[Show abstract][Hide abstract] ABSTRACT: The use of marker-based on-line image guided radiotherapy for prostate cancer has considerably reduced the treatment margins to sub-cm. In this study we have quantified the residual set-up errors remaining after isocenter correction, studied their development during beam delivery and estimated their impact on margins.
After initial on-line patient set-up based on orthogonal kV x-ray images of implanted fiducial markers, continuous electronic portal imaging was performed during treatment delivery in 10 of 39 treatment sessions for 20 prostate cancer patients. The cranio-caudal (CC) position of the centre-of-mass of the three markers was found using a threshold technique on every single image frame for all patients, typically 12-14 images for 5 treatment beams in every fraction. The CC prostate position was determined relative to its initial position at treatment onset and relative to its planned position within the field aperture. These results allowed determination of the CC intrafraction prostate motion and the intrafraction progression of the geometrical CC error, respectively.
At treatment onset the standard deviation (SD) of the set-up error was 1.0mm in the lateral direction and 1.5mm in the cranio-caudal (CC) direction. It did not depend significantly on the duration of the set-up procedure (mean: 3.0 min, span 1.2-14.6 min). The distribution of CC prostate positions relative to the position at treatment onset broadened from 0 to 1.4mm during the treatment session, while the corresponding CC setup error distribution broadened from 1.5 to 1.9 mm. This broadening means that the necessary CC setup margin increased by around 1mm during the treatment fraction.
Large differences in the intrafraction CC prostate motion patterns were found, however, intrafraction motion only results in a modest additional CC set-up margin of around 1mm relative to the margins needed for the residual set-up error at treatment start.
[Show abstract][Hide abstract] ABSTRACT: Brachytherapy dose distributions can be optimised by modulation of source dwell times. In this study dose optimisation in single planar interstitial implants was evaluated in order to quantify the potential benefit in patients.
In 14 patients, treated for recurrent rectal and cervical cancer, flexible catheters were sutured intra-operatively to the tumour bed in areas with compromised surgical margin. Both non-optimised, geometrically and graphically optimised CT -based dose plans were made. The overdose index (OI), homogeneity index (HI), conformal index (COIN), minimum target dose, and high dose volumes were evaluated. The dependence of OI, HI, and COIN on target volume and implant regularity was evaluated. In addition, 12 theoretical implant configurations were analyzed.
Geometrical and graphical optimisation improved the dose plans significantly with graphical optimisation being superior. Graphically optimised dose plans showed a significant decrease of 18%+/-9% in high dose volume (p<0.001). HI, COIN, and OI were significantly improved from 0.50+/-0.05 to 0.60+/-0.05, from 0.65+/-0.04 to 0.71+/-0.04, and from 0.19+/-0.03 to 0.15+/-0.03, respectively (p<0.001 for all). Moreover, minimum target dose increased significantly from 71%+/-5% to 80%+/-5% (p<0.001). The improvement in OI and HI obtained by optimisation depended on the regularity of the implant, such that the benefit of optimisation was larger for irregular implants. OI and HI correlated strongly with target volume limiting the usability of these parameters for comparison of dose plans between patients.
Dwell time optimisation significantly improved the dose distribution regarding homogeneity, conformity, minimum target dose, and size of high dose volumes. Graphical optimisation is fast, reproducible and superior to geometric optimisation.
No preview · Article · Oct 2006 · Radiotherapy and Oncology
[Show abstract][Hide abstract] ABSTRACT: Low dose hyper-radiosensitivity (HRS) has been observed in both normal tissues and tumours. This modelling study explores the possible impact of HRS on radiation treatment planning.
The interplay between volume-effect and HRS was studied in an idealized comparison of partial versus whole organ irradiation. In the further studies, CT scans of three previously scanned patients were used to estimate normal tissue complication probability (NTCP) for the kidneys after a conformal and a conventional treatment plan with and without consideration of HRS.
Idealized treatment plans were compared as pairs of a conventional and a conformal plan both treating the same target volume to the same dose per fraction. Contour maps of the difference in NTCP between paired plans showed a strong dependence on the magnitude of both the volume effect and the HRS effect. For more clinically realistic treatment plans with NTCP calculated for the kidney, the balance between the sparing due to the LQ effect and the increased sensitivity due to the HRS effect was dependent on both the dose distribution and the fractionation.
HRS may potentially affect radiotherapy treatment planning and the relative importance of HRS is larger in a tissue or organ with a pronounced volume effect. If HRS is expressed in some normal tissues or organs, this could offset much of the sparing predicted by the LQ formalism. However, in some clinical situations the NTCP calculated with correction for HRS may still be lower than the NTCP calculated from the uncorrected physical doses.
No preview · Article · May 2006 · Radiotherapy and Oncology
[Show abstract][Hide abstract] ABSTRACT: To correlate long-term colorectal dysfunctions following radical radiotherapy for bladder or prostate cancer with clinical parameters and dose-volume histogram parameters of the small intestine, rectum, and anal canal volume.
Seventy-one patients previously treated for bladder or prostate cancer were interviewed following CT-based radiotherapy of 60-70 Gy with questions concerning long-term colorectal dysfunctions. Median follow-up time was 30 months (range 12-109 months). Clinical parameters and parameters from the dose-volume histograms were correlated with colorectal dysfunctions (Spearman's test). Median and quartile values of all parameters were used as cut-off values for statistical analyses. A logistic regression model was used for analysis of urgency and incontinence in relation to median or maximum radiation dose to the anal canal volume.
Rectum length, volume and several dose-volume parameters from the anal canal volume and rectal volume were correlated with late organ dysfunctions. In a logistic model, fecal urgency and incontinence were dependent of dose-volume parameters from the anal canal volume. No relation between age or follow-up time and late effects were found. Dose-volume parameters of the small intestine were not related to any late dysfunctions.
A relationship between several late anorectal dysfunctions and dose-volume parameters from the rectum and anal canal volume was demonstrated. It is recommended to exclude the anal canal volume from the high dose-volume and to apply rectal shielding whenever possible to prevent late anorectal dysfunctions.
No preview · Article · Mar 2005 · Radiotherapy and Oncology
[Show abstract][Hide abstract] ABSTRACT: To determine the impact of filling volume changes of the urinary bladder and rectum on organ motion and dose distribution of the bladder and rectum during radical radiotherapy for bladder cancer and to calculate the internal margins to secure target coverage.
In 15 patients with muscle-invasive bladder cancer, a planning CT scan was performed with a bladder and rectal catheter, followed by three immediate CT scans with various filling of the urinary bladder and rectum. After 20 fractions, a fifth CT scan, without catherization, was performed. In each CT study, the bladder and rectum volumes were delineated and matched to the planning CT scan to measure the organ motion and calculate internal margins. These margins were compared with an isotropic standard margin of 2 cm. Dose-volume histograms were analyzed to describe the dose distribution in the bladder and rectum corresponding to various filling volumes.
Bladder movement was most pronounced in the anterior and cranial directions. The internal margins required to cover the bladder movements due to filling of the bladder and rectum in 87% of the patients were 2.4 cm in the anterior, 1.1 cm in the posterior, 3.5 cm in the cranial, 0.5 cm in the caudal, and 1.3 cm in the lateral direction.
The filling volumes of the bladder and rectum have a large impact on bladder movements, especially in the anterior and cranial directions. This should be included in the internal target volume with the introduction of anisotropic margins in conformal radiotherapy for bladder cancer.
No preview · Article · Jul 2004 · International Journal of Radiation OncologyBiologyPhysics
[Show abstract][Hide abstract] ABSTRACT: Sensori-neural hearing loss (SNHL) is a common complication to radiation therapy in the upper head and neck region. In this study, we estimated the dose response relationship for SNHL with adjustment for pre-therapeutic risk factors.
The pre- and post-therapeutic hearing levels were recorded in a previously published study of 20 patients receiving radiotherapy for nasopharyngeal carcinoma. In the present study, the dose to the inner ear of these patients was estimated with a computed tomography (CT) based treatment planning system. CT data from a 'proxy patient' were used for patients with no available CT scan. SNHL was analyzed as a function of radiation dose and potential risk factors were tested.
The incidence of SNHL increased significantly with increasing dose to the cochlea. Increasing patient's age, and decreasing pre-therapeutic hearing level were statistically significantly associated with an increased risk of SNHL. A nomogram is presented for estimating individualized dose constraints of potential use in treatment planning.
The inner ear is a critical structure in patients with nasopharyngeal carcinoma and the dose to the inner ear should be carefully considered when planning radiation treatment in this region.
Full-text · Article · Nov 2002 · Radiotherapy and Oncology