IMRT Planning System Research Articles

Under-dosing of potential microscopic prostate cancer with IMRT after neoadjuvant hormonal therapy

Neoadjuvant hormonal therapy (NHT) does not guarantee a symmetric tumor volume reduction or the absence of peripheral microscopic disease. This study determined the dosimetric impact of IMRT on potential microscopic disease when using a planning CT scan following NHT. Twenty prostate cancer patients were CT-scanned at the start of NHT. A repeat treatment planning CT scan was performed approximately 7 weeks later. Prior to each CT scan, patients were instructed to use an enema to empty their rectums and drink 24–30 ounces of fluid to fill their bladders. A single physician (AKL) contoured the prostate, seminal vesicles (SV), bladder and rectum for each study. NHT began within one week of the first CT scan and consisted of leuprolide 22.5mg IM with concurrent bicalutamide for 14–21 days. IMRT treatment plans were developed for each of the 40 paired CT data sets using an IMRT planning system. The CTV included the prostate and SV. The prostate PTV was expanded 7.6mm laterally, 5.8mm posteriorly, and 6.7mm in all other directions. The SV PTV was expanded 5mm in the anterior and posterior dimensions and 4.2mm in all other dimensions. The prescription dose to the PTV was 75.6Gy given in 42 fractions. Pre-NHT scans (CT-1) were co-registered with the corresponding post-NHT scans (CT-2) based on patient-specific bony anatomy to minimize setup uncertainties for this study. The deliverable leaf sequences for the IMRT plan based on the post-NHT CT images (PLAN-2) were applied to CT-1 (pre-NHT CT) for the same patient. This resulted in a dose distribution on pre-NHT anatomy but using the IMRT plan based on post-NHT anatomy. The pre-NHT plans (PLAN-1) also were applied to CT-2. This resulted in 80 IMRT plans for 20 patients. Quantitative and qualitative changes in organ volume, shape, and dosimetric parameters regarding target coverage and normal tissue were collected. The median interval between scans was 52 days [range 30–109], and the median reduction in prostate volume was 28% [4.5–58%]. There was a correlation between elapsed days and prostate volume reduction (p = 0.033). The mean change in the volumetric center of the prostate from CT-1 vs. CT-2 was 0.02 ± 0.11cm in the lateral direction, −0.070 ± 0.26cm in the AP direction, and 0.23 ± 0.28cm in the SI direction, which suggests asymmetric shrinkage of the prostate gland. Overall, 80% (16/20) of patients had <90% of their pre-NHT PTV covered by 75.6Gy using the post-NHT plan (PLAN-2), 40% (8/20) had <90% of the PTV covered by 70Gy, and 25% (5/20) had <90% covered by 66Gy. Patients with <90% PTV coverage at 70Gy and 66Gy were more likely to have prostate volume reductions of >30% (Fisher exact p < 0.025). Correlation is shown in Fig. 1 for 70Gy. CT planning acquired after NHT may result in significant under-dosing of potential microscopic disease at the periphery of the original tumor bed when using IMRT. The potential for under-dosing was more pronounced in patients that had prostate volume reductions of >30% and was related to HT duration, and was dimensionally unpredictable. Co-registered pre-NHT and post-NHT CT scans should be considered for IMRT planning, especially in patients with locally-advanced prostate cancer.

Automatic verification of step‐and‐shoot IMRT field segments using portal imaging

In step-and-shoot IMRT, many individual beam segments are delivered. These segments are generated by the IMRT treatment planning system and subsequently transmitted electronically through computer hardware and software modules before they are finally delivered. Hence, an independent system that monitors the actual field shape during treatment delivery is an added level of quality assurance in this complicated process. In this paper we describe the development and testing of such a system. The system verifies the field shape by comparing the radiation field detected by the built-in portal imaging system on the linac to the actual field shape planned on the treatment planning system. The comparison is based on a software algorithm that detects the leaf edge positions of the radiation field on the portal image and compares that to the calculated positions. The process is fully automated and requires minimal intervention of the radiation therapists. The system has been tested with actual clinical plan sequences and was able to alert the operator of incorrect settings in real time.

Dose calculation accuracy of lung planning with a commercial IMRT treatment planning system

Dose calculation accuracy of lung planning with a commercial IMRT treatment planning system

Intensity modulated irradiation of a thorax phantom: comparisonsbetween measurements, Monte Carlo calculations and pencil beamcalculations

The present study investigates the application of compensators forthe intensity modulated irradiation of a thorax phantom. Measurements arecompared with Monte Carlo and standard pencil beam algorithm dosecalculations. Compensators were manufactured to produce the intensity profilesthat were generated from the scientific version of the KonRad IMRTtreatment-planning system for a given treatment plan. The comparison of dosedistributions calculated with a pencil beam algorithm, with the Monte Carlocode EGS4 and with measurements is presented. By measurements in a waterphantom it is demonstrated that the method used to manufacture thecompensators reproduces the intensity profiles in a suitable manner.Monte Carlo simulations in a water phantom show that theaccelerator head model used for simulations is sufficient. No significantoverestimations of dose values inside the target volume by the pencil beamalgorithm are found in the thorax phantom. An overestimation of dose values inlung by the pencil beam algorithm is also not found. Expected dose calculationerrors of the pencil beam algorithm are suppressed, because the dose to thelow density region lung is reduced by the use of a non-coplanar beamarrangement and by intensity modulation.

Quantitative dosimetric verification of an IMRT planning and delivery system

Background and purpose: The accuracy of dose calculation and delivery of a commercial serial tomotherapy treatment planning and delivery system (Peacock, NOMOS Corporation) was experimentally determined. Materials and methods: External beam fluence distributions were optimized and delivered to test treatment plan target volumes, including three with cylindrical targets with diameters ranging from 2.0 to 6.2 cm and lengths of 0.9 through 4.8 cm, one using three cylindrical targets and two using C-shaped targets surrounding a critical structure, each with different dose distribution optimization criteria. Computer overlays of film-measured and calculated planar dose distributions were used to assess the dose calculation and delivery spatial accuracy. A 0.125 cm 3 ionization chamber was used to conduct absolute point dosimetry verification. Thermoluminescent dosimetry chips, a small-volume ionization chamber and radiochromic film were used as independent checks of the ion chamber measurements. Results: Spatial localization accuracy was found to be better than ±2.0 mm in the transverse axes (with one exception of 3.0 mm) and ±1.5 mm in the longitudinal axis. Dosimetric verification using single slice delivery versions of the plans showed that the relative dose distribution was accurate to ±2% within and outside the target volumes (in high dose and low dose gradient regions) with a mean and standard deviation for all points of −0.05% and 1.1%, respectively. The absolute dose per monitor unit was found to vary by ±3.5% of the mean value due to the lack of consideration for leakage radiation and the limited scattered radiation integration in the dose calculation algorithm. To deliver the prescribed dose, adjustment of the monitor units by the measured ratio would be required. Conclusions: The treatment planning and delivery system offered suitably accurate spatial registration and dose delivery of serial tomotherapy generated dose distributions. The quantitative dose comparisons were made as far as possible from abutment regions and examination of the dosimetry of these regions will also be important. Because of the variability in the dose per monitor unit and the complex nature of the calculation and delivery of serial tomotherapy, patient-specific quality assurance procedures will include a measurement of the delivered target dose.