. A seven-field technique was used with gantry angles of 0, 45, 90, 130, 220, 270 and 315 degrees, as shown in Figure 1. Directly opposing fields are not usually recommended in IMRT planning ; however, in this case, opposed lateral beams were used as the patient's prostheses limited the angles available to adequately avoid the rectum. Beam angles 0, 130 and 220 degrees were chosen as they allowed complete treatment of the PTV without entering through the metal hips. The other angles were chosen to provide rectal and bladder avoidance. They were constrained from entering via the prostheses by locking the primary collimator (jaws) before allowing IMRT optimisation (Figs 2a,b and 3). However, beams were allowed to deliver exit dose to the prostheses. Locking the primary jaws allows partial treatment of the PTV that is not obstructed by the prosthesis. This forces the optimisation to only use beams with these restricted jaw positions in the inverse planning process.
|Site||Volume (%)||Dose (cGy)||Priority|
The novel element in the technique described in this report is the use of IMRT inverse planning together with a geometric, rather than a dosimetric, constraint. Previous reports of IMRT planning for treatment of similar cases have utilised dosimetric constraints such as allowing little to no dose to enter or exit the prostheses[10, 11] by using, for example, non-coplanar beams.
The AAPM TG 63 primary recommendation to avoid beams passing through hip prostheses is well founded, given the dosimetric problems well described in the literature.[6, 7] However, this recommendation is primarily directed at beams ‘shadowing’ the PTV (recommendation 4.2). Avoiding beam entry alone will provide certainty of PTV dose and expand the possible gantry angles for coplanar IMRT beams which would otherwise be constrained to a narrow anterior and posterior range (Fig. 4). A comparison of dose–volume histograms from such a limited plan with the technique described earlier is presented in Figure 5.
The attenuation as the beam passes through the patient, and the ability to fan the beams out over more angles than would be possible by constraining beams to avoid prostheses entirely, results in a low dose to the prosthesis. With respect to normal tissue dose, this would be well below the tolerance threshold of the surrounding bone in which the prosthesis is situated.
This method of planning has been used in our department for the treatment of post-prostatectomy courses of radiation therapy, for which we do not use implanted fiducial markers. In accordance with AAAPM TG 63, verification portal imaging is utilised to confirm that the ipsilateral (entry side) prosthesis is not in the beam's eye view. A possible drawback of this method could arise in the treatment of patients with first-line radiation therapy, as the isocentre shifts to target implanted fiducial markers may cause the prosthesis-shielding beam constraints to be violated. Our department is working on a protocol to avoid this situation in such patients.
We contend that a constrained-beam inverse plan for IMRT is a more effective method of planning bilateral hip prosthesis cases than either constraining dose to the prostheses or constraining beam angles to avoid the prostheses. Allowing low exit dose to the prosthesis beyond the PTV increases the search space for the inverse planning algorithm, increasing the probability of finding the best plan and the efficiency of the process.
The authors would like to acknowledge the work of the Planning Department at RPAH Radiation Oncology.
- 1NSW Central Cancer Registry. Prostate Cancer Incidence, 2006–2008, NSW. Cancer Institute NSW; 2011.
- 2Population Health Division. The Health of the People of New South Wales – Report of the Chief Health Officer, Data Book – Burden of Disease. NSW Department of Health, Sydney, 2008.
- 3Australian Cancer Network Working Party on Management of Localised Prostate Cancer. Clinical Practice Guidelines: Evidence-Based Information and Recommendations for the Management of Localised Prostate Cancer. National Health and Medical Research Council, Commonwealth of Australia, Canberra, 2002.
- 4Australian Cancer Network Management of Metastatic Prostate Cancer Working Party. Clinical Practice Guidelines for the Management of Locally Advanced and Metastatic Prostate Cancer. Cancer Council Australia and Australian Cancer Network, Sydney, 2010.
- 9AP-PA field orientation followed by IMRT reduces lung exposure in comparison to conventional 3D conformal and sole IMRT in centrally located lung tumors. Radiother Oncol 2012; 7 (23): 1–6., , et al.