Niek van Wieringen

Comparison of biologically equivalent dose–volume parameters for the treatment of prostate cancer with concomitant boost IMRT versus IMRT combined with brachytherapy

BACKGROUND AND PURPOSE The two main modalities to deliver high dose to the prostate and prevent high doses to neighboring organs are intensity modulated radiotherapy (IMRT) or external beam radiotherapy combined with brachytherapy. Because of the different biological effectiveness the physical dose distributions were converted to 3-dimensional linear quadratic dose at 2 Gy per fraction (EQD(2)). From the latter, cumulative EQD(2)-volume histograms were determined for comparison of the modalities. MATERIAL AND METHODS An IMRT plan was made on the contoured planning target volume (PTV1) and organs at risk (OAR) of 20 patients (IMRT-only). A dose of 70 Gy was prescribed on the PTV1 with a concomitant boost to a total of 76 Gy on a subvolume (PTV2). Also a 46 Gy IMRT plan was made combined with either a pulsed dose-rate (PDR) or a high dose-rate (HDR) brachytherapy boost. The EQD(2) on the PTV1 of the combined IMRT-PDR and IMRT-HDR plans were made equivalent to the EQD(2) of the 70 Gy IMRT-only plan. The alpha/beta-ratio for prostate was set to 1.5 Gy and 10 Gy. For normal tissues an alpha/beta-ratio of 3.0 Gy was taken. Several EQD(2)-volume histogram parameters were calculated for comparison and analyzed by two-way ANOVA. RESULTS The mean EQD(2) to 95% of the prostate volume was slightly higher for the IMRT-only plan than for the brachytherapy modalities (P<0.001), in contrast to the mean EQD(2) to 50% of the prostate volume in which the opposite was the case (P<0.001). Rectum and bladder doses for IMRT-only are significantly higher (P<0.001). The urethra dose for IMRT-HDR was much higher than the other modalities only when the alpha/beta-ratio for prostate was 10 Gy. CONCLUSION Because of the high doses within an implant, the dose in 50% of the prostate volume is much higher with the brachytherapy modalities than IMRT-only which may have clinical consequences. With brachytherapy the doses to the OAR are lower or similar to IMRT-only. Dose escalation for prostate tumors is more easily achieved with brachytherapy than with IMRT alone. Therefore, brachytherapy might be the preferred modality to achieve further dose escalation.

Dosimetric evaluation of prostate rotations and their correction by couch rotations

PURPOSE To investigate the dosimetric effect of prostate rotations and limited on-line corrections by couch rotations (<or=3 degrees ) for prostate, seminal vesicles and organs at risk. METHODS For 5 patients IMRT plans were made, treating the prostate plus base of the seminal vesicles. Realistic and idealised dose distributions were considered, the latter demonstrating extreme effects for rotations and their corrections. Translation errors were assumed to be corrected on-line. For each patient 20 treatments with different rotation errors were simulated: 20 systematic errors were generated and 20 times 35 random deviations were superimposed to simulate day-to-day variations. Using a research module of PLATO-RTS treatments with rotation errors, with and without on-line corrections, were simulated. RESULTS The largest dosimetric effect of rotation errors and corrections was found for the seminal vesicles with idealised dose distribution: coverage improved from 92.6% (range 89.9-96.0%) to 95.9% (94.7-98.1%). The gain for the idealised prostate was less: 95.9% (94.4-97.0%) to 97.5% (95.5-98.4%). For the femoral heads the dose increase could be as large as 12.2% (6.2-19.3%). CONCLUSIONS On-line correction of rotations can improve target coverage slightly. For organs at risk at a large distance from the isocentre the result can be a significant increase in dose.

A comparison of inverse optimization algorithms for HDR/PDR prostate brachytherapy treatment planning

PURPOSE Graphical optimization (GrO) is a common method for high-dose-rate/pulsed-dose-rate (PDR) prostate brachytherapy treatment planning. New methods performing inverse optimization of the dose distribution have been developed over the past years. The purpose is to compare GrO and two established inverse methods, inverse planning simulated annealing (IPSA) and hybrid inverse treatment planning and optimization (HIPO), and one new method, enhanced geometric optimization-interactive inverse planning (EGO-IIP), in terms of speed and dose-volume histogram (DVH) parameters. METHODS AND MATERIALS For 26 prostate cancer patients treated with a PDR brachytherapy boost, an experienced treatment planner optimized the dose distributions using four different methods: GrO, IPSA, HIPO, and EGO-IIP. Relevant DVH parameters (prostate-V100%, D90%, V150%; urethra-D(0.1cm3) and D(1.0cm3); rectum-D(0.1cm3) and D(2.0cm3); bladder-D(2.0cm3)) were evaluated and their compliance to the constraints. Treatment planning time was also recorded. RESULTS All inverse methods resulted in shorter planning time (mean, 4-6.7 min), as compared with GrO (mean, 7.6 min). In terms of DVH parameters, none of the inverse methods outperformed the others. However, all inverse methods improved on compliance to the planning constraints as compared with GrO. On average, EGO-IIP and GrO resulted in highest D90%, and the IPSA plans resulted in lowest bladder D2.0cm3 and urethra D(1.0cm3). CONCLUSIONS Inverse planning methods decrease planning time as compared with GrO for PDR/high-dose-rate prostate brachytherapy. DVH parameters are comparable for all methods.

Improved tumour control probability with MRI-based prostate brachytherapy treatment planning

Abstract Backgroun. Due to improved visibility on MRI, contouring of the prostate is improved compared to CT. The aim of this study was to quantify the benefits of using MRI for treatment planning as compared to CT-based planning for temporary implant prostate brachytherapy. Material and methods. CT and MRI image data of 13 patients were used to delineate the prostate and organs at risk (OARs) and to reconstruct the implanted catheters (typically 12). An experienced treatment planner created plans on the CT-based structure sets (CT-plan) and on the MRI-based structure sets (MRI-plan). Then, active dwell-positions and weights of the CT-plans were transferred to the MRI-based structure sets (CT-planMRI-contours) and resulting dosimetric parameters and tumour control probabilities (TCPs) were studied. Results. For the CT-planMRI-contours a statistically significant lower target coverage was detected: mean V100 was 95.1% as opposed to 98.3% for the original plans (p < 0.01). Planning on CT caused cold-spots that influence the TCP. MRI-based planning improved the TCPs by 6–10%, depending on the parameters of the radiobiological model used for TCP calculation. Basing the treatment plan on either CT- or MRI-delineations does not influence plan quality. Conclusion. Evaluation of CT-based treatment planning by transferring the plan to MRI reveals underdosage of the prostate, especially at the base side. Planning on MRI can prevent cold-spots in the tumour and improves the TCP.

Deviations from the planned dose during 48 hours of stepping source prostate brachytherapy caused by anatomical variations

BACKGROUND AND PURPOSE To determine the uncertainties in planned dose associated with catheter and organ movement during 48 hours of stepping source prostate brachytherapy. MATERIAL AND METHODS Pulsed-dose rate (PDR) prostate brachytherapy as a boost is given in 24 pulses every 2 hours, making the total treatment last 48 hours. The entire treatment is based on one plan, created on the planning CT (CT1). Two follow-up CTs (CT2 and CT3) were acquired; halfway through the treatment and at the end of treatment. On these repeat scans the catheters were reconstructed and PTV and OARs were delineated. The original treatment plan was calculated on the repeat CTs. Target coverage V(100%), D(90), dose to 2cm(3) (D2cm(3)) of the rectum and bladder and dose to 0.1cm(3) of the urethra were recorded from the recalculated DVHs. RESULTS On the two repeat CTs the V100% decreased -1.5% and -2.3% as compared to the planning CT. For the rectum D2cm(3), the average increase was 14.8% (CT1-CT2) and 17.3% (CT1-CT3). Increase in bladder D2cm(3) was on average 23.1% (CT1-CT2) and 24.8% (CT1-CT3). For the urethra D0.1cm(3) an average decrease of -2% (CT1-CT2) and -3.2% (CT2-CT3) was observed. CONCLUSIONS Changes in target coverage during treatment were small and considered clinically irrelevant. However, an overall increase in dose to the OARs was found as compared to the planned dose, which should be taken into account during treatment planning.

Safety Aspects of Pulsed Dose Rate Brachytherapy: Analysis of Errors in 1,300 Treatment Sessions

PURPOSE To determine the safety of pulsed-dose-rate (PDR) brachytherapy by analyzing errors and technical failures during treatment. METHODS AND MATERIALS More than 1,300 patients underwent treatment with PDR brachytherapy, using five PDR remote afterloaders. Most patients were treated with consecutive pulse schemes, also outside regular office hours. Tumors were located in the breast, esophagus, prostate, bladder, gynecology, anus/rectum, orbit, head/neck, with a miscellaneous group of small numbers, such as the lip, nose, and bile duct. Errors and technical failures were analyzed for 1,300 treatment sessions, for which nearly 20,000 pulses were delivered. For each tumor localization, the number and type of occurring errors were determined, as were which localizations were more error prone than others. RESULTS By routinely using the built-in dummy check source, only 0.2% of all pulses showed an error during the phase of the pulse when the active source was outside the afterloader. Localizations treated using flexible catheters had greater error frequencies than those treated with straight needles or rigid applicators. Disturbed pulse frequencies were in the range of 0.6% for the anus/rectum on a classic version 1 afterloader to 14.9% for orbital tumors using a version 2 afterloader. Exceeding the planned overall treatment time by >10% was observed in only 1% of all treatments. Patients received their dose as originally planned in 98% of all treatments. CONCLUSIONS According to the experience in our institute with 1,300 PDR treatments, we found that PDR is a safe brachytherapy treatment modality, both during and outside of office hours.

Marker-based quantification of interfractional tumor position variation and the use of markers for setup verification in radiation therapy for esophageal cancer.

PURPOSE The aim of this study was to quantify interfractional esophageal tumor position variation using markers and investigate the use of markers for setup verification. MATERIALS AND METHODS Sixty-five markers placed in the tumor volumes of 24 esophageal cancer patients were identified in computed tomography (CT) and follow-up cone-beam CT. For each patient we calculated pairwise distances between markers over time to evaluate geometric tumor volume variation. We then quantified marker displacements relative to bony anatomy and estimated the variation of systematic (Σ) and random errors (σ). During bony anatomy-based setup verification, we visually inspected whether the markers were inside the planning target volume (PTV) and attempted marker-based registration. RESULTS Minor time trends with substantial fluctuations in pairwise distances implied tissue deformation. Overall, Σ(σ) in the left-right/cranial-caudal/anterior-posterior direction was 2.9(2.4)/4.1(2.4)/2.2(1.8) mm; for the proximal stomach, it was 5.4(4.3)/4.9(3.2)/1.9(2.4) mm. After bony anatomy-based setup correction, all markers were inside the PTV. However, due to large tissue deformation, marker-based registration was not feasible. CONCLUSIONS Generally, the interfractional position variation of esophageal tumors is more pronounced in the cranial-caudal direction and in the proximal stomach. Currently, marker-based setup verification is not feasible for clinical routine use, but markers can facilitate the setup verification by inspecting whether the PTV covers the tumor volume adequately.

Reduction in cardiac volume during chemoradiotherapy for patients with esophageal cancer

We investigated the change in cardiac volume over the course of chemoradiotherapy in 26 patients treated for esophageal cancer, using cone beam CT imaging. The cardiac volume reduced significantly, with a median reduction of 8%. A significant relationship with planned cardiac dose was not found.

Potential dosimetric benefit of an adaptive plan selection strategy for short-course radiotherapy in rectal cancer patients

PURPOSE An adaptive plan selection strategy can account for daily target volume variations for radiotherapy in rectal cancer patients. The aim was to quantify the daily dosimetric consequences of plan selection compared to a non-adaptive approach. MATERIALS AND METHODS Ten patients with rectal cancer, treated with 25Gy in five fractions to the mesorectum and pelvic lymph nodes, were selected. The adaptive strategy was simulated by creating three plans per patient, with varying upper ventral PTV margins, and selecting the smallest PTV covering the entire mesorectum on every daily CBCT scan. Subsequently, mesorectum, bladder, and bowel cavity were delineated on these scans. Daily dose-volume histograms were calculated for both the adaptive and non-adaptive plan, with a ventral PTV margin of 20mm. Coverage of the mesorectum, defined as V95%>99%, was calculated, as well as bladder and bowel cavity V95% and V15Gy. RESULTS In one patient, mesorectum coverage improved. A reduction in bladder V95% and bowel cavity V15Gy was found, of 6.9% and 18.4cm(3) (p<0.01), respectively. CONCLUSION Plan selection for radiotherapy in rectal cancer can improve coverage of the target volume. Overall dosimetric sparing of bladder and bowel cavity was limited but could be beneficial for individual patients.

Dose-Guided Radiotherapy: Potential Benefit of Online Dose Recalculation for Stereotactic Lung Irradiation in Patients With Non-Small-Cell Lung Cancer

PURPOSE To determine whether dose-guided radiotherapy (i.e., online recalculation and evaluation of the actual dose distribution) can improve decision making for lung cancer patients treated with stereotactic body radiotherapy. METHODS AND MATERIALS For this study 108 cone-beam computed tomography (CBCT) scans of 10 non-small-cell lung cancer patients treated with stereotactic body radiotherapy were analyzed retrospectively. The treatment plans were recalculated on the CBCT scans. The V(100%) of the internal target volume (ITV) and D(max) of the organs at risk (OARs) were analyzed. Results from the recalculated data were compared with dose estimates for target and OARs by superposition of the originally planned dose distribution on CBCT geometry (i.e., the original dose distribution was assumed to be spatially invariant). RESULTS Before position correction was applied the V(100%) of the ITV was 100% in 65% of the cases when an ITV-PTV margin of 5 mm was used and 52% of the cases when a margin of 3 mm was used. After position correction, the difference of D(max) in the OARs with respect to the treatment plan was within 5% in the majority of the cases. When the dose was not recalculated but estimated assuming an invariant dose distribution, clinically relevant errors occurred in both the ITV and the OARs. CONCLUSION Dose-guided radiotherapy can be used to determine the actual dose in OARs when the target has moved with respect to the OARs. When the workflow is optimized for speed, it can be used to prevent unnecessary position corrections. Estimating the dose by assuming an invariant dose instead of recalculation of the dose gives clinically relevant errors.