Dose-surface Histograms Research Articles

Bladder distension improves the dosimetry of organs at risk during intracavitary cervical high-dose-rate brachytherapy

To evaluate dose-volume histograms (DVHs) and dose-surface histograms (DSHs) to analyze bladder distension during cervical brachytherapy. Twenty brachytherapy fractions from five cervical cancer patients were selected. For each fraction, empty and full (200cc of contrasted saline) bladder simulation CT scans existed, one of which was used to plan treatment. An alternative plan was then created with the unused scan. DVH for each fraction was generated for the bladder, rectum, sigmoid colon, and small bowel. Mean DVH dose, D0.1cc, and D2cc were calculated for each organ at risk. Plans were then exported to a MATLAB-based program to generate a DSH. Full bladder plans showed no difference in bladder D2cc or D0.1cc compared with empty bladder plans; however, bladder mean DVH dose and DSH dose were both significantly reduced. Full bladder plans showed a significant reduction in small intestine D2cc from 2.81Gy to 1.83Gy and reduction in D0.1cc from 4.07Gy to 2.57Gy (p < 0.05); similarly, sigmoid D2cc was significantly reduced from 4.24Gy to 3.87Gy (p < 0.05) and D0.1cc was reduced from 6.12Gy to 5.61Gy (p < 0.05) in full bladder plans. Both small intestine and sigmoid also showed reduced mean DVH and DSH dose in full bladder plans. The rectum showed no significant difference in D2cc, D0.1cc, mean DVH, or DSH dose between plans. Bladder distension during cervical brachytherapy significantly reduced dose in all DVH and DSH parameters for sigmoid and small intestine with no change in bladder parameters. It reduces dose to organ at risk, but the correlation to toxicity requires further investigation.

Parametrized rectal dose and associations with late toxicity in prostate cancer radiotherapy.

We investigated possible associations between planned dose-volume parameters and rectal late toxicity in 170 patients having radical prostate cancer radiotherapy. For each patient, the rectum was outlined from anorectal junction to sigmoid colon, and rectal dose was parametrized using dose-volume (DVH), dose-surface (DSH) and dose-line (DLH) histograms. Generation of DLHs differed from previous studies in that the rectal dose was parametrized without first unwrapping onto 2-dimensional dose-surface maps. Patient-reported outcomes were collected using a validated Later Effects in Normal Tissues Subjective, Objective, Management and Analytic questionnaire. Associations between dose and toxicity were assessed using a one-sided Mann-Whitney U test. Associations (p < 0.05) were found between equieffective dose (EQD23) and late toxicity as follows: overall toxicity with DVH and DSH at 13-24 Gy; proctitis with DVH and DSH at 25-36 Gy and with DVH, DSH and DLH at 61-67 Gy; bowel urgency with DVH and DSH at 10-20 Gy. None of these associations met statistical significance following the application of a Bonferroni correction. Independently confirmed associations between rectal dose and late toxicity remain elusive. Future work to increase the accuracy of the knowledge of the rectal dose, either by accounting for interfraction and intrafraction rectal motion or via stabilization of the rectum during treatment, may be necessary to allow for improved dose-toxicity comparisons. This study is the first to use parametrized DLHs to study associations with patient-reported toxicity for prostate radiotherapy showing that it is feasible to model rectal dose mapping in three dimensions.

Multi-variable models predicting specific patient-reported acute urinary symptoms after radiotherapy for prostate cancer: Results of a cohort study

PurposeA prospective trial started in 2010, aiming at developing models for urinary toxicity and erectile dysfunction after radiotherapy for prostate cancer. This analysis is finalised at highlighting correlations between clinical/dosimetric factors and acute urinary specific symptoms, as measured by single questions of the International Prostate Symptom Score (IPSS). Materials/methodsIPSS was prospectively collected before and at the end of radiotherapy; absolute weekly bladder dose–surface histograms (DSHw) were chosen as dosimetric descriptors. Relevant clinical factors were prospectively gathered. Backward feature selection was used to identify variables to be included in logistic models for moderate–severe (scores⩾4) urinary symptoms. ResultsComplete data of 262 patients (120 conventional fractionation, 142 hypofractionation) were available. Smoking was a strong predictor for feeling of incomplete emptying, frequency, intermittency, urgency and straining; neoadjuvant hormonal therapy and use of antihypertensive drugs were risk factors for intermittency and weak stream, respectively. The baseline score was a major predictor for all symptoms with the exception of intermittency. DSHw were correlated to increased risk of frequency, intermittency, urgency and nocturia. Most models showed moderate–high discrimination (AUC≈0.60–0.79). ConclusionsSmoking and other clinical and dosimetric factors predict for specific moderate–severe acute urinary symptoms; baseline condition heavily modulated the risk in most endpoints.

Effect of Bladder Distension on Dosimetry of Organs at Risk During Intracavitary Cervical HDR Brachytherapy Using Dose-Volume and Dose-Surface Histograms

Effect of Bladder Distension on Dosimetry of Organs at Risk During Intracavitary Cervical HDR Brachytherapy Using Dose-Volume and Dose-Surface Histograms

Acute Toxicity After Image-Guided Intensity Modulated Radiation Therapy Compared to 3D Conformal Radiation Therapy in Prostate Cancer Patients

Image-guided intensity modulated radiation therapy (IG-IMRT) allows significant dose reductions to organs at risk in prostate cancer patients. However, clinical data identifying the benefits of IG-IMRT in daily practice are scarce. The purpose of this study was to compare dose distributions to organs at risk and acute gastrointestinal (GI) and genitourinary (GU) toxicity levels of patients treated to 78 Gy with either IG-IMRT or 3D-CRT. Patients treated with 3D-CRT (n=215) and IG-IMRT (n=260) receiving 78 Gy in 39 fractions within 2 randomized trials were selected. Dose surface histograms of anorectum, anal canal, and bladder were calculated. Identical toxicity questionnaires were distributed at baseline, prior to fraction 20 and 30 and at 90 days after treatment. Radiation Therapy Oncology Group (RTOG) grade ≥1, ≥2, and ≥3 endpoints were derived directly from questionnaires. Univariate and multivariate binary logistic regression analyses were applied. The median volumes receiving 5 to 75 Gy were significantly lower (all P<.001) with IG-IMRT for anorectum, anal canal, and bladder. The mean dose to the anorectum was 34.4 Gy versus 47.3 Gy (P<.001), 23.6 Gy versus 44.6 Gy for the anal canal (P<.001), and 33.1 Gy versus 43.2 Gy for the bladder (P<.001). Significantly lower grade ≥2 toxicity was observed for proctitis, stool frequency ≥6/day, and urinary frequency ≥12/day. IG-IMRT resulted in significantly lower overall RTOG grade ≥2 GI toxicity (29% vs 49%, respectively, P=.002) and overall GU grade ≥2 toxicity (38% vs 48%, respectively, P=.009). A clinically meaningful reduction in dose to organs at risk and acute toxicity levels was observed in IG-IMRT patients, as a result of improved technique and tighter margins. Therefore reduced late toxicity levels can be expected as well; additional research is needed to quantify such reductions.

Quantifying cell migration distance as a contributing factor to the development of rectal toxicity after prostate radiotherapy

Spatial information is usually neglected in mathematical models of radiation-induced toxicity. In the presence of inhomogeneous dose distributions produced by intensity modulated radiation therapy (IMRT) and volumetric arc therapy, this may be a limitation. We present a model to quantify the spatial characteristics of the dose distribution on the rectum through the quantification of the distribution of distances between dose points on the surface of the rectum in three-dimensions. The method allows us to evaluate the hypothesis that distances between lower and higher dose regions on the rectum influence radiation damage repair due to the migration of normal cells into damaged areas, and consequently, the development of radiation-induced toxicity in patients treated with radiation for prostate cancer. We present a method to compute distances between dose points on the surface of the rectum in three dimensions (3D) and to generate distance maps representing the distances between specific dose regions on the rectum. We introduce the concept of the distance dose surface histogram (DDSH), which is computed from the distance maps. The DDSH is a 2D histogram of rectum area on a grid defined by pairwise combinations of dose and distance. Each bin in the DDSH quantifies the area of the rectum exposed to a given dose and at a given distance from other another dose region on the rectum. By summing across the columns and rows of the DDSH, the authors can generate the dose surface histogram (DSH) and distance surface histogram (DiSH) for a particular dose region. The DiSH is a marginal histogram showing the distribution of distances for the dose points in a specific dose region from another region. We computed the DDSH, DiSH, and DSH for 33 patients treated with IMRT for prostate cancer, nine of whom developed late Grade 2 or higher late rectal toxicity. We show how even though the total area of the rectum exposed to a given dose may be the same for different patients, the distribution of distances for points receiving that dose can be different depending on the shape and contiguity of the region(s) containing those dose points. We also show that area of the rectum in the region receiving more than 75 Gy and at a distance of 16 to 22 mm from the 50 Gy isodose line was significantly correlated to the development of toxicity (p = 0.004, two sided t-test). This suggests that examining the distance distribution of points in specific dose regions could provide valuable additional information in predicting the risk of a patient developing radiation-induced rectal toxicity. We present a new method to quantify the spatial distribution of points in a given region relative to other regions on the rectum. The method provides a means to evaluate the hypothesis that distances between lower and higher dose regions on the rectum influence radiation damage repair due to the migration of normal cells into damaged areas, and may be a contributing factor to the development of radiation-induced toxicity in patients treated with radiation for prostate cancer.

Reduction of rectal doses by removal of gas in the rectum during vaginal cuff brachytherapy

The goal of this work was to evaluate whether the volume reduction related to removal of gas in the rectum could be translated in lower doses to organs at risk (OAR) during vaginal cuff brachytherapy (VBT). Fourteen pairs of brachytherapy planning CT scans derived from 11patients were re-segmented and re-planned using the same parameters. The only difference between pairs of CTs was the presence or lack of gas in the rectum. The first CT showed the basal status and the second was carried out after gas removal with a tube. A set of values derived from bladder and rectum dose-volume histograms (DVH) and dose-surface histograms (DSH) were extracted. Moreover the cylinder position related to the patient craniocaudal axis was recorded. Rectum volume decreased significantly from 77.8 ± 45 to 55.43 ± 17.6ml (p = 0.0052) after gas removal. Such volume diminution represented a significant reduction on all rectal DVH parameters analyzed except D25 % and D50 %. DSH parameter results were similar to previous ones. A nonsignificant increase of the bladder volume was observed and was associated with an increase of the DVH metrics analyzed. Removal of gas pockets is a simple and inexpensive maneuver that decreases rectal dose parameters on VBT, which can be translated as a better therapeutic ratio. It also suggests that other actions directed to empty the rectum could have a similar effect.

SU-E-J-110: A Novel Metric to Evaluate Dose Deformation Error for Deformable Image Registration Algorithms

Purpose: To propose a novel metric to estimate the dosimetric error associated with deforming dose cloud using the mapping generated by deformable image registration (DIR). Methods: The increasing use of the DIR techniques in daily clinical practice requires extensive quality assurance. Deforming dose using the deformation map generated by deforming one CT image to the other is controversial. We propose here a novel method that can serve as a quantitative QA metric for DIR. First, various isodoses in the original dose are converted into structures. Then a deformation map is generated by deforming the original CT to new CT scan. The subsequent mapping is then used to deform the isodose level structures and dose into the new CT (Dose/structure/deform set). The same isodose levels in the deformed dose are converted into structures (Dose/deform/structure set). Then, differential dose surface histograms (DDSH) for each contour are generated using the deformed dose. The difference between the full-width-half-maximum (FWHM) of the DDSHs at the corresponding isodose levels in both sets is defined as the dose deformation factor (DDF). For demonstration purposes, DDF was calculated at 14 different isodose levels for a representative head & neck and representative lung cancer patient that were rescanned during treatment. Results: The mean DDF was 0.38 Gy (range: 0.06 to 1.28Gy) for lung case and was 0.14 Gy (range: 0.02 to 0.85Gy) for the HN case. The maximum DDF values occurred at the 50% and 30% isodose levels for lung and HN cases, respectively. Conclusion: The proposed DDF can be used to evaluate the dose deformation error at different dose levels, independent of the deformation algorithm. The unit of the DDF is Gy and is useful in understanding the clinical effect of dose deformation at different isodose levels. This metric can be used to evaluate commercially available DIR software.

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).

Dose-distance metric that predicts late rectal bleeding in patients receiving radical prostate external-beam radiotherapy

The relationship between rectal dose distribution and the incidence of late rectal complications following external-beam radiotherapy has been previously studied using dose-volume histograms or dose-surface histograms. However, they do not account for the spatial dose distribution. This study proposes a metric based on both surface dose and distance that can predict the incidence of rectal bleeding in prostate cancer patients treated with radical radiotherapy. One hundred and forty-four patients treated with radical radiotherapy for prostate cancer were prospectively followed to record the incidence of grade ≥2 rectal bleeding. Radiotherapy plans were used to evaluate a dose-distance metric that accounts for the dose and its spatial distribution on the rectal surface, characterized by a logistic weighting function with slope a and inflection point d0. This was compared to the effective dose obtained from dose-surface histograms, characterized by the parameter n which describes sensitivity to hot spots. The log-rank test was used to determine statistically significant (p < 0.05) cut-off values for the dose-distance metric and effective dose that predict for the occurrence of rectal bleeding. For the dose-distance metric, only d0 = 25 and 30 mm combined with a > 5 led to statistical significant cut-offs. For the effective dose metric, only values of n in the range 0.07–0.35 led to statistically significant cut-offs. The proposed dose-distance metric is a predictor of rectal bleeding in prostate cancer patients treated with radiotherapy. Both the dose-distance metric and the effective dose metric indicate that the incidence of grade ≥2 rectal bleeding is sensitive to localized damage to the rectal surface.

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.

TU-C-BRA-02: The Relevance of the Spatial Distribution of Dose for Complications after Head and Neck and Prostate Radiotherapy

A thorough understanding of the dose‐response of individual organs‐at‐risk is essential for being able to choose the best radiotherapytreatment for a patient. Therefore, the spatial distribution of dose to the rectum, the anal canal and the parotid glands and its role for radiation‐induced late toxicities were assessed by performing a retrospective analysis of data from the randomised controlled trials MRC RT01 and PARSPORT. Interpretable geometrical features were introduced to quantify the spatial distribution of dose and correlations between the dose to the anorectal wall and seven clinically relevant late rectal toxicities were assessed. Novel risk‐factors based on the spatial distribution of dose could be identified for different endpoints. It was demonstrated that the type of dose‐response relationship differed considerably between different endpoints. Rectal bleeding was more strongly correlated to the lateral extent than to any other measure, including the dose‐surface histogram (DSH). The dosimetric measure with the strongest correlation to loose stools and proctitis were longitudinal extent and DSHs respectively. Based on this statistical analysis,dosimetric constraints taking spatial information into account were derived. An interpretable, non‐linear normal‐tissue‐complication‐probability model (NTCP model) explicitly taking spatial information into account was presented and its predictive power was evaluated and compared to NTCP models used in current clinical practice. Our novel NTCP models predicted late rectal toxicities significantly better than conventional models. Using the novel NTCP models 3D dose patterns related to a low risk of complications have been identified. These dose patterns may be useful for optimising and assessing treatment plans, potentially yielding better treatments with a reduced risk of late rectal toxicities. Furthermore, the role of spatial information and clinical risk factors for xerostomia after head‐and‐neck radiotherapy was assessed. Multivariate NTCP models taking the shape and the location of the dose to the parotid glands into account were derived based on data from the PARSPORT trial. Results were externally validated using data from two independent patient cohorts. A “directed” bath‐and‐shower effect was identified, indicating that a high relative concentration of the dose in the medial‐inferior part of the ipsi‐lateral gland and a high skewness of the dose in cranio‐caudal direction in the superficial lobe of the ipsi‐lateral parotid are beneficial for patients in terms of reducing the risk of xerostomia. Our novel NTCP model taking the “directed” bath‐and‐shower effect into account can be used to rank treatment plans more reliably than standard mean‐dose models. We have also shown that it is possible to implement this model in an inverse‐treatment‐planning system and take morphological information into account when generating IMRTtreatment plans. Learning Objectives: 1. Understand issues related to quantifying the spatial distribution of dose to organs‐at risk 2. Understand issues related to generating and validating NTCP models based on these risk‐factors 3. Understand issues related to identifying risk factors related to the spatial distribution of dose

Modeling late rectal toxicities based on a parameterized representation of the 3D dose distribution

Many models exist for predicting toxicities based on dose-volume histograms (DVHs) or dose-surface histograms (DSHs). This approach has several drawbacks as firstly the reduction of the dose distribution to a histogram results in the loss of spatial information and secondly the bins of the histograms are highly correlated with each other. Furthermore, some of the complex nonlinear models proposed in the past lack a direct physical interpretation and the ability to predict probabilities rather than binary outcomes. We propose a parameterized representation of the 3D distribution of the dose to the rectal wall which explicitly includes geometrical information in the form of the eccentricity of the dose distribution as well as its lateral and longitudinal extent. We use a nonlinear kernel-based probabilistic model to predict late rectal toxicity based on the parameterized dose distribution and assessed its predictive power using data from the MRC RT01 trial (ISCTRN 47772397). The endpoints under consideration were rectal bleeding, loose stools, and a global toxicity score. We extract simple rules identifying 3D dose patterns related to a specifically low risk of complication. Normal tissue complication probability (NTCP) models based on parameterized representations of geometrical and volumetric measures resulted in areas under the curve (AUCs) of 0.66, 0.63 and 0.67 for predicting rectal bleeding, loose stools and global toxicity, respectively. In comparison, NTCP models based on standard DVHs performed worse and resulted in AUCs of 0.59 for all three endpoints. In conclusion, we have presented low-dimensional, interpretable and nonlinear NTCP models based on the parameterized representation of the dose to the rectal wall. These models had a higher predictive power than models based on standard DVHs and their low dimensionality allowed for the identification of 3D dose patterns related to a low risk of complication.

SU-GG-T-38: Dosimetry and Inverse Treatment Planning for 3D Intensity Modulated Brachytherapy

Purpose: To propose a dosimetry algorithm for three‐dimensional (3D) intensity modulated brachytherapy (IMBT) and to develop an inverse treatment planning method applying the dosimetry algorithm. Method and Materials: A Matlab based prototype 3D IMBT treatment planning system was developed. The system consisted of three main components: (1) a comprehensive source commissioning method for intensity modulated sources based on Monte Carlo (EGSnrc) simulations; (2) a “modified TG43” (mTG43) dose calculation algorithm for IMBT dosimetry; (3) an inverse IMBT treatment planning method based on Dose Volume Histogram (DVH) or DoseSurface Histogram (DSH) constraints and simulated annealingoptimization algorithm. The system was applied for planning of an intracavitary accelerated partial breast irradiation (APBI) case treated with Xoft Axxent electronic brachytherapy. Plan quality, planning and delivery time of the IMBT plan were compared with the original plan used for the patient's treatment. Results: For the patient studied, IMBT plan showed better plan quality compared with the original plan. With similar coverage to the target, high dose region V200 was decrease by 16.1%. Maximum doses to skin and ribs were reduced by 56 cGy and 104 cGy in one fraction respectively. Mean dose to ipsilateral and contralateral breasts and lungs were also slightly reduced by IMBT. Conclusion: Application of three‐dimensional intensity modulation in brachytherapytreatment planning is both feasible and promising. 3D IMBT improves the quality of APBI brachytherapy treatment plan, increasing dose uniformity in target and reducing the dose to critical structures.

A computational tool for the efficient analysis of dose‐volume histograms for radiation therapy treatment plans

A Histogram Analysis in Radiation Therapy (HART) program was primarily developed to increase the efficiency and accuracy of dose–volume histogram (DVH) analysis of large quantities of patient data in radiation therapy research. The program was written in MATLAB to analyze patient plans exported from the treatment planning system (Pinnacle3) in the American Association of Physicists in Medicine/Radiation Therapy Oncology Group (AAPM/RTOG) format. HART‐computed DVH data was validated against manually extracted data from the planning system for five head and neck cancer patients treated with the intensity‐modulated radiation therapy (IMRT) technique. HART calculated over 4000 parameters from the differential DVH (dDVH) curves for each patient in approximately 10–15 minutes. Manual extraction of this amount of data required 5 to 6 hours. The normalized root mean square deviation (NRMSD) for the HART–extracted DVH outcomes was less than 1%, or within 0.5% distance‐to‐agreement (DTA). This tool is supported with various user‐friendly options and graphical displays. Additional features include optimal polynomial modeling of DVH curves for organs, treatment plan indices (TPI) evaluation, plan‐specific outcome analysis (POA), and spatial DVH (zDVH) and dose surface histogram (DSH) analyses, respectively. HART is freely available to the radiation oncology community.PACS numbers: 87.53.‐j; 87.53.Tf; 87.53.Xd.

Assessing correlations between the spatial distribution of the dose to the rectal wall and late rectal toxicity after prostate radiotherapy: an analysis of data from the MRC RT01 trial (ISRCTN 47772397)

Many studies have been performed to assess correlations between measures derived from dose–volume histograms and late rectal toxicities for radiotherapy of prostate cancer. The purpose of this study was to quantify correlations between measures describing the shape and location of the dose distribution and different outcomes. The dose to the rectal wall was projected on a two-dimensional map. In order to characterize the dose distribution, its centre of mass, longitudinal and lateral extent, and eccentricity were calculated at different dose levels. Furthermore, the dose–surface histogram (DSH) was determined. Correlations between these measures and seven clinically relevant rectal-toxicity endpoints were quantified by maximally selected standardized Wilcoxon rank statistics. The analysis was performed using data from the RT01 prostate radiotherapy trial. For some endpoints, the shape of the dose distribution is more strongly correlated with the outcome than simple DSHs. Rectal bleeding was most strongly correlated with the lateral extent of the dose distribution. For loose stools, the strongest correlations were found for longitudinal extent; proctitis was most strongly correlated with DSH. For the other endpoints no statistically significant correlations could be found. The strengths of the correlations between the shape of the dose distribution and outcome differed considerably between the different endpoints. Due to these significant correlations, it is desirable to use shape-based tools in order to assess the quality of a dose distribution.

Dose–volume histogram and dose–surface histogram analysis for skin reactions to carbon ion radiotherapy for bone and soft tissue sarcoma

Background and purpose To evaluate the usefulness of the dose–volume histogram (DVH) and dose–surface histogram (DSH) as clinically relevant and available parameters that helped to identify bone and soft tissue sarcoma patients at risk of developing late skin reactions, including ulceration, when treated with carbon ion radiotherapy. Materials and methods Thirty-five patients with bone and soft tissue sarcoma treated with carbon ion beams were studied. The clinical skin reactions were evaluated. Some pretreatment variables were compared with the grade of late skin reactions. Results Average DVH and DSH were established in accordance with the grading of the skin reactions. Prescribed dose, the difference in depths between the skin surface and the proximal extent of the tumor, and some DVH/DSH parameters were correlated with late skin reaction ( ≧ grade 3) according to univariate analysis. Furthermore, the area irradiated with over 60 GyE ( S 60 > 20 cm 2) on DSH was the most important factor by multivariate analysis. Conclusions The area irradiated with over 60 GyE ( S 60 > 20 cm 2) on DSH was found to be a parameter for use as a predictor of late skin reactions.

Using dose-surface maps to predict radiation-induced rectal bleeding: a neural network approach

The incidence of late-toxicities after radiotherapy can be modelled based on the dose delivered to the organ under consideration. Most predictive models reduce the dose distribution to a set of dose–volume parameters and do not take the spatial distribution of the dose into account. The aim of this study was to develop a classifier predicting radiation-induced rectal bleeding using all available information on the dose to the rectal wall. The dose was projected on a two-dimensional dose-surface map (DSM) by virtual rectum-unfolding. These DSMs were used as inputs for a classification method based on locally connected neural networks. In contrast to fully connected conventional neural nets, locally connected nets take the topology of the input into account. In order to train the nets, data from 329 patients from the RT01 trial (ISRCTN 47772397) were split into ten roughly equal parts. By using nine of these parts as a training set and the remaining part as an independent test set, a ten-fold cross-validation was performed. Ensemble learning was used and 250 nets were built from randomly selected patients from the training set. Out of these 250 nets, an ensemble of expert nets was chosen. The performances of the full ensemble and of the expert ensemble were quantified by using receiver-operator-characteristic (ROC) curves. In order to quantify the predictive power of the shape, ensembles of fully connected conventional neural nets based on dose-surface histograms (DSHs) were generated and their performances were quantified. The expert ensembles performed better than or equally as well as the full ensembles. The area under the ROC curve for the DSM-based expert ensemble was 0.64. The area under the ROC curve for the DSH-based expert ensemble equalled 0.59. This difference in performance indicates that not only volumetric, but also morphological aspects of the dose distribution are correlated to rectal bleeding after radiotherapy. Thus, the shape of the dose distribution should be taken into account when a predictive model for radiation-induced rectal bleeding is developed.

SU‐FF‐J‐37: Cumulative Rectal Dose Surface Histogram (DSH) Using Homologous Point Sets

Purpose: Present a metric to compare the outer rectal wall surface cumulative daily treatment dose with the original plan. Materials and Methods: The rectum outer surface was contoured for 28 fractions of a patient undergoing prostate tomotherapy without bowel preparation. The rectal variations could be divided into three ranges corresponding to a rectal filling of “empty”, “half” and “full” with approximately 61%, 21% and 18% of the data in each respective region. We use points on the outer surface of the rectum to calculate a Dose Surface Histogram (DSH). The set of points on the planning surface is the reference for all daily treatments. Doses on the daily contour are mapped onto the reference contour. Every point on the planning contour has a homologous point on the daily contour. Rapid calculations are performed using an in‐house MATLAB algorithm. For each treatment fraction, the dose D(i) to point i on the planning contour is equal to the dose D(j) where j is the previously determined corresponding point on the daily contour. The cumulative dose to point i is the sum of all the daily D(j). Results: A DSH was calculated for the plan and a representative daily MVCT slice. The cumulative DSH was compared to the nominal “average” DSH computed by simply averaging the plan and daily DSHs. There are significant differences above 80% of the prescribed dose. DSHs for a representative contour in each of the empty, half, and full rectum regions indicated 5% difference between the cumulative and planned rectum for doses below 70% and variations of 10 to 20% at higher doses. Conclusions: Cumulative DSHs accurately indicate the dosimetric effect of daily variations in rectal filling. A general technique to compute cumulative DSHs has been developed as an efficient metric to determine if adaptive planning is required.

SU‐GG‐J‐95: Feasibility to Use Daily Cone‐Beam‐CT to Determine Dose‐Surface Histogram of Oral Mucosa in Radiotherapy of H&amp;N Cancer

Purpose: to develop a novel approach for estimating dose delivered to the oral mucosa — dose‐surface histogram (DSH)‐ to facilitate assessment of radiation induced mucositis from radiotherapy for head and neck (H&amp;N) cancer. Method and Materials: Mucositis, Xerostomia, and dental caries are common oral complications from radiotherapy. We hypothesized that an improved method for describing dose distribution to oral mucosa and parotid glands would better enable prediction of these dose‐limiting effects. Due to large uncertainties related to mobility of oral tongue and month, we used daily CBCT to delineate the oral cavity and the gum, which were then expanded by 1‐mm. The shell of the expended 1‐mm should represent the mucosal layer. A dose‐surface‐histogram (DSH) was then calculated from the difference between the DVHs for the oral cavity from the 1‐mm expansion. In addition, the daily CBCT would also allow us to calculate the DVHs to the parotid glands that could be displaced and shrunken during the course of treatment. Results: We have successfully applied this method to two patients currently receiving IMRT for H&amp;N cancer. The absolute mucosal surface area receiving a high dose (&gt;80% of prescription dose) is significantly different from the volume within the oral cavity. Conclusion: DSH for oral mucosa is achievable using daily CBCT images. The dose distribution to the mucosa may provide us with a more robust predictor of radiation‐induced mucositis and Xerostomia.