Dosimetric comparison of artificial walls of bladder and rectum with real walls in common prostate IMRT techniques: Patient and Monte Carlo study

Abstract

BACKGROUND:

Rectum and bladder are hallow structures and considered as critical organs in prostate cancer intensity modulated radiotherapy (IMRT). Therefore, dose received by these organ walls must be considered for prediction of radiobiological effects. Contouring the real organ walls is quite difficult and time consuming in CT/MRI images, so the easy contouring artificial walls with uniform thickness could be appropriated alternatives.

OBJECTIVE:

To compare reconstructed artificial walls with real walls of bladder and rectum in common prostate IMRT techniques based on dose volume-histograms (DVHs) derived from artificial and real walls.

METHODS:

Artificial walls were reconstructed with 2–10 mm and 2–8 mm thicknesses for bladder and rectum, respectively. Four common IMRT techniques were applied to each patient. Spearman correlation was used to find the relation between the DVHs of true walls with artificial walls and whole organs. Monte Carlo (MC) simulations of the IMRT techniques and dosimetric comparison were also performed on a standard patient data.

RESULTS:

The 2 mm thickness artificial walls showed the minimum differences with the true bladder and rectum walls based on absolute evaluations (the maximum difference < 10cc and standard deviation < 15cc). However, relative evaluations showed that all the artificial walls had high correlations with real walls for selecting dose volume parameters. There was also good agreement between the treatment planning system and MC simulations results.

CONCLUSION:

The DVH of whole organs was not a good surrogate of the true wall. The 2 mm artificial walls can be regarded as good alternatives for both of rectum and bladder. However, in relative dose evaluations all studied artificial walls were appropriate.

Keywords

Prostate cancer intensity modulated radiotherapy bladder wall rectum wall Monte Carlo simulation

1 Introduction

Prostate cancer is one of the most common cancers among men [1] for which external radiotherapy is used as one of the treatment options for the patients with non-metastatic cancers [2, 3]. With improving in radiotherapy techniques, such as intensity modulated radiotherapy (IMRT) and image guided radiotherapy, the dose to normal tissues and organs at risks (OARs) can be reduced with consequent improvements in radiation toxicity [3 –7]. Therefore, computing the precise and accurate dose distribution of critical organs before the radiotherapy treatment have an essential importance. It could enable us to reduce the OARs dose and predict future problems following radiotherapy procedure.

Bladder and rectum are important critical organs in prostate radiotherapy. These structures are hollow organs that can be visualized in CT and MRI images. However, they are usually contoured with filling materials and their fillings dose distributions does not have any effect on their radiation toxicity [8 –11]. Therefore, the prediction of future complications of these organs is not reliable.

Fortunately, unlike the CT scans, the bladder and rectum walls may be well visualizes using MRI [12, 13]. It is possible to delineate these walls in MRI scans and match them with CT scans that gathered from patients at the same time, for dose calculations. However, delineation of these walls is hard and time consuming in MRI images. Consequently, this method is usually not practical, especially at busy departments. Thus, other alternative parameters modeling the real walls with easy and fast method to reconstruct are necessary for estimating the doses delivered to these organs.

There are alternative methods for modeling the bladder and rectum walls which are fast and easy and predicting the radiation induced injuries based on the dose-volume of the whole rectum and bladder [14 –16]. However, it will be more efficient to calculate these injuries only based on the bladder and rectum walls or other structures behaving quite comparable to these two organs.

To overcome the problem resulted for lack of visibility of the bladder and rectum walls on CT scans, other surrogates such as DVH, DSH and DVH of the artificial walls have been introduced. Based on our literature review, it seems that no any previous investigations assessed various dosimetric models for estimating bladder and rectum walls dose distribution in various IMRT techniques. Although there are some investigations about dosimetric modeling of rectum and bladder [8 , 17–22], Most of them have focused mainly on the bladder to find the appropriate alternative artificial wall mimicking the real bladder wall, since its radiation interaction probability is quite close to its filling material [11 , 17–20]. However, they have not studied different IMRT techniques. Carillo et al. [17], investigated the correlation between the dose volume histogram (DVH) of bladder wall with dose surface histogram (DSH) and artificial bladder walls with 5 and 10 mm thicknesses. In another study, Magio et al. [11], assessed correlation between the DVH of the bladder wall with these of the whole bladder as well as the artificial walls with 5, 7 and 10 mm thicknesses. In another study [21], the dosimetric behavior of rectum and rectum surface histograms were evaluated against the rectum wall in patient with prostate cancer.

Inhomogeneity of air and soft tissue existing in rectum can induce the dose calculation uncertainties in commercial TPSs, therefore, application of MC method for dose calculation seems to be more appropriate for preventing the errors related to inhomogenities [23]. In this study we tried to evaluate the correlation and differences between the rectum/bladder real walls and defined artificial walls histograms in prostate cancer patients undergoing IMRT, using TPS calculations made on patient data and also MC simulations made on the data of a standard patient (visible human project or VHP) [24]. We also aimed to find and propose an artificial wall with appropriate thickness mimicking the real bladder and rectum walls in common IMRT techniques in prostate cancer therapy based on the DVHs parameters.

2 Materials and methods

2.1 Patient selection, imaging, and treatment planning

A single center, retrospective study was done following national research ethics board approval. CT and MRI-T2 w scans of 25 prostate cancer patients with stages ranged from T2a to T3b were considered. The patients were randomly selected and had the same prescribed IMRT techniques with the same dose. Patients’ characteristics were shown in Table 1. These patients were routinely treated with 9 fields IMRT technique, and we just used the CT and MRI images of them in our study. T2-weighted MRI and CT scans were matched on bony structures, implanted gold markers in prostate and outer contour of the patients’ body. The patients’ MRI imaging was acquired immediately after their CT scan with the same position.

Table 1
Patient characteristics or demographics

Characteristics or demographics Number of patients/Value

Median age (range) 74 (61–85)

Mean±SD of PSA (ng/dl) 6.6±4.2

Median PSA (ng/dl) 6.5

GS: 6 10

GS: 7 10

GS: 8–9 5

AJCC T-stage:

T2a 6

T2b 7

T2c 7

T3a 3

T3b 2

Intermediate recurrence risk group 20

High recurrence risk group 5

Characteristics or demographics	Number of patients/Value
Median age (range)	74 (61–85)
Mean±SD of PSA (ng/dl)	6.6±4.2
Median PSA (ng/dl)	6.5
GS: 6	10
GS: 7	10
GS: 8–9	5
AJCC T-stage:
T2a	6
T2b	7
T2c	7
T3a	3
T3b	2
Intermediate recurrence risk group	20
High recurrence risk group	5

PSA: prostate-specific antigen; GS: Gleason score; AJCC: American Joint Committee on Cancer.

MRI-T2 w axial images (spin–echo sequence, TE = 100 ms, TR = 3000 ms) of the pelvis were obtained using a 1.5T Siemens Avanto scanner and a pelvic coil (Siemens Healthcare GmbH, Erlangen, Germany). CT scans were acquired using a spiral 16 slice Siemens Emotion System (Siemens Healthcare GmbH, Erlangen, Germany). All of the patients were scanned in the supine position with a comfortably full urinary bladder (mean: 115.7cc) and empty rectum (mean: 58.83cc) without any contrast medium.

Prostate for very low to intermediate risk patients and prostate with seminal vesicles for high risk patients were defined as clinical tumor volumes (CTVs). Four different IMRT plans were created including 9 fields (9F), 7 fields (7F), 5 fields (5F) and automatic beam angle optimization (BAO) for each patient, so 100 IMRT treatment plans were created. The planning tumor volume (PTV) was generated adding 8 mm margin posteriorly and 10 mm margin in other directions. CTVs and OARs were contoured on the MR images, although the dose calculations were performed based on the CT images following the fusion of the CT and MRI images.

IMRT plans were created using the Eclipse treatment planning system (TPS), version 11 (Varian Medical Systems, Palo Alto, USA) with simulated 6 MV photon beam from Varian Linear accelerator, Clinac 600 with 120L multileaf collimator (MLCs). All plans were interactively optimized following our institutional planning protocol based on the previous study by Pollak et al. [25] in which more than or equal to 98% of PTV volume received 70.2 Gy and no more than 2% of the PTV received 75 Gy or higher doses. Furthermore, the volume of bladder and rectum that received 50 Gy or lower (V_50Gy) must be less than 25% and 17% respectively for bladder and rectum. In addition, the volume of bladder and rectum that received 31 Gy or lower (V_31Gy) must be less than 50% and 35% respectively and the maximum dose of 40 Gy was considered for femur as the dose constraint [25]. For each patient, IMRT treatment plans were evaluated by a physicist for assuring about appropriate dose distribution regarding the above protocol.

2.2 Defining real and artificial bladder and rectum walls

For each of 25 patients, bladder, rectum, and their real walls were contoured in MRI images by an experienced radiologist. Artificial walls with uniform thickness in all directions were obtained using the wall generating tool and Boolean tool in Eclipse TPS. Artificial bladder walls (ABW) with 2, 5, 7 and 10 mm (ABW2, ABW5, ABW7 and ABW10) and artificial rectum walls (ARW) with 2, 4, 6 and 8 mm (ARW2, ARW4, ARW6 and ARW8) were constructed for all patients. The accuracy of contouring the artificial walls was then tested by a medical physicist to ensure that these walls have uniform thickness in all directions. An example of bladder and rectum real and artificial walls contours in axial CT images are shown in Fig. 1.

Fig.1

The example of bladder and rectum real and artificial walls contours in axial CT images. (a) Bladder and rectum real walls contours; (b) artificial bladder wall contours; (c) artificial rectum wall contours.

2.3 Monte Carlo study on the visible human project model

Visible human project data (visible human male) included CT and MRI (T1, T2 and PD weighted) images from a standard male cadaver were downloaded freely with the agreement of American national library of medicine from the related site (https://www.nlm.nih.gov/research/visible/). These images were matched on each other as described in previous paragraphs. A MC software, PRIMO (version 0.1.5.1307) that is based on the PENELOPE 2011 code, was used to simulate the 4 IMRT techniques in our study in a way that was expressed in previous researches [26, 27]. The ability of simulating IMRT treatment plans and summation various radiation fields from different directions in this software, make it suitable for dose calculation in our study. In the first step, the VHP data were imported to the TPS. The prostate and seminal vesicles were considered as CTV and all organs at risks, which mentioned for the patients, were contoured in these data by an oncologist. The mentioned IMRT techniques were planed and resulted MLC positions in irradiation period (control points) were gathered for all of the radiation fields for each technique using the beam fluence option in the Eclipse software. In the second step, CT and structure data of VHP (in DICOM format) were imported to the PRIMO software along with the MLC positions data for each field. MC simulations for each of 4 mentioned different techniques were done with a 2% or less uncertainties and splitting factor of 20 (variance reduction method) in a desktop computer with a 32 core of 2.6 GHz processor and 32 GB RAM memory. Real and artificial bladder and rectum walls were contoured on the VHP images in Eclipse software using the method described before for patients.

2.4 Parameters and analysis

Volume of organs at risk that received 60 Gy (V60), 50Gy (V50), and 30 Gy (V30) for bladder and rectum in 4 mentioned different techniques were obtained for all patients, as well as for the VHP model. The V60 and V50 are the tolerance doses for both bladder and rectum regarding previous studies [25 , 29]. In addition, we assumed V30 indicating low doses. These parameters were compared between artificial and real walls in various IMRT techniques using one-way Annova statistical test. This comparison was done for all of the patients and also for VHP data in MC simulations. The Spearman’s rank correlation was used to test the correlation between the DWH (DVH of real wall) and all the other parameters.

The mean differences and standard deviations between the ‘true’ DWH and the DVH of the artificial walls, and the DVH of the bladder/rectum (including filling), both in percentage (%) and absolute volume (cm³), were calculated and considered for analysis the patients. All statistical tests were performed with the SPSS software (IBM company, USA, v.22).

3 Results

The absolute and the relative differences between the mean values of the DWH and artificial bladder/rectum walls are plotted as a function of dose for 9 fields IMRT technique in Fig. 2. Our findings indicated that, in all of the IMRT techniques, 2 mm thickness artificial walls showed minimum differences with the true bladder and rectum walls based on absolute evaluations (maximum difference < 10cc and standard deviation < 15cc). In relative differences curves, it cannot be concluded that ABW2/ARW2 had minimum difference with bladder/rectum wall histogram. All of the artificial bladder wall DVHs had low differences with DWH (mean of differences < 5.5%, maximum difference < 11% and also maximum SD < 6%). In a similar way, all of the artificial rectum wall DVHs had low differences with DWH (mean of differences < 4%, maximum difference < 5% and also maximum SD < 5%), however, ARW6 and ARW8 DVH showed lower differences with DWH, in comparison with ARW 2 and 4 (mean of differences < 2.6% and 2.7% for ARW6 and 8 in comparison with 3.2% and 2.9% for ARW 2 and 4).

Fig.2

The means of differences between the DVHs of the whole organ and artificial walls with that of the dose wall histogram for the 9 fields IMRT technique averaged over all the patients. (a) Absolute and (b) relative differences for the bladder (c) absolute and (d) relative differences for the rectum.

Tables 2 and 3 represent the Spearman’s correlation between the DWH and the other dosimetric parameters. For bladder, the results showed that ABW2-10 were highly correlated with the DWH, with ABW2 showing the best agreement, approximately in all IMRT techniques, in absolute and percentage evaluation. In the other hand, for rectum, all of the artificial rectum walls had high correlations with DWH.

Table 2

Spearman’s correlation coefficients between the bladder DWH and the other dose histograms, calculated for the fraction of volume receiving more than 30, 50 and 60 Gy, in percentage (%) and absolute volume (cc), for various IMRT techniques

Absolute (cc)	V30				V50				V60
	9F	7F	5F	BAO	9F	7F	5F	BAO	9F	7F	5F	BAO
DVH	0.577	0.574	0.609	0.537	0.461	0.508	0.623	0.694	0.443	0.42	0.464	0.618
ABW2	0.783	0.791	0.795	0.752	0.685	0.686	0.732	0.774	0.603	0.622	0.568	0.723
ABW5	0.756	0.767	0.761	0.747	0.549	0.647	0.701	0.707	0.515	0.588	0.523	0.653
ABW7	0.768	0.777	0.771	0.744	0.588	0.668	0.734	0.740	0.551	0.522	0.562	0.685
ABW10	0.751	0.759	0.762	0.748	0.582	0.645	0.705	0.742	0.548	0.521	0.545	0.713
Relative (%)
DVH	0.838	0.973	0.969	0.975	0.496	0.816	0.786	0.868	0.153	0.713	0.682	0.767
ABW2	0.982	0.984	0.982	0.983	0.892	0.913	0.887	0.921	0.771	0.795	0.823	0.797
ABW5	0.983	0.982	0.981	0.983	0.878	0.896	0.876	0.905	0.727	0.784	0.769	0.752
ABW7	0.983	0.985	0.982	0.983	0.890	0.898	0.876	0.906	0.693	0.797	0.799	0.791
ABW10	0.982	0.984	0.982	0.963	0.871	0.878	0.852	0.889	0.712	0.770	0.730	0.794

DVH: dose volume histogram; ABWx: artificial bladder wall with uniform thickness of x mm; ARWx: artificial bladder wall with uniform thickness of x mm; 9F: 9 fields IMRT technique; 7F: 7 fields IMRT technique; 5F: 5 fields IMRT technique; BAO: beam angle optimization IMRT technique.

Table 3

Spearman’s correlation coefficients between the rectum DWH and the other dose histograms, calculated for the fraction of volume receiving more than 30, 50 and 60 Gy, in percentage (%) and absolute volume (cc), for various IMRT techniques

Absolute (cc)	V30				V50				V60
	9F	7F	5F	BAO	9F	7F	5F	BAO	9F	7F	5F	BAO
DVH	0.676	0.804	0.689	0.759	0.541	0.836	0.789	0.776	0.822	0.904	0.837	0.824
ARW2	0.652	0.726	0.581	0.662	0.815	0.835	0.809	0.765	0.824	0.861	0.848	0.831
ARW4	0.711	0.766	0.625	0.711	0.678	0.817	0.781	0.756	0.836	0.896	0.857	0.832
ARW6	0.738	0.780	0.652	0.739	0.794	0.831	0.785	0.766	0.839	0.893	0.864	0.842
ARW8	0.730	0.772	0642	0.730	0.784	0.827	0.774	0.760	0.832	0.907	0.856	0.838
Relative (%)
DVH	0.890	0.939	0.880	0.913	0.858	0.958	0.947	0.588	0.871	0.961	0.962	0.955
ARW2	0.692	0.736	0.569	0.767	0.962	0.954	0.948	0.955	0.958	0.964	0.957	0.950
ARW4	0.951	0.932	0.909	0.915	0.967	0.961	0.957	0.949	0.967	0.963	0.961	0.953
ARW6	0.896	0.932	0.895	0.910	0.968	0.959	0.954	0.945	0.971	0.962	0.965	0.953
ARW8	0.929	0.934	0.903	0.910	0.963	0.957	0.953	0.939	0.969	0.960	0.963	0.948
DVH	0.890	0.939	0.880	0.913	0.858	0.958	0.947	0.588	0.871	0.961	0.962	0.955

MC simulation on a VHP male data, showed that ABW2 had a lowest absolute and relative differences with a bladder DWH for 4 mentioned IMRT techniques (mean absolute differences < 7.5 cc and mean relative differences < 1.7%). Similarly, ARW2 had a lowest absolute and relative differences with a DWH of rectum wall for all techniques (mean absolute differences < 2.8cc and mean relative differences < 2%). In addition, all artificial bladder and rectum walls DVHs in all of the IMRT techniques, had low differences with the DWHs in relative evaluation (maximum of relative differences < 10 % and < 8% for bladder and rectum, respectively). Differences’ values between the values of the DWH and artificial bladder/rectum walls as a function of dose, that resulted from the MC simulations for 9 fields IMRT technique were showed in Fig. 3. Other differences curves from the MC simulations based on the other IMRT techniques could be find in the Appendix section.

Fig.3

The differences between the DVHs of the whole organ and artificial walls with that of the dose wall histogram for the 9 fields IMRT technique resulted from Monte Carlo simulations. (a): absolute and (b): relative differences for the bladder; (c): absolute and (d): relative differences for the rectum.

The absolute mean and SD values (averaged over all the patients) of selected dosimetric parameters resulted from various IMRT techniques calculated based on the DVH of real and artificial bladder and rectum walls, as well as the DVH of whole structures are presented in Table 4. The relevant values of such parameters resulted from MC simulations for the same IMRT techniques on a standard patient data are reported in Table 5. The results show that there were no statistically significant differences observed between different techniques for relative and absolute histograms.

Table 4

The mean and standard deviation (in parentheses) of V30, V50 and V60 values resulted from various IMRT techniques averaged over all the patients (All values are in cc)

	V30				V50				V60
	9F	7F	5F	BAO	9F	7F	5F	BAO	9F	7F	5F	BAO
Bladder	141.61 (44.48)	133.56 (39.57)	131.86 (39.17)	135.49 (40.41)	81.31 (26.01)	84.52 (27.44)	76.06 (21.61)	75.91 (24.30)	58.32 (21.48)	59.80 (20.38)	55.58 (18.26)	56.54 (21.30)
Bladder wall	27.67 (5.54)	26.69 (5.26)	26.51 (5.20)	26.73 (5.05)	14.83 (3.13)	15.84 (3.63)	15.18 (3.60)	14.85 (3.32)	11.45 (2.89)	11.95 (2.97)	11.53 (2.68)	11.75 (2.78)
ABW2	23.42 (4.63)	22.75 (4.54)	22.70 (4.57)	22.59 (3.95)	14.59 (3.50)	15.18 (3.48)	14.46 (3.07)	14.21 (2.23)	11.82 (3.16)	12.03 (2.97)	11.59 (2.74)	11.83 (3.27)
ABW5	57.54 (10.24)	55.83 (9.91)	55.51 (9.89)	55.55 (9.48)	35.48 (7.11)	37.14 (7.70)	35.34 (7.02)	34.60 (7.24)	28.18 (6.96)	28.81 (6.58)	27.82 (6.33)	28.20 (7.44)
ABW7	71.63 (13.37)	69.35 (12.74)	68.94 (12.66)	69.12 (12.17)	43.72 (8.67)	45.91 (9.53)	43.43 (8.70)	42.66 (8.92)	34.40 (8.45)	35.20 (7.97)	34.02 (7.69)	34.44 (8.97)
ABW10	92.04 (21.75)	88.75 (12.74)	88.13 (20.20)	88.83 (20.21)	55.96 (12.75)	58.86 (14.16)	55.53 (12.23)	54.32 (13.01)	43.48 (11.81)	44.44 (11.10)	42.79 (10.70)	43.10 (12.45)
Rectum	44.77 (19.52)	44.59 (20.58)	45.89 (18.83)	48.95 (20.91)	26.30 (13.89)	24.79 (11.77)	24.90 (11.13)	23.51 (10.65)	15.83 (7.77)	16.58 (8.94)	16.76 (8.73)	15.84 (7.93)
Rectum wall	19.74 (8.55)	19.43 (8.94)	19.67 (7.76)	20.94 (9.47)	9.46 (4.25)	9.66 (4.44)	9.59 (4.12)	9.32 (4.02)	6.65 (3.29)	6.34 (3.42)	6.72 (3.58)	6.89 (3.34)
ARW2	13.76 (2.93)	13.56 (3.29)	13.74 (2.67)	14.65 (3.51)	7.30 (2.71)	7.42 (2.85)	7.33 (2.66)	7.16 (2.36)	5.13 (2.03)	5.10 (2.36)	5.18 (2.29)	5.32 (2.06)
ARW4	26.67 (7.06)	26.24 (7.85)	26.66 (6.26)	28.46 (8.16)	13.60 (5.24)	14.29 (5.81)	14.25 (5.36)	13.81 (4.94)	9.81 (4.21)	9.91 (4.83)	10.04 (4.71)	10.09 (4.26)
ARW6	35.28 (10.59)	34.50 (11.73)	35.18 (9.68)	37.62 (11.92)	18.57 (7.36)	18.93 (8.06)	18.91 (7.55)	18.18 (7.06)	12.80 (5.77)	12.88 (6.51)	13.25 (6.44)	13.03 (5.85)
ARW8	40.66 (13.21)	39.62 (14.53)	40.47 (12.12)	43.46 (14.63)	21.48 (8.78)	21.96 (9.56)	21.97 (8.97)	20.99 (8.50)	14.56 (6.80)	15.17 (7.78)	15.32 (7.59)	14.69 (6.90)

Table 5

The V30, V50 and V60 values resulted from various IMRT techniques using MC simulations on a standard patient data [24]. (All values are in cc)

	V30				V50				V60
	9F	7F	5F	BAO	9F	7F	5F	BAO	9F	7F	5F	BAO
Bladder	65.96	62.12	63.17	52.21	35.23	34.54	33.80	32.20	24.40	25.63	24.99	25.51
Bladder wall	25.87	24.75	25.83	20.43	16.54	15.64	15.79	14.22	12.68	12.62	12.77	12.63
ABW2	18.09	16.90	17.55	13.63	10.61	10.02	10.30	9.19	7.90	7.82	7.92	7.92
ABW5	38.40	35.91	36.93	29.34	21.71	20.48	20.96	18.78	15.75	15.76	15.93	15.98
ABW7	48.45	45.34	46.46	37.13	26.86	25.45	25.91	23.40	19.42	19.49	19.71	19.75
ABW10	59.82	56.09	57.21	26.54	32.71	31.42	31.42	28.96	23.23	23.87	23.72	23.77
Rectum	29.32	29.26	29.91	34.48	13.56	13.22	14.59	14.63	8.08	6.83	7.67	9.81
Rectum wall	15.44	16.19	15.55	16.38	7.74	8.07	8.55	7.99	5.17	4.72	4.96	5.86
ARW2	11.98	12.69	12.21	12.82	5.98	6.20	6.52	6.23	4.03	3.71	3.82	4.75
ARW4	20.05	20.76	20.82	22.48	9.68	9.82	10.54	10.36	6.35	5.64	5.99	7.31
ARW6	25.03	25.34	25.95	29.09	11.77	11.72	12.77	12.58	7.49	6.51	7.06	8.77
ARW8	28.00	28.00	28.70	33.15	13.01	12.84	14.07	13.98	7.98	6.80	7.57	9.58

4 Discussion

The effect of delineation method on bladder wall, observer variability, and contouring time on CT images was quantified in a study by Rosewall et al [10]. They reported that manual organ wall delineation was considerably time consuming and subjected to a broad inter observer variability. It appears that isotropic contractions from the external surface can provide a quicker, more reproducible and reasonably accurate substitute for the manual bladder and rectum walls delineation.

The correlation and differences between the rectum/bladder real walls and defined artificial walls histograms in prostate cancer patients undergoing IMRT were evaluated in this study using TPS calculations made on patient data and also MC simulations made on the data of a standard patient (VHP). We found artificial walls with appropriate thickness mimicking the real bladder and rectum walls in common prostate IMRT techniques. High correlations between the DWH and ABW2 of bladder were found in all the IMRT techniques with both of the TPS calculations and the MC simulations. Our findings were consistent with previous studies [11, 17], where DSH and ABW 5–10 had high correlations with DWH of bladder based on both absolute and relative histograms. In addition, DSH had the lowest absolute difference with DWH.

We evaluated the absolute differences and correlation values, because these parameters have higher predictive values compared to the relative ones as indicated in previous studies [18, 30]. Some of the previous studies [11 , 31], have investigated the correlation between the DSHs and DWHs of bladder and rectum. We introduced the ABW2 and ARW2, as artificial walls of bladder and rectum with the thickness of 2mm. Comparing the size of dosimetric voxels in the TPS (that were equal to 2mm in our study) and the DSH definition in previous studies [31, 32] (that is briefly an artificial wall with the thickness of one voxel constructed from the external surface using uniform contraction), we can conclude that the definition of ABW2 and ARW2 are almost the same as that of the DSHs of bladder and rectum respectively.

Finding the superiority of one IMRT technique or comparing different techniques was not the aim of this study, because it was evaluated in previous studies [33, 34]. However, it was interesting that the correlation values were approximately similar for all techniques with no significant differences. The results showed that ABW5,7 and 10 were as appropriate as ABW2 and had high correlation values with DWH. However, the absolute difference is an important parameter with ABW2 having the minimum value. Therefore, it is recommended to use ABW2 instead of the bladder DWH with absolute values, and all ABW2-10 instead of DWH with relative values. MC simulations confirmed these findings for bladder. Furthermore, previous studies [11, 17], had similar results by investigating other alternative parameters for bladder wall in IMRT or 3D conformal prostate radiotherapy.

TPS calculations and MC simulations indicated that ARW2 was the best alternative for the rectum DWH in absolute values. Nevertheless, regarding the relative values, the ABW6 showed better results based on the MC simulations for all of the IMRT techniques. The agreement between the Monte Carlo simulations and TPS calculations specially for rectum wall enables us to use the ARW2 as an alternative mimicking the real rectum wall even with TPS calculations. It was reported that the convolution and analytical algorithms which used in commercial TPSs calculate the rectum/rectum wall doses with some errors (2–10%) due to inhomogeneity existing in rectum [23]. The absolute findings from MC simulation were consistent with the TPS calculations in cancer patients. However, in a point of view of relative differences, ARW2 showed the lowest differences with DWH and it differs a little from patients TPS calculation findings. In summary, 4 various IMRT techniques were investigated in our study. Other IMRT techniques or other radiation therapy methods like volumetric modulated arc therapy or Tomotherapy could also be a subject of similar future studies.

5 Conclusion

In general, TPS calculations and MC simulations results were in a very good agreement. Artificial walls with 2 mm thickness had the lowest absolute difference with DWH of bladder and rectum. These can be appropriate alternatives for the bladder and rectum true walls for absolute dose evaluations. Nevertheless, for relative dose evaluations other artificial defined walls are as appropriate as the DSH in mimicking the bladder and rectum DWHs for all the IMRT techniques investigated in our study.

Funding

No funding was received for this study.

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Footnotes

Acknowledgments

The patients’ imaging and their radiotherapy planning procedures were carried out at the Radiotherapy and Oncology Department of Shohaday-e-Tajrish Hospital, Tehran, Iran. Therefore, the authors express their sincere appreciation to the above institutes for their financial help and technical assistance.

References

Cooperberg

M.R.

, Chan

J.M.

, Epidemiology of prostate cancer, Springer Press, New York, 2017.

Kupelian

P.A.

, Potters

, Khuntia

, et al., Radical prostatectomy, external beam radiotherapy <72 Gy, external beam radiotherapy ≥72 Gy, permanent seed implantation, or combined seeds/external beam radiotherapy for stage T1–T2 prostate cancer, Int J Radiat Oncol Biol Phys 58 (2004), 25–33.

Khan

F.M.

, Gerbi

B.J.

, Treatment planning in radiation oncology, Wolters Kluwer Health/Lippincott Williams & Wilkins, Philadelphia, 2012.

Doležel

, Odrazka

Vaculikova

, et al., Dose escalation in prostate radiotherapy up to 82 Gy using simultaneous integrated boost, Strahlenther Onkol 186 (2010), 197–202.

Sharma

N.K.

, Li

, Chen

D.Y.

, et al., Intensity-modulated radiotherapy reduces gastrointestinal toxicity in patients treated with androgen deprivation therapy for prostate cancer, Int J Radiat Oncol Biol Phys 80 (2011), 437–444.

Bekelman

J.E.

, Mitra

, Efstathiou

, et al., Outcomes after intensity-modulated versus conformal radiotherapy in older men with nonmetastatic prostate cancer, Int J Radiat Oncol Biol Phys 81 (2011), e325–e334.

Chen

C.Y.

, Lee

L.M.

, Yu

H.W.

, et al., Dosimetric and radiobiological comparison of Cyberknife and Tomotherapy in stereotactic body radiotherapy for localized prostate cancer, J Xray Sci Technol 25 (2017), 465–477.

Lebesque

J.V.

, Bruce

A.M.

, Kroes

A.G.

, et al., Variation in volumes, dose-volume histograms, and estimated normal tissue complication probabilities of rectum and bladder during conformal radiotherapy of T3 prostate cancer, Int J Radiat Oncol Biol Phys 33 (1995), 1109–1119.

Pinkawa

, Asadpour

, Gagel

, et al., Prostate position variability and dose–volume histograms in radiotherapy for prostate cancer with full and empty bladder, Int J Radiat Oncol Biol Phys 64 (2006), 856–861.

10.

Rosewall

, Bayley

A.J.

, Chung

, et al., The effect of delineation method and observer variability on bladder dose-volume histograms for prostate intensity modulated radiotherapy, Radiother Oncol 101 (2011), 479–485.

11.

Maggio

, Carillo

, Cozzarini

, et al., 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, Phys Med Biol 58 (2013), N115.

12.

Fisher

M.R.

, Hricak

, Tanagho

E.A.

, Urinary bladder MR imaging. Part II, Neoplasm Radiology 157 (1985), 471–477.

13.

Ren

, Huan

, Li

, et al., Combined T2-weighted and diffusion-weighted MRI for diagnosis of urinary bladder invasion in patients with prostate carcinoma, J Magn Reson Imaging Off J Int Soc Magn Reson Med 30 (2009), 351–356.

14.

Fiorino

, Valdagni

, Rancati

, et al., Dose–volume effects for normal tissues in external radiotherapy: pelvis, Radiother Oncol 93 (2009), 153–167.

15.

Rancati

, Fiorino

, Fellin

, et al., Inclusion of clinical risk factors into NTCP modelling of late rectal toxicity after high dose radiotherapy for prostate cancer, Radiother Oncol 100 (2011), 124–130.

16.

Michalski

J.M.

, Gay

, Jackson

, et al., Radiation dose–volume effects in radiation-induced rectal injury, Int J Radiat Oncol Biol Phys 76 (2010), S123–S129.

17.

Carillo

, Cozzarini

, Chietera

, et al., Correlation between surrogates of bladder dosimetry and dose–volume histograms of the bladder wall defined on MRI in prostate cancer radiotherapy, Radiother Oncol 105 (2012), 180–183.

18.

Hoogeman

M.S.

, Peeters

S.T.

, de Bois

, et al., Absolute and relative dose–surface and dose–volume histograms of the bladder: which one is the most representative for the actual treatment? Phys Med Biol 50 (2005), 3589.

19.

Cheung

M.R.

, Tucker

S.L.

, Dong

, et al., Investigation of bladder dose and volume factors influencing late urinary toxicity after external beam radiotherapy for prostate cancer, Int J Radiat Oncol Biol Phys 67 (2007), 1059–1065.

20.

Carillo

, Cozzarini

, Rancati

, et al., Relationships between bladder dose–volume/surface histograms and acute urinary toxicity after radiotherapy for prostate cancer, Radiother Oncol 111 (2014), 100–105.

21.

Scaife

J.E.

, Thomas

S.J.

, Harrison

, et al., Accumulated dose to the rectum, measured using dose–volume histograms and dose-surface maps, is different from planned dose in all patients treated with radiotherapy for prostate cancer, Br J Radiol 88 (2015), 20150243.

22.

Wachter-Gerstner

, Wachter

, Reinstadler

, et al., Bladder and rectum dose defined from MRI based treatment planning for cervix cancer brachytherapy: comon of dose–volume histograms for organ contours and organ wall, comparison with ICRU rectum and bladder reference point, Radiother Oncol 68 (2003), 269–276.

23.

Song

J.S.

, Court

L.E.

, Cormack

R.A.

, Monte Carlo calculation of rectal dose when using an intrarectal balloon during prostate radiation therapy, Med Dosim 32 (2007), 151–156.

24.

Ackerman

M.J.

, The visible human project, Proc IEEE 86 (1998), 504–511.

25.

Pollack

, Walker

, Horwitz

E.M.

, et al., Randomized trial of hypofractionated external-beam radiotherapy for prostate cancer, J Clin Oncol 31 (2013), 3860–3868.

26.

Rodriguez

, Sempau

, Brualla

, PRIMO: A graphical environment for the Monte Carlo simulation of Varian and Elekta linacs, Strahlenther Onkol 189 (2013), 881–886.

27.

Esposito

, Silva

, Oliveira

, et al., Primo software as a tool for Monte Carlo simulations of intensity modulated radiotherapy: a feasibility study, Radiat Oncol 13 (2018), 91.

28.

Emami

, Lyman

, Brown

, et al., Tolerance of normal tissue to therapeutic irradiation, Int J Radiat Oncol Biol Phys 21 (1991), 109–122.

29.

Milano

M.T.

, Constine

L.S.

, Okunieff

, Normal tissue tolerance dose metrics for radiation therapy of major organs, Seminars in Radiation Oncology 17 (2007), 131–140.

30.

Peeters

S.T.

, Hoogeman

M.S.

, Heemsbergen

W.D.

, et al., Volume and hormonal effects for acute side effects of rectum and bladder during conformal radiotherapy for prostate cancer, Int J Radiat Oncol Biol Phys 63 (2005), 1142–1152.

31.

, Li

, Spelbring

, et al., Dose-surface histograms as treatment planning tool for prostate conformal therapy, Med Phys 22 (1995), 279–284.

32.

, Boyer

, Lu

, et al., Analysis of the dose-surface histogram and dose-wall histogram for the rectum and bladder, Med Phys 24 (1997), 1107–1116.

33.

Wang

S.C.

, Wang

, He

Y.B.

, et al., A dosimetric comparison of the fixed-beam IMRT plans using different leaf width of multileaf collimators for the intermediate risk prostate cancer, Radiat Phys Chem 127 (2016), 210–221.

34.

Shawata

A.S.

, Akl

M.F.

, Elshahat

K.M.

, et al., Evaluation of different planning methods of 3DCRT, IMRT, and RapidArc for localized prostate cancer patients: planning and dosimetric study, Egypt J Radiol Nucl Med 50 (2019), s43055-019-0021-z.