Normal Tissue Dose-volume Histogram Research Articles

Dosimetric Evaluation of Adaptive Therapy in Non-Small Cell Lung Cancer Patients Undergoing Palliative Thoracic Radiotherapy

This study aimed to describe changes in gross tumour volume (GTV) that occurred during the course of radiotherapy (RT) in patients who underwent palliative thoracic radiotherapy (PTR), and to describe the role of adaptive treatment for protection of normal tissue. Twenty patients with non-small cell lung cancer (NSCLC) referred for PTR were treated using a total of 10 fractions and a dose of 300 cGy/day in accordance with the initial GTV, clinical target volume (CTV), and planning target volume (PTV). Computed tomography simulation (CTS) images were retaken for each patient at the end of the fifth fraction, and the second plan was created. The fractional volume reduction (FVR) of the GTV and the PTV were then calculated. The changes in normal tissue dose-volume histogram (DVH) parameters between the two plans were compared. Mean GTV and PTV values were 223.9 cc and 1113.3 cc for the first plan and 196.2 cc and 1029.7 cc for the second plan , respectively. After five fractionated treatments, the FVR was 15.9% of the GTV (p < 0.001) and 8% of the PTV (p< 0.001). The daily regression for GTV was 3.1 percent. A statistically non-significant decrease occurred for the normal tissue doses. The geometric changes in GTV and PTV positively influenced the DVH parameters, but were not statistically significant. The clinical implications of this approach to CTS plan assessment should be examined using prospective studies with adequate number of patients.

Fractionation in normal tissues: the (α/β)eff concept can account for dose heterogeneity and volume effects

The simple Linear–Quadratic (LQ)-based Withers iso-effect formula (WIF) is widely used in external-beam radiotherapy to derive a new tumour dose prescription such that there is normal-tissue (NT) iso-effect when changing the fraction size and/or number. However, as conventionally applied, the WIF is invalid unless the normal-tissue response is solely determined by the tumour dose. We propose a generalized WIF (gWIF) which retains the tumour prescription dose, but replaces the intrinsic fractionation sensitivity measure (α/β) by a new concept, the normal-tissue effective fractionation sensitivity, , which takes into account both the dose heterogeneity in, and the volume effect of, the late-responding normal-tissue in question. Closed-form analytical expressions for ensuring exact normal-tissue iso-effect are derived for: (i) uniform dose, and (ii) arbitrary dose distributions with volume-effect parameter n = 1 from the normal-tissue dose–volume histogram. For arbitrary dose distributions and arbitrary n, a numerical solution for exhibits a weak dependence on the number of fractions. As n is increased, increases from its intrinsic value at n = 0 (100% serial normal-tissue) to values close to or even exceeding the tumour (α/β) at n = 1 (100% parallel normal-tissue), with the highest values of corresponding to the most conformal dose distributions. Applications of this new concept to inverse planning and to highly conformal modalities are discussed, as is the effect of possible deviations from LQ behaviour at large fraction sizes.

Intravenous Contrast Agent Influence on Thoracic Computed Tomography Simulation Investigated Through A Heterogeneous Dose Calculation Method Using 5-Bulk Densities

Using 5-bulk-density heterogeneous dose calculation, we investigated whether contrast-enhanced (CE+) computed tomography (CT) will affect dose-calculation accuracy in the thoracic area. We analyzed 17 radiation-oncology patients who underwent thoracic CE+ CTs. Full-resolution CT and 5-bulk-density plans were generated using an adaptive convolution algorithm. Bulk densities for air, lung, fat, soft tissue, and bone were applied to regions identified by an isodensity segmentation tool. The population-averaged physical density of each region was calculated and compared with the reference value calculated from 66 noncontrast-enhanced (CE-) thoracic CT images. Using the 5-bulk densities, we created a new plan in which the physical densities of each area were forced to be the same as the CE- reference value, and we compared the dose-volume histograms (DVH). Average physical density for the segmented air, lung, fat, soft tissue, and bone for CE+ were 0.14, 0.29, 0.90, 1.03, and 1.13 g/cm(3), and the reference values for CE- were 0.14, 0.26, 0.89, 1.02, and 1.12 g/cm(3), respectively. In all the cases, the normal-tissue DVH agreed to better than 1%. In 15 cases, DVH of the planning target volume (PTV) agreed to better than 3%. In 2 patients, >3% difference in the PTV dose was observed. Only 2 patients with a strong injection artifact in the PTV or beam showed >3% discrepancy in the target dose. When using CE+ CT for treatment planning, strong injection artifacts must be excluded.

Consequences of additional use of PET information for target volume delineation and radiotherapy dose distribution for esophageal cancer

Background and purpose To determine the consequences of target volume (TV) modifications, based on the additional use of PET information, on radiation planning, assuming PET/CT-imaging represents the true extent of the tumour. Materials and methods For 21 patients with esophageal cancer, two separate TV’s were retrospectively defined based on CT (CT-TV) and co-registered PET/CT images (PET/CT-TV). Two 3D-CRT plans (prescribed dose 50.4 Gy) were constructed to cover the corresponding TV’s. Subsequently, these plans were compared for target coverage, normal tissue dose–volume histograms and the corresponding normal tissue complication probability (NTCP) values. Results The addition of PET led to the modification of CT-TV with at least 10% in 12 of 21 patients (57%) (reduction in 9, enlargement in 3). PET/CT-TV was inadequately covered by the CT-based treatment plan in 8 patients (36%). Treatment plan modifications resulted in significant changes ( p < 0.05) in dose distributions to heart and lungs. Corresponding changes in NTCP values ranged from −3% to +2% for radiation pneumonitis and from −0.2% to +1.2% for cardiac mortality. Conclusions This study demonstrated that TV’s based on CT might exclude PET-avid disease. Consequences are under dosing and thereby possibly ineffective treatment. Moreover, the addition of PET in radiation planning might result in clinical important changes in NTCP.

Three-Dimensional Imaging in Gynecologic Brachytherapy: A Survey of the American Brachytherapy Society

To determine current practice patterns with regard to three-dimensional (3D) imaging for gynecologic brachytherapy among American Brachytherapy Society (ABS) members. Registered physician members of the ABS received a 19-item survey by e-mail in August 2007. This report excludes physicians not performing brachytherapy for cervical cancer. Of the 256 surveys sent, we report results for 133 respondents who perform one or more implantations per year for locally advanced cervical cancer. Ultrasound aids 56% of physicians with applicator insertion. After insertion, 70% of physicians routinely obtain a computed tomography (CT) scan. The majority (55%) use CT rather than X-ray films (43%) or magnetic resonance imaging (MRI; 2%) for dose specification to the cervix. However, 76% prescribe to Point A alone instead of using a 3D-derived tumor volume (14%), both Point A and tumor volume (7%), or mg/h (3%). Those using 3D imaging routinely contour the bladder and rectum (94%), sigmoid (45%), small bowel (38%), and/or urethra (8%) and calculate normal tissue dose-volume histogram (DVH) analysis parameters including the D2cc (49%), D1cc (36%), D0.1cc (19%), and/or D5cc (19%). Respondents most commonly modify the treatment plan based on International Commission on Radiation Units bladder and/or rectal point dose values (53%) compared with DVH values (45%) or both (2%). More ABS physician members use CT postimplantation imaging than plain films for visualizing the gynecologic brachytherapy apparatus. However, the majority prescribe to Point A rather than using 3D image based dosimetry. Use of 3D image-based treatment planning for gynecologic brachytherapy has the potential for significant growth in the United States.

MO‐B‐211A‐01: Biological/Clinical Outcomes Models in Radiation Therapy Planning

Radiation therapy must strike a balance between clinically acceptable tumor control probability (TCP) and normal tissue complication probability (NTCP). Typical treatments expose several normal organs to incidental dose with an associated risk of a variety of radiation‐induced complications, each of which may depend differently on the spatial and temporal dose distribution. Dose/volume effects, wherein the ‘iso‐complication dose’ increases if the irradiated volume fraction decreases, are particularly important in designing and evaluating treatment plans. These effects are pronounced for some complications and weak for others. The classic “Emami” paper of 1991 summarized guidelines based on decades of clinical experience derived from the simple beam arrangements of the 2‐dimensional planning era when discrete parts of normal tissues typically received uniform doses at conventional fractionation schedules (partial organ irradiation). More recently, 3‐dimensional planning (including IMRT) has led to more conformal distributions with superior patient‐specific dosimetric information. Three‐dimensionl techniques have also encouraged treatment to higher doses and exposed normal tissues to a more variable and inhomogeneous range of dose distributions and fraction sizes. These complex relationships are often summarized in dose‐volume histograms (DVH).Depending on the suspected volume effect, treatment planners use various DVH‐based metrics to estimate complication risks. These include single DVH points (e.g. maximum dose or percent or absolute volume above a dose cut‐point), combinations of such points, generalized equivalent uniform dose (including mean dose) and models (e.g. Lyman model). Co‐morbidities, chemotherapies and other medical factors may also need to be considered. We will describe commonly used evaluation metrics, some of which date back to the early 1990's and others from more recent publications or in‐house experience.The current metrics used to evaluate normal tissue DVHs are sub‐optimal. Under the QUANTEC (Quantitative Analysis of Normal Tissue Effects in the Clinic) initiative, a project that is jointly funded by AAPM and ASTRO, approximately 60 physicists and physicians have critically surveyed the NTCP literature. Their updated consensus guidelines will be published in the near future. Some of their preliminary findings will be discussed here (check the proffered sessions for other presentations).Learning Objectives:1. Understand the general features of the volume effect as applied to NTCP.2. Be familiar with the dose/volume metrics commonly used for complications risk estimation in treatment planning.3. Understand some of the difficulties in determining reliable dosimetric predictors of normal tissue complications.

Case Study of Anti-1-Amino-3-F-18 Fluorocyclobutane-1-Carboxylic Acid (Anti-[F-18] FACBC) to Guide Prostate Cancer Radiotherapy Target Design

Anti-1-amino-3-F-18 fluorocyclobutane-1-carboxylic acid (FACBC) is a novel radiotracer, which has shown some promise for use with positron emission tomography (PET)/computed tomography (CT) for visualizing prostate cancer. Here we describe a case of a prostate cancer patient who underwent radiation treatment and had an FACBC scan obtained as part of a pilot study. We explored the potential impact of FACBC on treatment planning. We registered the FACBC acquisition with the PET/CT, which required a simple translation. Then, we did a deformable image registration of the PET/CT with the planning CT-this process allowed the FACBC-defined gross tumor volume (GTVFACBC) to be projected into the planning CT. An intensity-modulated radiotherapy (IMRT) plan (plan A) not including GTVFACBC (with final dose to 81.0 Gy) was generated, as was an IMRT plan including the GTVFACBC to a final dose of 86.4 Gy (plan B). Target coverage and normal tissue dose volume histogram (DVH) endpoints were tabulated. In this particular patient, bladder constraints could not be met on any plan due to anatomic limitations. However, the impact on the rectal DVH could be assessed, and inclusion of the GTVFACBC did permit rectal DVH constraints to be met in plan B while maintaining target coverage and inhomogeneity constraints. In our test case, it was feasible to use FACBC to guide IMRT, and highlights the role of deformable image registration of the PET/CT with the planning CT. These findings can guide future studies incorporating FACBC into treatment planning.

Accurate heterogeneous dose calculation for lung cancer patients without high-resolution CT densities.

The aim of this study was to investigate the relative accuracy of megavoltage photon‐beam dose calculations employing either five bulk densities or independent voxel densities determined by calibration of the CT Houndsfield number.Full‐resolution CT and bulk density treatment plans were generated for 70 lung or esophageal cancer tumors (66 cases) using a commercial treatment planning system with an adaptive convolution dose calculation algorithm (Pinnacle3, Philips Medicals Systems). Bulk densities were applied to segmented regions. Individual and population average densities were compared to the full‐resolution plan for each case. Monitor units were kept constant and no normalizations were employed. Dose volume histograms (DVH) and dose difference distributions were examined for all cases.The average densities of the segmented air, lung, fat, soft tissue, and bone for the entire set were found to be 0.14, 0.26, 0.89, 1.02, and 1.12 g/cm3, respectively. In all cases, the normal tissue DVH agreed to better than 2% in dose. In 62 of 70 DVHs of the planning target volume (PTV), agreement to better than 3% in dose was observed. Six cases demonstrated emphysema, one with bullous formations and one with a hiatus hernia having a large volume of gas. These required the additional assignment of density to the emphysemic lung and inflammatory changes to the lung, the regions of collapsed lung, the bullous formations, and the hernia gas.Bulk tissue density dose calculation provides an accurate method of heterogeneous dose calculation. However, patients with advanced emphysema may require high‐resolution CT studies for accurate treatment planning.PACS number: 87.53.Tf

Proton Radiotherapy for Childhood Ependymoma: Initial Clinical Outcomes and Dose Comparisons

To report preliminary clinical outcomes for pediatric patients treated with proton beam radiation for intracranial ependymoma and compare the dose distributions of intensity-modulated radiation therapy with photons (IMRT), three-dimensional conformal proton radiation, and intensity-modulated proton radiation therapy (IMPT) for representative patients. All children with intracranial ependymoma confined to the supratentorial or infratentorial brain treated at the Francis H. Burr Proton Facility and Harvard Cyclotron between November 2000 and March 2006 were included in this study. Seventeen patients were treated with protons. Proton, IMRT, and IMPT plans were generated with similar clinical constraints for representative infratentorial and supratentorial ependymoma cases. Tumor and normal tissue dose-volume histograms were calculated and compared. At a median follow-up of 26 months from the start date of radiation therapy, local control, progression-free survival, and overall survival rates were 86%, 80%, and 89%, respectively. Subtotal resection was significantly associated with decreased local control (p = 0.016). Similar tumor volume coverage was achieved with IMPT, proton therapy, and IMRT. Substantial normal tissue sparing was seen with proton therapy compared with IMRT. Use of IMPT will allow for additional sparing of some critical structures. Preliminary disease control with proton therapy compares favorably with the literature. Dosimetric comparisons show the advantage of proton radiation compared with IMRT in the treatment of ependymoma. Further sparing of normal structures appears possible with IMPT. Superior dose distributions were accomplished with fewer beam angles with the use of protons and IMPT.

Intensity modulated radiotherapy for localized prostate cancer: rigid compliance to dose-volume constraints as a warranty of acceptable toxicity?

BackgroundTo report the toxicity after intensity modulated radiotherapy (IMRT) for patients with localized prostate cancer, as a sole treatment or after radical prostatectomy.MethodsBetween August 2001 and December 2003, 132 patients with prostate cancer were treated with IMRT and 125 were evaluable to acute and late toxicity analysis, after a minimum follow-up time of one year. Clinical and treatment data, including normal tissue dose-volume histogram (DVH) constraints, were reviewed. Gastro-intestinal (GI) and genito-urinary (GU) signs and symptoms were evaluated according to the Radiation Therapy Oncology Group (RTOG) toxicity scales. Median prescribed dose was 76 Gy. Median follow-up time was of 26.1 months.ResultsFrom the 125 patients, 73 (58.4%) presented acute Grade 1 or Grade 2 GI and 97 (77.2%) presented acute Grade 1 or Grade 2 GU toxicity. Grade 3 GI acute toxicity occurred in only 2 patients (1.6%) and Grade 3 GU acute toxicity in only 3 patients (2.4%). Regarding Grade 1 and 2 late toxicity, 26 patients (20.8%) and 21 patients (16.8%) presented GI and GU toxicity, respectively. Grade 2 GI late toxicity occurred in 6 patients (4.8%) and Grade 2 GU late toxicity in 4 patients (3.2%). None patient presented any Grade 3 or higher late toxicity. Non-conformity to DVH constraints occurred in only 11.2% of treatment plans. On univariate analysis, no significant risk factor was identified for Grade 2 GI late toxicity, but mean dose delivered to the PTV was associated to higher Grade 2 GU late toxicity (p = 0.042).ConclusionIMRT is a well tolerable technique for routine treatment of localized prostate cancer, with short and medium-term acceptable toxicity profiles. According to the data presented here, rigid compliance to DHV constraints might prevent higher incidences of normal tissue complication.

A phase I/II trial to evaluate three-dimensional conformal radiation therapy confined to the region of the lumpectomy cavity for stage I/II breast carcinoma: Initial report of feasibility and reproducibility of Radiation Therapy Oncology Group (RTOG) Study 0319

This prospective study (Radiation Therapy Oncology Group Study 0319) examines the use of three-dimensional conformal external beam radiation therapy to deliver accelerated partial breast irradiation. Reproducibility, as measured by technical feasibility, was the primary end point with the goal of demonstrating whether the technique is widely applicable in a multicenter setting before a Phase III trial is undertaken. This study was designed such that if fewer than 5 cases out of the first 42 patients evaluable were scored as unacceptable, the treatment would be considered reproducible. Patients received 38.5 Gy in 3.85 Gy/fraction delivered twice daily. The clinical target volume included the lumpectomy cavity plus a 10-15-mm margin bounded by 5 mm within the skin surface and the lung-chest wall interface. The planning target volume (PTV) included the clinical target volume plus a 10-mm margin. Treatment plans were judged as follows: (1) No variations (total coverage), 95% isodose surface covers 100% of the PTV and all specified critical normal tissue dose-volume histogram (DVH) limits met. (2) Minor variation (marginal coverage), 95% isodose surface covers between > or = 95% and <100% of the PTV. No portion of PTV receives <93% of prescription (isocenter) dose. All specified critical normal tissue DVH limits fall within 5% of the guidelines. (3) Major variation (miss), 95% isodose surface covers <95% of the PTV. Portion of PTV receives <93% of prescription isocenter dose. Any critical normal tissue DVH limit exceeds 5% of the specified value. A total of 58 patients were enrolled on this study between 8/15/03 and 4/30/04, 5 of whom were ineligible or did not receive protocol treatment. Two additional patients were excluded, one because the on-study form was not submitted, and the other because no treatment planning material was submitted. This primary end point analysis is based on the first 42 (out of 51) evaluable patients, which were accrued from 17 different institutions (31 centers were credentialed for case enrollment, but because of rapid accrual, not all centers were able to submit cases before trial closure). These 42 patients had the following characteristics: median age was 61 years; 48% had a maximum tumor dimension of <1 cm; 86% had invasive ductal carcinoma; 64% were postmenopausal; the location of tumor was upper outer for 40% and upper central for 21%; 79% had no chemotherapy, and 64% had no hormonal therapy. There were 4 cases with major variations (all 4 related to normal tissue DVHs exceeding 5% of the specified limit). A total of 32 cases with minor variations in treatment plans were detected (16 related to normal tissue DVHs exceeding the specified limits [by < or = 5%], 6 related to suboptimal coverage of the PTV, and 10 related to both). There were 6 cases with no variations. Of the 51 total evaluable patients, 1 additional major variation was noted (PTV receiving <93% of the prescription dose). An additional 5 cases with minor variations in treatment plans were detected (3 related to normal tissue DVHs exceeding the specified limits [by < or = 5%], 1 related to suboptimal coverage of the PTV, and 1 related to both). There were 3 more cases with no variations. Accelerated partial breast irradiation using three-dimensional conformal external beam radiation therapy was shown in this preliminary analysis of the first 42 evaluable patients to be technically feasible and reproducible in a multi-institutional trial using exceptionally strict dosimetric criteria.

Adjuvant intensity modulated pelvic radiation therapy in gynecologic malignancies: Survey of the gynecologic IMRT working group

Intensity modulated radiation therapy (IMRT) is receiving increasing attention in gynecologic malignancies. Dosimetric studies have demonstrated that IMRT reduces the volume of normal tissues irradiated compared to conventional techniques. Moreover, recent outcome studies have suggested that IMRT is associated with less acute and chronic toxicity. A concern, however, is that no standards exist on how gynecologic IMRT should be performed. To develop guidelines, the Gynecologic IMRT Working Group was formed comprised of investigators from the United States, Canada, Asia and Europe. Recently, members were sent a survey on intensity modulated pelvic RT (IM-PRT). The 45-question survey focused on the use of adjuvant IM-PRT in endometrial and cervical cancer. It did not address IMRT in women with an intact uterus or more comprehensive fields. The survey consisted of 6 sections: patient selection, equipment, simulation, target delineation, planning and delivery. Members were asked to give their recommendations. Twenty-seven members (18 USA, 4 Europe, 3 Asia, 2 Canada) completed the survey. Number of sections completed varied by gynecologic IMRT experience. All respondents were academic physicians (median years in practice, 13). Most (89%) were IMRT-users; 63% had treated a gynecology patient with IMRT. Most common contraindications to gynecologic IMRT cited were obesity and poor cooperation. Most respondents (78%) recommended inverse planning. All commercial planning systems were considered acceptable. Recommended positioning was supine (46%), prone (15%) or either (39%). Most (81%) recommended customized immobilization. All recommended thin (3–5 mm) planning CT slices and 88% recommended contrast at simulation, primarily intravenous, rectal, and oral contrast. Most (80%) recommended a vaginal marker. All respondents recommended contouring of the bladder, rectum, and small bowel as avoidance structures. Other tissues contoured included the femoral heads (84%), bone marrow (38%), sigmoid (4%), vulva (2%), and sacrum (2%). Most respondents (72%) recommended inclusion of the vagina, parametria, presacral region, and external, internal, and common iliac lymph nodes in the clinical target volume (CTV). Fifty-two percent, however, recommended different CTVs for cervical and endometrial cancer, the main difference being the exclusion of the presacral region in endometrial cancer. Sixteen percent did not recommend inclusion of the common iliacs in either disease. Additional radiographic studies (predominantly PET and MR) were recommended by 46% to aid in CTV delineation. All recommended a CTV-to-PTV expansion, primarily 5–10 mm. Eighteen percent recommended expansion of the normal tissues. Recommended beam configurations were: 5–9 beams (94%), equally-spaced angles (70%) and coplanar beams (88%). Sixteen respondents provided detailed input planning parameters and acceptance criteria for both PTV and normal tissue dose-volume histograms. Minimum and maximum PTV doses of 95% and 105–110% of the prescription dose were recommended by 62% and 71%, respectively. All recommended quality assurance prior to and throughout treatment. This is the first survey of the application of IMRT to gynecologic tumors and provides insight into its use at a wide variety of centers. While differences in recommended practice were observed, considerable agreement exists between investigators. These results will form the basis of an upcoming consensus statement on adjuvant IM-PRT from the Gynecologic IMRT Working Group. Current work is focused on developing guidelines for target delineation

Can we predict pulmonary complications in lung cancer treated with 3D conformal radiotherapy (3D-CRT): prospective of normal tissue dose volume histograms (DVHs) and pulmonary function relations

L. Claude, L. Falchero, D. Arpin, D. Perol, M. Vincent, J.F. Cordier, J.C. Guerin, C. Carrie Department of Radiation Oncology, Centre Leon Berard, Lyon, France, Department of Pulmonary Medicine, Centre Hospitalier, Villefranche/Saone, France, Department of Pulmonary Medicine, Hopital des Chanaux, Macon, France, Department of Pulmonary Medicine, Hopital Saint Joseph, Lyon, France, Department of Pulmonary Medicine, Hopital Louis Pradel, Lyon, France, Department of Pulmonary Medicine, Hopital de la Croix-Rousse, Lyon, France

Intensity-modulated stereotactic radiosurgery using dynamic micro-multileaf collimation

Purpose: The implementation of dynamic leaf motion on a micro-multileaf collimator system provides the capability for intensity-modulated stereotactic radiosurgery (IMSRS), and the consequent potential for improved dose distributions for irregularly shaped tumor volumes adjacent to critical organs. This study explores the use of IMSRS to provide improved tumor coverage and normal tissue sparing for small cranial tumors relative to plans based on multiple fixed uniform-intensity beams or traditional circular collimator arc-based stereotactic techniques. Methods and Materials: Four patient cases involving small brain lesions are presented and analyzed. The cases were chosen to include a representative selection of target shapes, number of targets, and adjacent critical areas. Patient plans generated for these comparisons include standard arcs with multiple circular collimators, and fixed noncoplanar static fields with uniform-intensity beams and IMSRS. Parameters used for evaluation of the plans include the percentage of irradiated volume to tumor volume (PITV), normal tissue dose–volume histograms, and dose–homogeneity ratios. All IMSRS plans were computed using previously established IMRT techniques adapted for use with the BrainLAB M3 micro-multileaf collimator. The algorithms comprising the IMRT system for optimization of intensity distributions and conversion into leaf trajectories of the BrainLab M3 were developed at our institution. The ADAC Pinnacle 3 radiation treatment-planning system was used for dose calculations and for input of contours for target volumes and normal critical structures. Results: For all cases, the IMSRS plans showed a high degree of conformity of the dose distribution with the target shape. The IMSRS plans provided either ( 1) a smaller volume of normal tissue irradiated to significant dose levels, generally taken as doses greater than 50% of the prescription, or ( 2) a lower dose to an important adjacent critical organ. The reduction in volume of normal tissue irradiated in the IMSRS plans ranged from 10% to 50% relative to the other arc and uniform fixed-field plans. Conclusion: The case studies presented for IMSRS demonstrate significant dosimetric improvements for small, irregularly shaped lesions of the brain when compared to treatments using multiple static fields or standard SRS arc techniques with circular collimators. For all cases, the IMSRS plan yielded a smaller volume of normal tissue irradiated, and/or a reduction in the volume of an adjacent critical organ (i.e., brainstem) irradiated to significant dose levels.

35 Tumor and normal tissue dose-volume histograms in interstitial head and neck brachytherapy

35 Tumor and normal tissue dose-volume histograms in interstitial head and neck brachytherapy

Selection of coplanar or noncoplanar beams using three-dimensional optimization based on maximum beam separation and minimized nontarget irradiation

The design of an appropriate set of multiple fixed fields to achieve a steep dose gradient at the tumor edge, with minimal normal tissue exposure, is a very difficult problem, since a virtually infinite number of possible beam orientations exists. In practice we have selected beams in an iterative and often time-consuming process. This work proposes an optimization method, based on geometric and dose elements, to effectively arrive at a set of beam orientations. Beams are selected by minimizing a goal function including an angle function (beam separation for steep dose gradient at target edge) and a length function (related to normal tissue dose volume histogram). The relative importance of these two factors may be adjusted depending on the clinic situation. The model is flexible and can include case specific practical anatomic and physical considerations. In extremely simple situations, the goal function yields results consistent with well-known analytical solutions. When applied to more complex clinical situations, it provides clinically reasonable solutions similar to those empirically developed by the clinician. The optimization process takes approximately 25 min on a UNIX workstation. The optimization scheme provides a practical means for rapidly designing multiple field coplanar or noncoplanar treatments. It overcomes limitations in human three-dimensional visualization such as trying to visualize beam directions and keeping track of the hinge angle between beams while accounting for anatomic/machine constraints. In practice, it has been used as a starting point for physicians to make modifications, based on their clinical judgment.

Strategy for dose escalation using 3-dimensional conformal radiation therapy for lung cancer

Local failure is a major obstacle to the cure of locally advanced non-small-cell lung cancer. 3-Dimensional conformal radiation therapy (3-DCRT) selects optimal treatment parameters to increase dose to tumor and reduce normal tissue dose, potentially permitting dose escalation. There are several ongoing trials of dose escalation using 3-Dimensional conformal radiation therapy for non-small-cell lung cancer. We performed this analysis to determine if data derived from dose volume histograms could be used as the basis for designing the method of dose escalation in these trials. Between 1990 and 1993, 31 patients were treated with 3-DCRT and had complete normal tissue dose volume histograms created as part of the planning process. The stage distribution was stage I/II 13%, stage IIIa in 45%, and stage IIIb in 42%. The median radiation dose to gross disease was 70.2 Gy (52.2-72 Gy). Elective mediastinal irradiation (50.4 Gy) was administered to 52% (16/31) of patients. The major toxicity encountered in this experience was pulmonary. Dose-volume-histogram data were used to analyze the predictors of toxicity and showed a correlation between risk of pulmonary toxicity and indices of dose to lung parenchyma. Grade 3 or higher pulmonary toxicity occurred in 38% (3/8) of pts with >30%of lung volume receiving > or =25 Gy, versus 4% (1/23) of pts. with < or = 30% lung receiving > or = 25 Gy (p=0.04). Grade 3 or higher pulmonary toxicity occurred in 29% (4/14) of patients with a predicted pulmonary normal tissue complication probability of 12% or higher versus 0% (0/17) in patients with a predicted probability of less than 12% (p=0.03). The single fatality occurred in a patient with a calculated pneumonitis probability of 85% and a high percent (49%) lung volume receiving >= 25 GY. This preliminary experience demonstrates a correlation between lung dose-volume-histogram data and the risk of severe pulmonary toxicity. This provides an opportunity to modify the method of radiation dose escalation. Dose-volume-histogram data can allow escalation according to the risk to the lung parenchyma (which is the major organ of concern) rather than escalation according to tumor dose levels. Because of teh major inter-patient variability of intrathoracic tumor bulk and anatomic distribution, this strategy is intuitively appropriate. This approach may facilitate completion of dose escalation studies and identification of maximum tolerable pulmonary dose levels.