Mean Planning Target Volume Dose Research Articles

Prediction of DVH parameter changes due to setup errors for breast cancer treatment based on 2D portal dosimetry

Electronic portal imaging devices (EPIDs) are increasingly used for portal dosimetry applications. In our department, EPIDs are clinically used for two-dimensional (2D) transit dosimetry. Predicted and measured portal dose images are compared to detect dose delivery errors caused for instance by setup errors or organ motion. The aim of this work is to develop a model to predict dose-volume histogram (DVH) changes due to setup errors during breast cancer treatment using 2D transit dosimetry. First, correlations between DVH parameter changes and 2D gamma parameters are investigated for different simulated setup errors, which are described by a binomial logistic regression model. The model calculates the probability that a DVH parameter changes more than a specific tolerance level and uses several gamma evaluation parameters for the planning target volume (PTV) projection in the EPID plane as input. Second, the predictive model is applied to clinically measured portal images. Predicted DVH parameter changes are compared to calculated DVH parameter changes using the measured setup error resulting from a dosimetric registration procedure. Statistical accuracy is investigated by using receiver operating characteristic (ROC) curves and values for the area under the curve (AUC), sensitivity, specificity, positive and negative predictive values. Changes in the mean PTV dose larger than 5%, and changes in V90 and V95 larger than 10% are accurately predicted based on a set of 2D gamma parameters. Most pronounced changes in the three DVH parameters are found for setup errors in the lateral-medial direction. AUC, sensitivity, specificity, and negative predictive values were between 85% and 100% while the positive predictive values were lower but still higher than 54%. Clinical predictive value is decreased due to the occurrence of patient rotations or breast deformations during treatment, but the overall reliability of the predictive model remains high. Based on our predictive model, 2D transit dosimetry measurements can now directly be translated in clinically more relevant DVH parameter changes for the PTV during conventional breast treatment. In this way, the possibility to design decision protocols based on extracted DVH changes is created instead of undertaking elaborate actions such as repeated treatment planning or 3D dose reconstruction for a large group of patients.

ICRU reference dose in an era of intensity-modulated radiation therapy clinical trials: Correlation with planning target volume mean dose and suitability for intensity-modulated radiation therapy dose prescription

Background and Purpose IMRT clinical trials lack dose prescription and specification standards similar to ICRU standards for two- and three-dimensional external beam planning. In this study, we analyzed dose distributions for patients whose treatment plans incorporated IMRT, and compared the dose determined at the ICRU reference point to the PTV doses determined from dose–volume histograms. Additionally, we evaluated if ICRU reference type single-point dose prescriptions are suitable for IMRT dose prescriptions. Materials and methods For this study, IMRT plans of 117 patients treated at our institution were randomly selected and analyzed. The treatment plans were clinically applied to the following disease sites: abdominal (11), anal (10), brain (11), gynecological (15), head and neck (25), lung (15), male pelvis (10) and prostate (20). The ICRU reference point was located in each treatment plan following ICRU Report 50 guidelines. The reference point was placed in the central part of the PTV and at or near the isocenter. In each case, the dose was calculated and recorded to this point. For each patient – volume and dose (PTV, PTV mean, median and modal) information was extracted from the planned dose–volume histogram. Results The ICRU reference dose vs PTV mean dose relationship in IMRT exhibited a weak positive association (Pearson correlation coefficient 0.63). In approximately 65% of the cases studied, dose at the ICRU reference point was greater than the corresponding PTV mean dose. The dose difference between ICRU reference and PTV mean doses was ⩽2% in approximately 79% of the cases studied (average 1.21% (±1.55), range −4% to +4%). Paired t-test analyses showed that the ICRU reference doses and PTV median doses were statistically similar ( p = 0.42). The magnitude of PTV did not influence the difference between ICRU reference and PTV mean doses. Conclusions The general relationship between ICRU reference and PTV mean doses in IMRT is similar to that in 3D CRT distributions. Point doses in IMRT are influenced by the degree of intensity modulation as well as calculation grid size utilized. Although the ICRU reference point type prescriptions conceptually may be extended for IMRT dose prescriptions and used as a representative of tumor dose, new universally acceptable dose prescription and specification standards for IMRT based on RTOG IMRT prescription model incorporating dose–volume specification would likely lead to greater consistency among treatment centers.

Monte Carlo evaluation of the AAA treatment planning algorithm in a heterogeneous multilayer phantom and IMRT clinical treatments for an Elekta SL25 linear accelerator

The Anisotropic Analytical Algorithm (AAA) is a new pencil beam convolution/superposition algorithm proposed by Varian for photon dose calculations. The configuration of AAA depends on linear accelerator design and specifications. The purpose of this study was to investigate the accuracy of AAA for an Elekta SL25 linear accelerator for small fields and intensity modulated radiation therapy (IMRT) treatments in inhomogeneous media. The accuracy of AAA was evaluated in two studies. First, AAA was compared both with Monte Carlo (MC) and the measurements in an inhomogeneous phantom simulating lung equivalent tissues and bone ribs. The algorithm was tested under lateral electronic disequilibrium conditions, using small fields (2 x 2 cm(2)). Good agreement was generally achieved for depth dose and profiles, with deviations generally below 3% in lung inhomogeneities and below 5% at interfaces. However, the effects of attenuation and scattering close to the bone ribs were not fully taken into account by AAA, and small inhomogeneities may lead to planning errors. Second, AAA and MC were compared for IMRT plans in clinical conditions, i.e., dose calculations in a computed tomography scan of a patient. One ethmoid tumor, one orophaxynx and two lung tumors are presented in this paper. Small differences were found between the dose volume histograms. For instance, a 1.7% difference for the mean planning target volume dose was obtained for the ethmoid case. Since better agreement was achieved for the same plans but in homogeneous conditions, these differences must be attributed to the handling of inhomogeneities by AAA. Therefore, inherent assumptions of the algorithm, principally the assumption of independent depth and lateral directions in the scaling of the kernels, were slightly influencing AAA's validity in inhomogeneities. However, AAA showed a good accuracy overall and a great ability to handle small fields in inhomogeneous media compared to other pencil beam convolution algorithms.

Sci-Thur PM Therapy-10: Is there an optimal starting gantry angle for equiangular-spaced beam arrangements in IMRT?

Equiangular‐spaced beam arrangements are the standard for intensity‐modulated radiotherapy(IMRT). However, since the choice of starting gantry angle is arbitrary, the question arises of whether there exist optimal starting gantry angles which will produce clinically significant improvements. To investigate the effect of beam orientation, the starting gantry angles in a 5‐beam equiangular‐spaced arrangement were incremented by 5°. Fifteen IMRT plans were generated per patient using 6 MV photon beams with static MLC delivery for 5 prostate, 5 lung, and 5 head‐and‐neck cancer patients. The plans were compared based on Planning Target Volume (PTV) and Organ‐at‐Risk (OAR) dose‐volume histograms, PTV mean dose, OAR maximum dose, total number of segments, and total number of monitor units. A fundamental geometric PTV conformity analysis was also conducted for each increment in gantry angle. To achieve this, divergent ray tracing was performed from the beam source coordinates through the PTV Beam's‐Eye‐View to produce a 3D cone beam matrix. The geometric conformity index (ratio of beam overlap volume to PTV volume) was then calculated. Clinically insignificant variations in PTV mean dose were found for all treatment sites as the starting gantry angles were incremented. This observation was further supported by the small variation in geometric conformity index with gantry increment. The variations in rectal V 75 Gy, spinal cord DVH, and parotid V 30 Gy were clinically significant for the prostate, lung, and head‐and‐neck patients, respectively. It is evident that the choice of starting gantry angle in IMRT equiangular‐spaced beam arrangements must be chosen with care.

The potential for dose escalation in lung cancer as a result of systematically reducing margins used to generate planning target volume

To determine how much the radiation dose to lung tumors could be increased as the margins used to generate planning target volume (PTV) are reduced. Treatment plans for 18 patients with non-small-cell lung carcinoma were retrospectively generated. Dose escalation was performed in two phases: The dose was increased as long as healthy tissue dose-volume constraints did not exceed (1) the values from the treatment plan originally used for the patients and (2) clinically acceptable values. No correlation of dose escalation was observed with tumor location, tumor stage, tumor motion, and tumor volume. An increase in dose was observed for many of the patients with as little as 2-mm uniform reduction in PTV margin, with increases in mean PTV dose exceeding 15 Gy for 5 patients. Sixteen of 18 patients experienced a decrease in mean heart, esophagus, and lung dose when margins were reduced and prescription doses were increased. Reduced margins allowed an increased dose to the tumors. However, a much larger dose escalation was possible for some patients but not for others, demonstrating that each patient is different, so individual treatment plans must be tailored for maximum tumor coverage and minimum exposure of healthy tissue.

Dosimetry and preliminary acute toxicity in the first 100 men treated for prostate cancer on a randomized hypofractionation dose escalation trial

The alpha/beta ratio for prostate cancer is postulated to be between 1 and 3, giving rise to the hypothesis that there may be a therapeutic advantage to hypofractionation. The dosimetry and acute toxicity are described in the first 100 men enrolled in a randomized trial. The trial compares 76 Gy in 38 fractions (Arm I) to 70.2 Gy in 26 fractions (Arm II) using intensity modulated radiotherapy. The planning target volume (PTV) margins in Arms I and II were 5 mm and 3 mm posteriorly and 8 mm and 7 mm in all other dimensions. The PTV D95% was at least the prescription dose. The mean PTV doses for Arms I and II were 81.1 and 73.8 Gy. There were no differences in overall maximum acute gastrointestinal (GI) or genitourinary (GU) toxicity acutely. However, there was a slight but significant increase in Arm II GI toxicity during Weeks 2, 3, and 4. In multivariate analyses, only the combined rectal DVH parameter of V65 Gy/V50 Gy was significant for GI toxicity and the bladder volume for GU toxicity. Hypofractionation at 2.7 Gy per fraction to 70.2 Gy was well tolerated acutely using the planning conditions described.

Dose heterogeneity in the target volume and intensity-modulated radiotherapy to escalate the dose in the treatment of non–small-cell lung cancer

To quantify the dose escalation achievable in the treatment of non-small-cell lung cancer (NSCLC) by allowing dose heterogeneity in the target volume or using intensity-modulated radiotherapy (IMRT), or both. Computed tomography data and contours of 10 NSCLC patients with limited movements of the tumor and representing a broad spectrum of clinical cases were selected for this study. Four irradiation techniques were compared: two conformal (CRT) and two IMRT techniques, either prescribing a homogeneous dose in the planning target volume (PTV) (CRT(hom) and IMRT(hom)) or allowing dose heterogeneity (CRT(inhom) and IMRT(inhom)). The dose heterogeneity was allowed only toward high doses, i.e., the minimum dose in the target for CRT(inhom) and IMRT(inhom) could not be lower than for the corresponding homogeneous plan. The dose in the PTV was escalated (fraction size of 2.25 Gy) until either an organ at risk reached the maximum allowed dose or the mean PTV dose reached a maximum level set at 101.25 Gy. When small and convex tumors were irradiated, CRT(hom) could achieve the maximum dose of 101.25 Gy, whereas for bigger and/or concave PTVs the dose level achievable with CRT(hom) was significantly lower, in 1 case even below 60 Gy. The CRT(inhom) allowed on average a 6% dose escalation with respect to CRT(hom). The IMRT(hom) achieved in all except 1 case a mean PTV dose of at least 75 Gy. The gain in mean PTV dose of IMRT(hom) with respect to CRT(hom) ranged from 7.7 to 14.8 Gy and the IMRT(hom) plans were always more conformal than the corresponding CRT(hom) plans. The IMRT(inhom) provided an additional advantage over IMRT(hom) of at least 5 Gy. For all CRT plans the achievable dose was determined by the lung dose threshold, whereas for more than half of the IMRT plans the esophagus was the dose-limiting organ. The IMRT plans were deliverable with 10-12 segments per beam and did not produce an increase of lung volume irradiated at low doses (<20 Gy). The dose in NSCLC treatments can be escalated by loosening the constraints on maximum dose in the target volume or using IMRT, or both. For large and concave tumors, an average dose escalation of 6% and 17% was possible when dose heterogeneity and IMRT were applied alone. When they were combined, the average dose increase was as high as 35%. Intensity-modulated RT delivered in a static mode can produce homogeneous dose distributions in the target and does not lead to an increase of lung volume receiving (very) low doses, even down to 5 Gy.

10. The comparison between the three – field and four-field techniques of planning of radiotherapy in prostate cancer

Purpose evaluation 3-field(3F) and 4-field(4F) planning techiniques for patients with localized prostate cancer. Materials/methods: Five patients with prostate cancer (T3N0M0) were evaluated. CT images were obtained at 5 mm increments and were transferred to CadPlan_planning_workstation. The planning target volume (PTV) was defined as prostate and seminal vesicles with 15mm margins around clinical target volume (CTV) except prostate-rectum interface where 5 mm margin was applied. CTV was defined as prostate and seminal vesicles. Following organs at risk (OAR) were outlined: rectum, bladder, right femoral head. Following 3F and 4F plans were performed: 3F with angles (0deg-120deg-240deg; 0deg-90deg-270deg) and 4F (Odeg-90deg-180deg-270deg). We also created two versions of treatment plans including of energy; 6 MV and 20 MV for Clinac2300CD. Total dose was 74 Gy. Mean total doses of thirty plans in irradiated organs at risk (rectum, bladder and righ femoral head) were compared. For PTV mean and minimum dose were criteria for comparision of treatment plans. Results: There were no significant dose differenes between evaluated plans of treatment in PTV (0.05). Because mean dose in femoral head in each treatment plan was below tolerance dose, main dose-limiting organ was rectum and bladder. Lowest mean dose 42.7 Gy in rectum was achived by application of 3F technique of 20 MV(0deg-90deg-270deg). Bladder was also spared with the same 3F technique of 20 MV, where mean dose was 45.2 Gy. Conclusions: This study showed that the, T” three-field technique (an anterior and two opposing lateral fields) provided with 20 MV is optimal and assures the lowest rectal dose.