Dose-wall Histograms Research Articles

Comparison of dose statistics for bladder wall and rectum wall vs whole organs for VMAT prostate treatment

Dose-wall histograms (DWHs) have been used as alternatives to dose-volume histograms (DVHs) for hollow organs, with the rationale that the dose delivered to the interior of a hollow organ would be unrelated to the level of radiation damage. The purpose of this study is to conduct a statistical comparison of dose statistics for both walled and solid structure contours for both bladder and rectum in the treatment of intermediate risk prostate cancer with volumetric arc therapy (VMAT). Ten intermediate risk prostate cases were randomly selected. Rectum and bladder were first contoured as solid structures, and then the corresponding wall structures were generated using either a slice-by-slice cropping (2D method), or with a full 3D cropping tool (3D method). Each case was then inverse planned using a 2-arc VMAT technique. Two plans per case were created, 1 with a hypofractionated treatment and 1 with a standard fractionated treatment. DVHs were calculated for solid structure contours, and DWHs were calculated for the walled structure contours generated using 2D and 3D contouring tools. A nonparametric Spearman statistic correlation test was used to compare a large number of relevant dose histogram points, and to establish the relationship between dose statistics for walled and solid structures. Several notable relationships were observed. Maximum rectal dose was strongly correlated between the solid structure and both the 2D-generated (Spearman's correlation rs = 0.988, p < 0.01) and 3D-generated (rs = 0.952 p< 0.01) wall structures. This indicates that the rectal hot spot occurred in or near the wall for all cases, suggesting that both structure types give similar maximum dose information for rectum. Maximum bladder dose was not significantly correlated between solid structures and the 2D (rs = 0.596, p= 0.069) and 3D-generated (rs = 0.681, p= 0.03) counterparts. This suggests that the maximum dose is not consistently in or near the bladder wall. This favors the use of bladder wall contours when considering bladder toxicity, with the maximum dose to the wall potentially being more relevant radiobiologically. This analysis was extended to many other relevant points on the rectum and bladder histogram curves. Where correlations are strong, equations of best-fit are presented. This work establishes several statistically-significant relationships between bladder and rectum DVHs and DWHs for VMAT irradiation of intermediate-risk prostate cancer. This information may be used to inform contouring requirements for clinical trial design as well as for standard patient care.

Interfraction variation and dosimetric changes during image-guided radiation therapy in prostate cancer patients.

PurposeThe aim of this study was to identify volume changes and dose variations of rectum and bladder during radiation therapy in prostate cancer (PC) patients.Materials and MethodsWe analyzed 20 patients with PC treated with helical tomotherapy. Daily image guidance was performed. We re-contoured the entire bladder and rectum including its contents as well as the organ walls on megavoltage computed tomography once a week. Dose variations were analyzed by means of Dmedian, Dmean, Dmax, V10 to V75, as well as the organs at risk (OAR) volume. Further, we investigated the correlation between volume changes and changes in Dmean of OAR.ResultsDuring treatment, the rectal volume ranged from 62% to 223% of its initial volume, the bladder volume from 22% to 375%. The average Dmean ranged from 87% to 118% for the rectum and 58% to 160% for the bladder. The Pearson correlation coefficients between volume changes and corresponding changes in Dmean were -0.82 for the bladder and 0.52 for the rectum. The comparison of the dose wall histogram (DWH) and the dose volume histogram (DVH) showed that the DVH underestimates the percentage of the rectal and bladder volume exposed to the high dose region.ConclusionRelevant variations in the volume of OAR and corresponding dose variations can be observed. For the bladder, an increase in the volume generally leads to lower doses; for the rectum, the correlation is weaker. Having demonstrated remarkable differences in the dose distribution of the DWH and the DVH, the use of DWHs should be considered.

A plan quality classifier derived with overlap-wall-histogram of hollow organs for automatic IMRT plan quality control of prostate cancer cases

Purpose: We developed a plan quality classification model to assess IMRT plan quality of prostate cancer patients for automatic plan quality control. Methods : For hollow organs such as rectum and bladder, dose-wall-histogram (DWH) was used to evaluate OAR dose sparing in our institution. Correspondingly, we proposed a new descriptor called overlap-wall-histogram (OWH) to describe the complex spatial relationship between PTV and a hollow organ. Two metrics calculated from the OWH and DWH are introduced to quantitatively evaluate the difficulty of patient geometry for planning and plan quality in terms of OAR sparing, respectively. A linear correlation between these two metrics was observed after plotting plan quality metric as a function of geometry difficulty metric studied from a database of prostate cases treated in our institution with acceptable plan quality. Thus, a fitting line was built acting as the boundary of high quality and poor quality plans. A query plan falling above the boundary is assessed as high quality, vice versa poor quality. Results: 15 prostate IMRT plans were used to test our model. One was identified as poor quality and the others were common-level. After re-planning all plans, the dose constraints for bladder wall W75 (percentage of wall receiving more than 75Gy), W 70 , W 65 and W 60 can be reduced by 3.34%, 3%, 6.99%, 6.54% for that poor quality plan and 1.11%, 0.95%, 1.45% and 1.81% averagely for the common-level quality group, without sacrificing PTV coverage and rectum dose sparing. Conclusion : An effective model was built to provide automatic IMRT plan quality control by evaluating hollow OAR dose sparing for prostate cancer patients. Furthermore, for the query plan with poor quality, potential improvement of plan quality can be estimated and a good reference plan with similar or harder geometry can be automatically chosen from our database to help guide the re-planning if necessary. --------------------------- Cite this article as : Song T, Tian Z, Jia X, Zhou L, Jiang SB, Gu X. A plan quality classifier derived with overlap-wall-histogram of hollow organs for automatic IMRT plan quality control of prostate cancer cases. Int J Cancer Ther Oncol 2014; 2(2):020241. DOI: 10.14319/ijcto.0202.41

Impact of the radiotherapy technique on the correlation between dose–volume histograms of the bladder wall defined on MRI imaging and dose–volume/surface histograms in prostate cancer patients

The aim of this study was to evaluate the correlation between the ‘true’ absolute and relative dose–volume histograms (DVHs) of the bladder wall, dose–wall histogram (DWH) defined on MRI imaging and other surrogates of bladder dosimetry in prostate cancer patients, planned both with 3D-conformal and intensity-modulated radiation therapy (IMRT) techniques. For 17 prostate cancer patients, previously treated with radical intent, CT and MRI scans were acquired and matched. The contours of bladder walls were drawn by using MRI images. External bladder surfaces were then used to generate artificial bladder walls by performing automatic contractions of 5, 7 and 10 mm. For each patient a 3D conformal radiotherapy (3DCRT) and an IMRT treatment plan was generated with a prescription dose of 77.4 Gy (1.8 Gy/fr) and DVH of the whole bladder of the artificial walls (DVH-5/10) and dose–surface histograms (DSHs) were calculated and compared against the DWH in absolute and relative value, for both treatment planning techniques. A specific software (VODCA v. 4.4.0, MSS Inc.) was used for calculating the dose–volume/surface histogram. Correlation was quantified for selected dose–volume/surface parameters by the Spearman correlation coefficient. The agreement between %DWH and DVH5, DVH7 and DVH10 was found to be very good (maximum average deviations below 2%, SD < 5%): DVH5 showed the best agreement. The correlation was slightly better for absolute (R = 0.80–0.94) compared to relative (R = 0.66–0.92) histograms. The DSH was also found to be highly correlated with the DWH, although slightly higher deviations were generally found. The DVH was not a good surrogate of the DWH (R < 0.7 for most of parameters). When comparing the two treatment techniques, more pronounced differences between relative histograms were seen for IMRT with respect to 3DCRT (p < 0.0001).

Correlation between surrogates of bladder dosimetry and dose–volume histograms of the bladder wall defined on MRI in prostate cancer radiotherapy

The correlation between bladder dose–wall-histogram (DWH) and dose–volume-histogram (DVH), dose–surface-histogram (DSH), and DVH-5/10 was investigated in a group of 28 patients; bladder walls were drawn on T2-MRI. DVH showed the poorest correlation with DWH; DSH or DVH-5/10 should be preferred in planning; absolute DVH may be used for radical patients, although less robust.

Late Rectal Toxicity on RTOG 94-06: Analysis Using a Mixture Lyman Model

To estimate the parameters of the Lyman normal-tissue complication probability model using censored time-to-event data for Grade ≥2 late rectal toxicity among patients treated on Radiation Therapy Oncology Group 94-06, a dose-escalation trial designed to determine the maximum tolerated dose for three-dimensional conformal radiotherapy of prostate cancer. The Lyman normal-tissue complication probability model was fitted to data from 1,010 of the 1,084 patients accrued on Radiation Therapy Oncology Group 94-06 using an approach that accounts for censored observations. Separate fits were obtained using dose-volume histograms for whole rectum and dose-wall histograms for rectal wall. With a median follow-up of 7.2 years, the crude incidence of Grade ≥2 late rectal toxicity was 15% (n = 148). The parameters of the Lyman model fitted to dose-volume histograms data, with 95% profile-likelihood confidence intervals, were TD(50) = 79.1 Gy (75.3 Gy, 84.3 Gy), m = 0.146 (0.107, 0.225), and n = 0.077 (0.041, 0.156). The fit based on dose-wall histogram data was not significantly different. Patients with cardiovascular disease had a significantly higher incidence of late rectal toxicity (p = 0.015), corresponding to a dose-modifying factor of 5.3%. No significant association with late rectal toxicity was found for diabetes, hypertension, rectal volume, rectal length, neoadjuvant hormone therapy, or prescribed dose per fraction (1.8 Gy vs. 2 Gy). These results, based on a large cohort of patients from a multi-institutional trial, are expected to be widely representative of the ability of the Lyman model to describe the long-term risk of Grade ≥2 late rectal toxicity after three-dimensional conformal radiotherapy of prostate cancer.

Investigation of bladder dose and volume factors influencing late urinary toxicity after external beam radiotherapy for prostate cancer

We sought to identify the bladder dose-volume factors associated with an increased risk of late urinary toxicity among prostate cancer patients treated with radiotherapy. This retrospective analysis included data from 128 prostate cancer patients treated on protocol with 2 Gy/fraction to 46 Gy followed by a boost to 78 Gy. The endpoint for this analysis was Grade 1 or greater late genitourinary (GU) toxicity occurring within two years of treatment. The Lyman-Kutcher-Burman, mean dose, threshold dose, and hottest volume models were fitted to the toxicity data using the maximum likelihood method. Model fits based on dose-volume histograms tended to fit the toxicity data better than models based on dose-wall histograms. The hottest volume (hotspot) model was found to be the best-fitting model investigated. The best fit was for the hottest 2.9% of bladder (95% CI, 1.1-6.8%). This model has an area under the receiver operating characteristic curve of 0.74. The hotspot model separated the patients into clinically meaningful subgroups with approximately 25% of the patients who received <78 Gy to the hottest 2.9% of bladder had GU toxicity at eight years compared with approximately 50% when the dose was > or =78 Gy (p = 0.002). This provides the first evidence supporting that bladder "hotspots" are related to GU toxicity within two years after external beam radiotherapy for prostate cancer. Confirming data are needed from other investigators. Particular attention should be given to hotspots higher than 78 Gy in bladder in radiation treatment planning.

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.

Comparison of Rectal Dose-Wall Histograms and Dose-Volume Histograms in Predicting Rectal Grade Toxicity After Prostate Cancer Radiotherapy

Purpose/Objective: To compare rectal dose-wall histograms and dose-volume histograms for the same patient population and their respective ability to predict rectal complications after prostate cancer radiotherapy.

Statistical analysis of dose‐volume and dose‐wall histograms for rectal toxicity following 3D‐CRT in prostate cancer

The purpose of this paper is to determine the correlation between dose-volume histogram (DVH) and dose wall-histogram (DWH) in the evaluation of rectal complications for prostate cancer patients treated with three-dimensional conformal radiotherapy (3D-CRT). A retrospective analysis of DVHs and DWHs of a subset of 25 prostate cancer patients treated with 3D-CRT was performed. For every patient the rectum and the rectal wall (inner and outer surface) were contoured. Median ICRU radiation dose of 79.4 Gy was administered. Correlation between DVHs and DWHs parameters was investigated by the nonparametric Spearman test and by linear regression analysis. The results showed a statistically significant linear correlation between pairs of DVH and DWH dosimetric parameters with Spearman correlation values (S) bigger than 0.8, with p values better than 0.0005 (two-sided) when the emptied rectum is considered. The variation of S and linear fit slope values [b(1)] showed a very similar functional shape with a minimum at 91% ICRU dose [S =0.83, b(1)=0.65]. The present study confirms a high correlation (>80%) between DVH and DWH of the rectum following 3D-CRT for prostate cancer. The derived advantage is that the contouring of inner surface of rectum could be obviated in almost 90% of patients when performing predictive models for rectal complications based on dosimetric variables under the standard treatment conditions specified in this study.

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.

Comparison of rectal dose–wall histogram versus dose–volume histogram for modeling the incidence of late rectal bleeding after radiotherapy

To compare the fits of normal-tissue complication probability (NTCP) models based on rectal dose-wall histograms (DWHs) vs. dose-volume histograms (DVHs) when the two are used to analyze a common set of late rectal toxicity data. Data were analyzed from 128 prostate cancer patients treated with 3-dimensional conformal radiotherapy (3D-CRT) at The University of Texas M.D. Anderson Cancer Center (UTMDACC). The DVH for total rectal volume, including contents, was obtained for each patient from the treatment-planning system. A DWH was also computed, using the outer rectal contour plus an autogenerated inner contour that corresponds to an assumed 3-mm rectal wall thickness. The endpoint for analysis was Grade 2 or higher late rectal bleeding within 2 years of treatment; all patients had at least 2 years of follow-up. Four different NTCP models were fitted to the response data by using either the DVH or the DWH to describe the dose distribution to rectum or rectal wall, respectively. The 4 models considered were the Lyman model, the mean dose model, the parallel-architecture model, and a model based on the volume of a organ receiving more than a specified dose (the "cutoff-dose" model). For each of the models, the fit to the late rectal bleeding data was slightly improved when the analysis was based on the rectal DWH instead of on the DVH. In addition, the results of the cutoff dose and parallel architecture models were consistent with one another for the DWH data but not for the DVH data. For the DWH data, both models predict a 50% or higher incidence of Grade 2 or worse late rectal bleeding within 2 years if 80% or more of the rectal wall is exposed to doses greater than 32 Gy. A 50% or higher incidence of rectal bleeding is also predicted if the mean dose to rectal wall exceeds 53.2 Gy. A consistent, although modest, improvement occurs in the fits of NTCP models to the UTMDACC 2-year late rectal bleeding data when the fit is based on the rectal dose-wall histogram instead of on the dose-volume histogram for entire rectum, including contents.

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 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.

A model to simulate day-to-day variations in rectum shape

Purpose: To develop a model that predicts possible rectum configurations that can occur during radiotherapy of prostate cancer on the basis of a planning CT scan and patient group data.Materials and Methods: We used a stochastic shape description model with a limited number of parameters (area, area difference, and curvature) on a slice-by-slice basis to simulate rectum motion. The probability distributions of the chosen parameters were obtained from a group of 9 reference patients, who each received 15–17 repeat CT scans. We used a Monte Carlo technique to generate different rectum configurations from the probability distributions. We verified the model by comparing dose-wall histograms (DWHs) of the originally delineated rectal contours and simulated rectums for a three-field treatment technique with a prescription dose of 78 Gy. The 15–17 sets of rectal contours of each patient are regarded as the golden standard and provide a good estimate of the actual dose received during the treatment. We determined the equivalent uniform dose (EUD) for a quantitative comparison between the actual dose, the dose predicted on the basis of the simulations, and the dose predicted on the basis of a single planning CT scan.Results: The simulated rectum configurations yield a better estimate of the actual dose in the rectal wall than the rectum in the planning CT scan alone. The differences between the EUD based on the planning CT scan and the actual EUD ranged between −1.1 Gy and 2.1 Gy, with respect to a mean actual EUD of 69.8 Gy. This range is smaller for the EUD based on the simulated rectums, namely −0.4 Gy to 0.6 Gy. Furthermore, the simulation generates a set of rectum configurations that provides an estimate of the variation in DWHs during the course of the treatment. This estimate can be used in addition to the DWH of the planning CT scan in the analysis of gastrointestinal toxicity.Conclusions: To simulate rectum shapes, we have developed a model that can be used in addition to the information available in the planning CT scan in the analysis of the received dose to the rectal wall during radiotherapy of prostate cancer.

Comparison of dose–volume histograms and dose–wall histograms of the rectum of patients treated with intracavitary brachytherapy

The correlation between dose values from dose–volume histograms (DVHs) and dose values from dose–wall histograms (DWHs) of the rectum tissue of patient with uterine cervix cancer was determined. The minimum dose in 2 cm3 in the high-dose region of the DVH is a good estimate of the dose in the rectum wall.

Radical radiation therapy for carcinoma of the prostate in patients with a single hip prosthesis: a technique analysis using dose-volume histograms

Radiation therapy of the prostate patient is sometimes complicated by the presence of a hip prosthesis. Using dose-volume histogram (DVH) data, various techniques were assessed in terms of rectal and bladder sparing, as well as total dose to the femoral head and neck. An attempt was made to isolate a suitable technique in prosthesis patients that was comparable to the standard 4-field box technique in nonprosthesis cases. A 3-field coplanar technique is identified as equally efficacious in terms of rectal sparing, while avoiding the hip prosthesis. The importance of using dose-wall histograms for the rectum in plan ranking is discussed, as are the limitations of DVH data analyses without biological endpoints.

Dose–wall histograms and normalized dose–surface histograms for the rectum: a new method to analyze the dose distribution over the rectum in conformal radiotherapy

Purpose: To develop an accurate method to generate a dose–volume histogram (DVH) of the rectum wall, solely based on the outer contours of the rectum wall. Methods and Materials: A mathematical model for the rectum wall is developed, incorporating the stretching of the rectum wall due to variable rectal filling and neighboring structures. The model is based on the assumption that the amount of intersected rectum wall tissue normal to the central axis of the rectum is constant. The main objective of the model is to determine the thickness of the rectum wall in each wall element. Two approaches are described, each yielding a DVH of the rectum wall, based only on the delineated outer contours of the rectum. In the first approach, the model is used to create a set of inner contours out of the axial outer contours. Both sets of contours are used to derive a dose–wall histogram (DWH) of the rectum. In the second approach, the model is used to generate a normalized 2D sampling space, which is subsequently binned into a normalized dose–surface histogram (NDSH). The model is verified using 20 sets of CT data (5 patients × 4 scans) in which both outer and inner contours of the rectum are carefully delineated. The DWHs and NDSHs are compared with DVHs of the rectum wall, which require contouring of the outer and inner surfaces of the rectum wall, and with DVHs of the total rectum (including rectal filling). The variation between DWHs, NDSHs, and DVHs is investigated using normal tissue complication probability (NTCP) calculations. Results: The local wall thickness of the rectum as outlined on CT data was in conformity with the described rectum model. The amount of rectum wall tissue per unit length rectum varied considerably between patients (27%, 1 SD). In all analyzed patients, the DWHs and NDSHs corresponded well to the DVHs of the rectum wall. Much more discrepancies were observed between the DVHs of the total rectum and the DVHs of the rectum wall. Conclusion: The applied methods yield accurate dose distributions of the rectum wall, without delineating the inner surface of the rectum. This reduces both the workload and variations due to inaccurate delineation of the rectum wall. The DWH and NDSH are effective tools to evaluate 3D dose distributions of the rectum wall and to estimate the complication probability of the rectum in high-dose conformal radiotherapy.

Analysis of the dose-surface histogram and dose-wall histogram for the rectum and bladder.

Dose distribution throughout a hollow organ is represented by either a dose-surface histogram (DSH) or a dose-wall histogram (DWH). A spherical shell model for the bladder and a cylindrical shell model for the rectum were introduced for quantifying the difference between the DWH and DSH. The difference was given by subtraction of the percent volume of the wall from the percent area of the surface for any specific dose level. Taking the dose-grid size and contour-delineation uncertainties into account, the DSH and DWH calculation errors were estimated by simplified formulas. The DSHs and DWHs for the rectum and bladder in patients undergoing four-field-prostate treatment and gynecological intracavitary brachytherapy were computed with a refined numerical algorithm. Results of the analytic models and the numerical calculations demonstrated that the difference between the DWH and DSH was small (about 5%) for a fully filled bladder or rectum but large (about 10%) for an empty rectum or a contracted bladder. The error of DSH was about 3%, which is smaller than that of DWH.