Dose-surface Histograms Research Articles

Prospective Clinical Trial of Bladder Filling and Three-Dimensional Dosimetry in High-Dose-Rate Vaginal Cuff Brachytherapy

To investigate the effect of bladder filling on dosimetry and to determine the best bladder dosimetric parameter for vaginal cuff brachytherapy. In this prospective clinical trial, a total of 20 women underwent vaginal cylinder high-dose-rate brachytherapy. The bladder was full for Fraction 2 and empty for Fraction 3. Dose-volume histogram and dose-surface histogram values were generated for the bladder, rectum, and urethra. The midline maximal bladder point (MBP) and the midline maximal rectal point were recorded. Paired t tests, Pearson correlations, and regression analyses were performed. The volume and surface area of the irradiated bladder were significantly smaller when the bladder was empty than when full. Of the several dose-volume histogram and dose-surface histogram parameters evaluated, the bladder maximal dose received by 2 cm(3) of tissue, volume of bladder receiving >/=50% of the dose, volume of bladder receiving >/=70% of the dose, and surface area of bladder receiving >/=50% of the dose significantly predicted for the difference between the empty vs. full filling state. The volume of bladder receiving >/=70% of the dose and the maximal dose received by 2 cm(3) of tissue correlated significantly with the MBP. Bladder filling did not alter the volume or surface area of the rectum irradiated. However, an empty bladder did result in the nearest point of bowel being significantly closer to the vaginal cylinder than when the bladder was full. Patients undergoing vaginal cuff brachytherapy treated with an empty bladder have a lower bladder dose than those treated with a full bladder. The MBP correlated well with the volumetric assessments of bladder dose and provided a noninvasive method for reporting the MBP dose using three-dimensional imaging. The MBP can therefore be used as a surrogate for complex dosimetry in the clinic.

Optimization of intravascular brachytherapy treatment planning in peripheral arteries

This work deals with the treatment planning optimization for intravascular brachytherapy (IVB) in peripheral arteries. The objective is both to quantitatively study the validity of different hypotheses required for a reliable application of the treatment with current techniques, and to contribute to the definition and the specification of a new optimized procedure taking into account the actual patient's vessel geometry. The detection of vascular luminal surface was performed by an image analysis process, i.e., virtual active navigation, applied to standard CT data. Dose distribution was calculated according to the formalism proposed and recommended by the AAPM in TG43 and TG60. A method combining simulated annealing and BFGS algorithms was applied to optimize the parameters associated with the dwell points such as their number, positions, and dwell times. Dose-surface histogram (DSH) was used to evaluate the dose distribution results. Four levels of accuracy in target surface description were tested. The application of this optimization method to four different CT data sets including patient data, phantom and animal models showed that the treatment plan can be improved when the actual vessel geometry has been taken into account.

SU-FF-T-351: Predicting Late Lung Complications Following Lung Tumor Radiosurgery with Cyber Knife Using Biologically Effective Doses and Normalized Dose-Surface Histograms

Purpose: To evaluate prospectively the acute and late morbidities (toxicity) of hypofractionated stereotactic radiosurgery with cyberknife with stage I/II lung cancer: predictors for radiation pneumonitis and fibrosis. End Point is to determine the radiation induced lung toxicity (pneumonitis and fibrosis) and tumor response using biological-effective dose histogram. Methods and Materials: The model evaluates the biological consequences of inhomogenities in the physical dose to the surface of the lung for a given fractionation scheme. A method of normalizing the surface area of the lung is employed so that the predicted NTCP is independent of the differing cross-sectional size of the lung. The surface area of the lung is normalized to ensure that all sections of the lung contribute equally to the NTCP. Results: The model has been used to assess late lung complications and the milder RTOG grade 2 and 3 reactions. This model was found to predict late lung toxicity levels of 1.5 ± 0.6 % for an accelerated treatment of 60 Gy in 3 fractions commonly employed at our center. The model predicts that the average NTCP for late effects for four lung patients becomes greater than 5 % with a fractionation scheme of 60 Gy in 3 fractions. Conclusion: The relative seriality model will be extended to incorporate dose-surface histograms and BED. The model assesses the biological effects of inhomogeneities in the dose delivered to the lung. Dose-surface histograms provide a convenient means of studying the differences in the biologically effective dose to the lung.

Comparison of rectal dose histograms under conditions of nonlinear isodoses

Cumulative dose–volume histograms of organs-at-risk are commonly used as predictors of the likelihood and severity of toxicity. In the case of hollow organs such as the rectum, the dose–volume histogram of the organ disregarding the lumen (dose–wall histogram, DWH) can be difficult to outline, and where the voxel size approaches the wall thickness, poor dose statistics can result. Various surrogates for DWH have been proposed, including the dose–volume histogram of organ plus the lumen (DVH), the dose–surface histogram (DSH), the normalized dose–volume histogram of organ plus lumen (NDVH) and the normalized dose–surface histogram (NDSH). In order to test the relative relationships between these histogram types in the case of prostate radiotherapy, a cylindrical model of the rectum has been combined with an idealized nonlinear isodose model in order to allow typical isodose shapes from both conventional three-field radiotherapy and prostate IMRT to be studied. Analytic expressions for histogram values are derived, and implemented through a spreadsheet. Isodose shapes and positions from five patients are combined to generate typical isodose distributions for which histograms may be generated and compared. It is found that the plan type has little effect on the relative values of the histogram points, and that no other histogram acts as a robust estimate of the DWH under all conditions. However, if the irradiated portion of the rectum is empty, NDVH is a good surrogate for DWH.

TU-EE-A1-01: Capabilities of a CT-Suitable, Patient-Adaptive HDR/PDR Intracavitary Brachytherapy Applicator for the Treatment of Cervical Cancer

Purpose: To demonstrate the capability of a prototype, high-dose-rate/pulsed-dose-rate (HDR/PDR) intracavitary brachytherapy cervical applicator to be (a) imaged using computed tomography (CT) with minimal artifacts as compared to current, clinically-utilized applicators and (b) deliver dose to a simulated disease plane that is at least equivalent to that delivered by the FW applicator while delivering less dose to multiple, simulated rectal planes for equivalent loadings. Capability (b) demonstrates the prototype applicator's ability to adapt to varying patient anatomies utilizing a remotely adjustable shield contained within the colpostat. Method and Materials: CT image sets were acquired of the Fletcher-Suite-Delclos (FSD), Fletcher-Williamson (FW), and prototype applicators (utilizing a step-and-shoot technique) and artifacts generated by each were compared qualitatively. Images were acquired of the applicators positioned parallel to the table and in positions that simulated applicator placement during treatment. Using film validated MCNPX Monte Carlo (MC) models of the prototype and FW applicators, dose comparisons (min, max, average, and dose-surface histograms) were made in simulated disease (1cm medial to colpostat) and multiple rectal (1cm distal and 0, 0.5, and 1cm medial to the colpostat) planes. Results: Preliminary results indicate that the prototype applicator is CT-friendly; qualitatively minimizing anatomy-obscuring artifacts compared to equivalent FW and FSD image sets. Additionally, the prototype applicator is able to deliver comparable dose levels (within +/− 5%) to a simulated disease plane while reducing dose (32% average) to varying simulated rectal planes when compared to equivalent FW treatments. Conclusion: The prototype applicator is able to be CT imaged with minimal artifacts and substantially reduce dose delivered to the rectum while maintaining dose delivered to disease when compared to current ICBT technologies. Currently, the model is being extended to include two prototype applicators and a 15° tandem so that dose-volume histograms can be generated using specific-patient cases of varying anatomical geometries.

Cluster model analysis of late rectal bleeding after IMRT of prostate cancer: A case–control study

Cluster models are newly developed normal-tissue complication probability models in which the spatial aspects of radiation-induced injury are taken into account by considering the size of spatially contiguous aggregates of damaged tissue units. The purpose of this study was to test the validity of a two-dimensional cluster model of late rectal toxicity based on maximum cluster size of damage to rectal surface. A paired case-control study was performed in which each of 9 patients experiencing Grade 2 or higher late rectal toxicity after intensity-modulated radiation therapy of localized prostate cancer was paired with a patient having a similar rectal dose-surface histogram but free of rectal toxicity. Numeric simulations were performed to determine the distribution of maximum cluster size on each rectal surface for each of many different choices of possible model parameters. Model parameters were found for which patients with rectal toxicity were consistently more likely to have a significantly larger mean maximum cluster size than their matched controls. These parameter values correspond to a 50% probability of tissue-unit damage at doses near 30 Gy. This study suggests that a cluster model based on maximum cluster size of damage to rectal surface successfully incorporates spatial information beyond that contained in the rectal dose-surface histogram and may therefore provide a useful new tool for predicting rectal normal-tissue complication probability after radiotherapy.

Rectal wall sparing effect of three different endorectal balloons in 3D conformal and IMRT prostate radiotherapy

To investigate the dosimetric consequences and rectal wall (Rwall) sparing effect of three different endorectal balloons (ERBs) for three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT) for prostate cancer. In 20 patients, 4 planning computed tomography scans were made: 1 without ERB and 3 with ERB1, ERB2, or ERB3 inserted. Two different planning target volumes were defined: prostate only, and prostate plus seminal vesicles. The 3D-CRT and IMRT planning techniques were used, and the prescription dose was 78 Gy. In 284 treatment plans, the Rwall mean dose, the Rwall normal tissue complication probability, and the absolute Rwall volumes exposed to > or =50 Gy (V(50)) and > or =70 Gy (V(70)) were calculated. For spatial dose distribution analysis, inner rectal wall dose maps and dose surface histograms were generated. Each ERB was tolerated well. In the case of 3D-CRT, each ERB showed a statistically significant reduction of all the measured parameters. ERB2 and ERB3 performed better than ERB1. In IMRT, a statistically significant reduction in the Rwall dose parameters could not be demonstrated for any of the ERBs. For 3D-CRT and IMRT, as a result of the rectal dilation, ranging from 8 to 20 cm in circumference, the ERBs resulted in a reduction of the relative inner Rwall surface exposed to intermediate and high doses. In 3D-CRT, any ERB showed a significant rectal wall sparing effect. ERB2 and ERB3 were superior to ERB1. For both 3D-CRT and IMRT, a reduction of the relative inner Rwall surface exposed to intermediate and high doses was found, which may lead to reduced late rectal toxicity. Development of user- and patient-friendly ERBs is warranted to increase their acceptability.

Absolute and relative dose–surface and dose–volume histograms of the bladder: which one is the most representative for the actual treatment?

The purpose of this study was to quantify to what extent relative and absolute bladder dose–volume and dose–surface histograms of the planning CT scan were representative for the actual treatment. We used data of 17 patients, who each received 11 repeat CT scans and a planning CT scan. The repeat CT scans were matched on the planning CT scan by the bony anatomy. Clinical treatment plans were used to evaluate the impact of bladder filling changes on the four histogram types. The impact was quantified by calculating for this patient group the correlation coefficient between the planning histogram and the treatment histogram. We found that the absolute dose–surface histogram was the most representative one for the actual treatment.

Volume and hormonal effects for acute side effects of rectum and bladder during conformal radiotherapy for prostate cancer

To identify dosimetric variables predictive of acute gastrointestinal (GI) and genitourinary (GU) toxicity and to determine whether hormonal therapy (HT) is independently associated with acute GI and GU toxicity in prostate cancer patients treated with conformal radiotherapy (RT). This analysis was performed on 336 patients participating in a multicenter (four hospitals) randomized trial comparing 68 Gy and 78 Gy. The clinical target volume consisted of the prostate with or without the seminal vesicles, depending on the risk of seminal vesicle involvement. The margin from the clinical target volume to the planning target volume was 1 cm. For these patients, the treatment plan for a total dose of 68 Gy was used, because nearly all toxicity appeared before the onset of the 10-Gy boost. Acute toxicity (<120 days) was scored according to the Radiation Therapy Oncology Group criteria. The dosimetric parameters were obtained from the relative and absolute dose-volume/surface histograms derived from the rectal wall (rectal wall volume receiving > or =5-65 Gy) and the bladder surface (bladder surface receiving > or =5-65 Gy). Additionally, relative and absolute dose-length histograms of the rectum were created, and the lengths of rectum receiving more than a certain dose over the whole circumference (rectal length receiving > or =5-65 Gy) were computed. The clinical variables taken into account for GI toxicity were neoadjuvant HT, hospital, and dose-volume group; for GU toxicity, the variables pretreatment GU symptoms, neoadjuvant HT, and transurethral resection of the prostate were analyzed. The variable neoadjuvant HT was divided into three categories: no HT, short-term neoadjuvant HT (started < or =3 months before RT), and long-term neoadjuvant HT (started >3 months before RT). Acute GI toxicity Grade 2 or worse was seen in 46% of the patients. Patients with long-term neoadjuvant HT experienced less Grade 2 or worse toxicity (27%) compared with those receiving short-term neoadjuvant HT (50%) and no HT (50%). The volumes of the prostate and seminal vesicles were significantly smaller in both groups receiving neoadjuvant HT compared with those receiving no HT. In multivariate logistic regression analysis, including the two statistically significant clinical variables neoadjuvant HT and hospital, a volume effect was found for the relative, as well as absolute, rectal wall volumes exposed to intermediate and high doses. Of all the length parameters, the relative rectal length irradiated to doses of > or =5 Gy and > or =30 Gy and absolute lengths receiving > or =5-15 and 30 Gy were significant. Acute GU toxicity Grade 2 or worse was reported in 56% of cases. For patients with pretreatment GU symptoms, the rate was 93%. The use of short-term and long-term neoadjuvant HT resulted in more GU toxicity (73% and 71%) compared with no HT (50%). In multivariate analysis, containing the variables pretreatment symptoms and neoadjuvant HT, only the absolute dose-surface histogram parameters (absolute surface irradiated to > or =40, 45, and 65 Gy) were significantly associated with acute GU toxicity. A volume effect was found for acute GI toxicity for relative, as well as absolute, volumes. With regard to acute GU toxicity, an area effect was found, but only for absolute dose-surface histogram parameters. Neoadjuvant HT appeared to be an independent prognostic factor for acute toxicity, resulting in less acute GI toxicity, but more acute GU toxicity. The presence of pretreatment GU symptoms was the most important prognostic factor for GU symptoms during RT.

SU-FF-T-387: Treatment Plan Evaluation and Optimization For SRS and IMRT Using Dose-Surface Histograms

Purpose: To introduce a new method in determination of the periphery dose and dose differences between the tumor and surrounding tissues, including organs at risk (OARs), for stereotactic radiosurgery (SRS) and/or intensity modulation radiotherapy (IMRT) using dose-surface histograms (DSHs). Method and Materials: Plan-target volume (PTV) and its expansion into surrounding healthy tissue (TISSUE) by 3 mm (or 5 mm) for a small (or large) PTV are created. The contours of the TISSUE on CT or MRI slices are modified at the region where the Tissue intersects with an OAR. The PTV and TISSUE are then expanded or shrunk, denoted as PTV' and TISSUE', by a pixel size (PS) within a fine dose grid, respectively. DSH for the PTV (DSHPTV) can then be determined by (DVHPTV*PTV − DVH PTV'*PTV')/(PTV − PTV')/PS. Similarly, DSH for the TISSUE (DSHTISSUE) is given by (DVHTISSUE*Tissue − DVH TISSUE'*TISSUE')/(TISSUE − TISSUE')/PS. The mean and variation of DSHPTV characterizes the periphery doses and the difference between the mean doses on DSHPTV and DSHVOI quantifies the dose drop-off from the PTV to the surrounding tissue. Results: Application of the new method on Gamma Knife plans and LINEAC-based SRS plans clearly shows that gamma knife plan has better dose drop-off but worse periphery doses for relative rounded target. However, the LINEAC-based MLC-shaped arcs plans are generally superior for irregular-shape lesions. In comparison of plans using MLC-shaped arcs, conformal beams, and IMRT, we found no improvement on periphery dose and dose drop-off in IMRT for some cases due to lack of DSHPTV in the inverse planning systems. Conclusion: The accurate quantification of the periphery dose and dose drop-off is extremely important because they are closely correlated to the treatment prescription and the clinical outcomes.

A comparative study of rectal dose histograms in prostate brachytherapy: Some analytic and numerical results

A cumulative dose histogram is the graph of an integral function integrated over a domain VCR3 and is dubbed the dose-volume histogram (DVH), the dose-surface histogram (DSH) or the dose-wall histogram (DWH), depending on the dimension and structure of the region V. This paper presents a comparative study of the three rectal dose histograms for sixty patients as well as for a cylindrical model of the rectum; in particular, the DSH and DVH for the cylindrical model with one point source are computed analytically in terms of elliptic integrals. The difference among the three relative dose histograms, averaged over the sixty patients, is less than 5%, whereas that between DVH and DWH for various wall-thickness can be as large as 3-12 cm3 in the range 60-100 Gy. The paper also contains an error analysis using two simple models of the rectum, for which the true DSH and DWH can be computed via numerical integration, to evaluate the effect of digitization. The digitized computation agrees quite well with the pre-digitization numerical integration, within 1% or 0.2 cm3, because of the low dose-gradient effect near the rectum in prostate brachytherapy.

A Monte Carlo study of the impact of the choice of rectum volume definition on estimates of equivalent uniform doses and the volume parameter

Calculations of normal tissue complication probability (NTCP) values for the rectum are difficult because it is a hollow, non-rigid, organ. Finding the true cumulative dose distribution for a number of treatment fractions requires a CT scan before each treatment fraction. This is labour intensive, and several surrogate distributions have therefore been suggested, such as dose wall histograms, dose surface histograms and histograms for the solid rectum, with and without margins. In this study, a Monte Carlo method is used to investigate the relationships between the cumulative dose distributions based on all treatment fractions and the above-mentioned histograms that are based on one CT scan only, in terms of equivalent uniform dose. Furthermore, the effect of a specific choice of histogram on estimates of the volume parameter of the probit NTCP model was investigated. It was found that the solid rectum and the rectum wall histograms (without margins) gave equivalent uniform doses with an expected value close to the values calculated from the cumulative dose distributions in the rectum wall. With the number of patients available in this study the standard deviations of the estimates of the volume parameter were large, and it was not possible to decide which volume gave the best estimates of the volume parameter, but there were distinct differences in the mean values of the values obtained.

A geometric model for evaluating the effects of inter-fraction rectal motion during prostate radiotherapy

A geometric model is presented which allows calculation of the dosimetric consequences of rectal motion in prostate radiotherapy. Variations in the position of the rectum are measured by repeat CT scanning during the courses of treatment of five patients. Dose distributions are calculated by applying the same conformal treatment plan to each imaged fraction and rectal dose-surface histograms produced. The 2D model allows isotropic expansion and contraction in the plane of each CT slice. By summing the dose to specific volume elements tracked by the model, composite dose distributions are produced that explicitly include measured inter-fraction motion for each patient. These are then used to estimate effective dose-surface histograms (DSHs) for the entire treatment. Results are presented showing the magnitudes of the measured target and rectal motion and showing the effects of this motion on the integral dose to the rectum. The possibility of using such information to calculate normal tissue complication probabilities (NTCP) is demonstrated and discussed.

A cylindrical model of the rectum: comparing dose–volume, dose–surface and dose–wall histograms in the radiotherapy of prostate cancer

The calculation of the percentage cumulative histogram of the rectal wall (DWH) in prostate cancer radiotherapy may be subject to large uncertainties due to the difficulty of assessing the wall thickness on CT images. For this reason often only the external contour is used to define the rectum and then the percentage cumulative dose–volume histogram (DVH) of the rectum including any filling is calculated as a ‘surrogate’ for the DWH. More recently, other approaches using only the external contour have been proposed to estimate the DWH such as the percentage normalized dose–surface histograms (NDSH). A similar concept can be used when considering the solid rectum (the percentage normalized DVH, NDVH). The purpose of this investigation was to assess the relationships between rectal DVH, NDVH, DSH, NDSH and DWH in the common case of three- and four-field techniques in prostate cancer irradiation. Analytical relationships between the above parameters have been derived for a cylindrical rectum model in the case of three- and four-field techniques. The model is applied to the case of an empty rectum, a full rectum and to the more realistic mixed full/empty rectum situation for a four-field technique delivering 76 Gy (ICRU dose) with 18 MV x-rays. Different positions of the lateral beam with respect to the rectum axis were simulated. In the case of no lumen variation along the z-axis, the DWH is found to be very close to the DVH and to the DSH for empty and full rectum, respectively. The largest differences (up to 15%) between DVH and DSH were seen in the high-dose region (>70 Gy). In the more realistic case of lumen variation along the z-axis, the DWH always lies between NDVH and NDSH and, excluding the full-rectum situation, the DWH differs from the DVH by less than 7% in the 50–75 Gy range. In the case of significant portions of rectum being completely shielded, the DVH may differ from the NDVH/NDSH/DWH by up to 10–15%. In most clinical situations NDVH is within a few per cent of DWH, whilst NDSH may differ from DWH by up to 15–20%, especially in the high-dose region (V70). In conclusion, for most situations, the DVH is highly correlated with NDVH and DWH. A high degree of consistency between NDVH and DWH was found in most clinical cases whilst largest deviations between NDSH and DWH were evident in the high-dose region (70–75 Gy). In the less common case of a very full rectum a poorer correlation between DVH/NDVH and DWH was found whilst NDSH mimicked the DWH very well. In summary, except for the case of a ‘very full’ rectum, NDVH may be used as a robust surrogate for DWH. The DVH seems to be sufficiently robust if the rectum is prevalently empty.

The dosimetric effect of organ movement in prostate radiotherapy: New models for hollow organs

In this work two new models were developed in order to calculate the effects of movement on the dose distribution for hollow organs. For the rectum a two-dimensional model is used where isotropic expansion or contraction is allowed in the plane of each CT slice. A fully three-dimensional (3D) deformations model was created for the bladder which was considered as an isotropic expanding/contracting contour about its center of mass. The idea is to determine the dose to specific pieces of tissue in the hollow organ wall for a series of outlines. The dose affecting the wall of the hollow organ (sensitive tissue volume) each fraction, was plotted as a dose-surface-histogram (DSH). Also, for the first time, a “final” DSH—which takes into account the organ movements over 9 or 10 fractions—was calculated for five patients and compared to each daily DSH. As a result of interfraction motion, the rectum and bladder are shifted systematically into or out of the high dose region, thus resulting in a “final” DSH which differs from the planning DSH. All the results obtained in this work are crucial for a better estimation of the uncertainty in the dose delivered to the normal tissue of the hollow organs at risk when organ motion is present. The use of DSHs instead of dose-volume-histograms (DVHs) for hollow organs and the explicit inclusion of geometrical information of the 3D dose distribution should lead to better predictions of late rectal or bladder complications after radiotherapy of prostate cancer.

CERR: a computational environment for radiotherapy research.

A software environment is described, called the computational environment for radiotherapy research (CERR, pronounced "sir"). CERR partially addresses four broad needs in treatment planning research: (a) it provides a convenient and powerful software environment to develop and prototype treatment planning concepts, (b) it serves as a software integration environment to combine treatment planning software written in multiple languages (MATLAB, FORTRAN, C/C++, JAVA, etc.), together with treatment plan information (computed tomography scans, outlined structures, dose distributions, digital films, etc.), (c) it provides the ability to extract treatment plans from disparate planning systems using the widely available AAPM/RTOG archiving mechanism, and (d) it provides a convenient and powerful tool for sharing and reproducing treatment planning research results. The functional components currently being distributed, including source code, include: (1) an import program which converts the widely available AAPM/RTOG treatment planning format into a MATLAB cell-array data object, facilitating manipulation; (2) viewers which display axial, coronal, and sagittal computed tomography images, structure contours, digital films, and isodose lines or dose colorwash, (3) a suite of contouring tools to edit and/or create anatomical structures, (4) dose-volume and dose-surface histogram calculation and display tools, and (5) various predefined commands. CERR allows the user to retrieve any AAPM/RTOG key word information about the treatment plan archive. The code is relatively self-describing, because it relies on MATLAB structure field name definitions based on the AAPM/RTOG standard. New structure field names can be added dynamically or permanently. New components of arbitrary data type can be stored and accessed without disturbing system operation. CERR has been applied to aid research in dose-volume-outcome modeling, Monte Carlo dose calculation, and treatment planning optimization. In summary, CERR provides a powerful, convenient, and common framework which allows researchers to use common patient data sets, and compare and share research results.

Probability of late rectal morbidity in 125I prostate brachytherapy

Purpose: Rectal toxicity is a concern in prostate brachytherapy because it is difficult to avoid delivering a dose equal to, or greater than, the prescription dose to the anterior surface of the rectum. The purpose of this study was to define the probability that a patient will experience Grade 2 (bleeding/ulceration) late rectal morbidity after 125I prostate brachytherapy according to the rectal dosimetry. Methods and Materials: Ninety-eight consecutive patients who received monotherapy 125I prostate implants for treatment of Stage T1-T2, favorable-risk adenocarcinoma of the prostate were evaluated for Radiation Therapy Oncology Group Grade 2 late rectal morbidity. All reported incidences of late morbidity were retrospectively confirmed by colonoscopy. All patients had at least 15 months follow-up after implantation. The median follow-up was 32 months (range 15–54). The rectal dosimetry was based on a CT scan obtained at 3–9 weeks after implantation. The rectum was contoured on each CT image between the base and apex of the prostate. A dose-surface histogram was compiled for each implant, and the relative surface area that received a dose ≥100, ≥150, ≥200, ≥300, ≥400, and ≥500 Gy was recorded. The probability of developing late rectal toxicity was calculated by logistic regression analysis as a function of dose and the percentage of the rectal surface that received that dose. Results: Of the 98 patients, 10 developed Grade 2 late rectal morbidity. The percentage of the rectal surface that received 100, 150, 200, and 300 Gy was significantly greater ( p ≤0.02) for patients who experienced late rectal morbidity. The probability of late rectal morbidity increased with both the dose and the percentage of the rectal surface that received that dose. The probability was ≤1% when 20%, 7%, and 0% of the rectal surface received 100, 150, and 200 Gy, respectively. The probability increased to ≤5% when 31%, 19%, and 9% of the rectal surface received these doses. The probability of late rectal morbidity can also be expressed in terms of the maximal rectal dose. The probability of late morbidity was 0.4%, 1.2%, and 4.7% when the maximal rectal dose was 150, 200, and 300 Gy, respectively. Conclusion: The percentage of the rectal surface that receives a dose ≥100 Gy is predictive of Grade 2 (bleeding/ulceration) late rectal morbidity after 125I prostate brachytherapy. The probability of late morbidity depends on both the dose and the percentage of the rectal surface that received that dose. Our results indicate that the rectum can tolerate doses of 100, 150, and 200 Gy to approximately 30%, 20%, and 10% of the rectal surface with a ≤5% risk of late morbidity. Our results also indicate that the practical guideline for limiting the incidence of late morbidity to 1%, 3%, or 5% is to keep the maximal rectal dose to <200, 250, and 300 Gy, respectively.

Comparison of methods for calculating rectal dose after 125I prostate brachytherapy implants

Purpose : To compare several different methods of calculating the rectal dose and examine how accurately they represent rectal dose surface area measurements and, also, their practicality for routine use. Methods and Materials : This study comprised 55 patients, randomly selected from 295 prostate brachytherapy patients implanted at the Vancouver Cancer Center between 1998 and 2000. All implants used a nonuniform loading of 0.33 mCi (NIST-99) 125I seeds and a prescribed dose of 144 Gy. Pelvic CT scans were obtained for each patient ∼30 days after implantation. For the purposes of calculating the rectal dose, several structures were contoured on the CT images: ( 1) a 1-mm-thick anterior rectal wall, ( 2) the anterior half rectum, and ( 3) the whole rectum. Point doses were also obtained along the anterior rectal surface. The thin wall contour provided a surrogate for a dose-surface histogram (DSH) and was our reference standard rectal dose measurement. Alternate rectal dose measurements (volume, surface area, and length of rectum receiving a dose of interest [DOI] of ≥144 Gy and 216 Gy, as well as point dose measures) were calculated using several methods (VariSeed software) and compared with the surrogate DSH measure ( SA DOI ). Results : The best correlation with SA 144 Gy was the dose volumes (whole or anterior half rectum) ( R = 0.949). The length of rectum receiving ≥144 Gy also correlated well with SA 144 Gy ( R ≥0.898). Point dose measures, such as the average and maximal anterior dose, correlated poorly with SA 144 Gy ( R ≤0.649). The 216-Gy measurements supported these results. In addition, dose-volume measurements were the most practical (∼6 min/patient), with our surrogate DSH the least practical (∼20 min/patient). Conclusion : Dose-volume measurements for the whole or anterior half rectum, because they were the most practical measures and best represented the DSH measurements, should be considered a standard method of reporting the rectal dose when calculating the DSH is not practical. Average or maximal anterior rectal doses are not reliable indicators of surface area dosimetry.

Correlations between dose–surface histograms and the incidence of long-term rectal bleeding following conformal or conventional radiotherapy treatment of prostate cancer

Background and Purpose: In a randomized trial, the incidence of rectal bleeding among patients treated for prostate cancer using conformal radiotherapy was significantly lower ( p = 0.002) than that among those treated conventionally. Here the relationship between rectal dose distributions and incidences of bleeding is assessed. Methods and Materials: Rectal dose–surface histograms (DSHs) have been calculated for 79 trial patients. The relationship between the DSHs and incidences of Grade 1–3 bleeding has been explored using both semiempiric and biologic (parallel) model-based approaches. Results: Semiempiric analysis of the trial data suggests that it is more useful to work with DSH fractional surface areas multiplied by outlined rectal lengths than with either raw DSH fractional areas or fractional areas multiplied by absolute total outlined rectal surface area. Fitting the parallel model to length-multiplied rectal DSHs and complication data reveals the existence of a significant volume effect, the rate of Grade 1–3 bleeding falling by 1.1% (95% confidence interval [0.04, 2.2]%) for each 1% decrease in the fraction of rectal wall (outlined over an 11-cm length) receiving a dose of more than 57 Gy. Conclusion: The existence of this volume effect suggests that dose escalation can be achieved using conformal techniques, although the extent to which doses may be safely escalated cannot be reliably estimated from the trial data.

A comparison of radiation dose to the neurovascular bundles in men with and without prostate brachytherapy-induced erectile dysfunction

Purpose: The etiology of erectile dysfunction after definitive local therapy for carcinoma of the prostate gland represents a multifactorial phenomenon including neurogenic compromise, venous insufficiency, local trauma, and psychogenic causes. It has been suggested that impotence after prostate brachytherapy is a consequence of excessive radiation dose to the neurovascular bundles (NVB). Herein we evaluate the potential relationship between radiation dose to the NVB and the development of erectile dysfunction following prostate brachytherapy. Methods and Materials: The radiation dose to the NVB was evaluated for 33 patients who developed erectile dysfunction (ED) following brachytherapy plus 21 additional patients who were potent before and subsequent to brachytherapy. Of the 54 patient study group, the median follow up was 37 months, and 25 patients were managed with 125I as a monotherapeutic approach and 29 received 103Pd as a boost following 45 Gy of external beam radiation therapy. Radiographic localization of the NVB was performed via a two-dimensional geometric model that placed 3-NVB calculation points on the left and right posterolateral side of each 5-mm CT slice. Parameters evaluated included dose–surface histograms, dose parameters via point doses on each slice, the magnitude of the dose in relationship to the distance from the base, and the relationship between NVB radiation dose in patients with and without ED, patient response to sildenafil and case sequence number. Results: In terms of percent prescribed minimum peripheral dose (% mPD), there was no significant difference in mean neurovascular bundle dose between potent and impotent patients, between the isotopes ( 125I or 103Pd), mono- or boost therapy, or side of the prostate for which the overall average was 217% ± 55% of mPD. There was also no significant dosimetric difference in terms of response to sildenafil based on a multivariate analysis which included % mPD and various dose thresholds and side of the gland. The dose distribution over the length of the prostate rose smoothly from the base and apex to peak at midgland in 125I implants while 103Pd implants had a relatively constant dose over the length of the prostate. Considering the calculation grid as forming a 6-mm wide ribbon along each side of the prostate, the average patient had 70 mm 2 area receiving at least 300% of mPD. Conclusion: In this study, no relationship between radiation dose to the NVB and the development of post brachytherapy erectile dysfunction was discernible. Such a difference may become evident with additional follow-up. If long-term brachytherapy-induced erectile dysfunction is related to the radiation dose to the NVB, the ultimate preservation of potency following prostate brachytherapy may be markedly inferior to what has been reported. Nevertheless, the majority of this patient population responded favorably to sildenafil.