A systematic evaluation of air cavity dose perturbation in megavoltage x-ray beams

Dosimetric Evaluation of Commercially Available Flat vs. Self-Produced 3D-Conformal Silicone Boluses for the Head and Neck Region

Background

Boluses are routinely used in radiotherapy to modify surface doses. Nevertheless, considerable dose discrepancies may occur in some cases due to fit inaccuracy of commercially available standard flat boluses. Moreover, due to the simple geometric design of conventional boluses, also surrounding healthy skin areas may be unintentionally covered, resulting in the unwanted dose buildup. With the fused deposition modeling (FDM) technique, there is a simple and possibly cost-effective way to solve these problems in routine clinical practice. This paper presents a procedure of self-manufacturing bespoke patient-specific silicone boluses and the evaluation of buildup and fit accuracy in comparison to standard rectangular commercially available silicone boluses.

Methods

3D-conformal silicone boluses were custom-built to cover the surgical scar region of 25 patients who received adjuvant radiotherapy of head and neck cancer at the University Hospital Würzburg. During a standard CT-based planning procedure, a 5-mm-thick 3D bolus contour was generated to cover the radiopaque marked surgical scar with an additional safety margin. From these digital contours, molds were 3D printed and poured with silicone. Dose measurements for both types of boluses were performed with radiochromic films (EBT3) at three points per patient—at least one aimed to be in the high-dose area (scar) and one in the lower-dose area (spared healthy skin). Surface–bolus distance, which ideally should not be present, was determined from cone-beam CT performed for positioning control. The dosimetric influence of surface–bolus distance was also determined on slab phantom for different field sizes. The trial was performed with hardware that may be routinely available in every radiotherapy department, with the exception of the 3D printer. The required number of patients was determined based on the results of preparatory measurements with the help of the statistical consultancy of the University of Würzburg. The number of measuring points represents the total number of patients.

Results

In the high-dose area of the scar, there was a significantly better intended dose buildup of 2.45% (95%CI 0.0014–0.0477, p = 0.038, N = 30) in favor of a 3D-conformal bolus. Median distances between the body surface and bolus differed significantly between 3D-conformal and commercially available boluses (3.5 vs. 7.9 mm, p = 0.001). The surface dose at the slab phantom did not differ between commercially available and 3D-conformal boluses. Increasing the surface–bolus distance from 5 to 10 mm decreased the surface dose by approximately 2% and 11% in the 6 × 6- and 3 × 3-cm² fields, respectively. In comparison to the commercially available bolus, an unintended dose buildup in the healthy skin areas was reduced by 25.9% (95%CI 19.5–32.3, p < 0.01, N = 37) using the 3D-conformal bolus limited to the region surrounding the surgical scar.

Conclusions

Using 3D-conformal boluses allows a comparison to the commercially available boluses’ dose buildup in the covered areas. Smaller field size is prone to a larger surface–bolus distance effect. Higher conformity of 3D-conformal boluses reduces this effect. This may be especially relevant for volumetric modulated arc therapy (VMAT) and intensity-modulated radiotherapy (IMRT) techniques with a huge number of smaller fields. High conformity of 3D-conformal boluses reduces an unintended dose buildup in healthy skin. The limiting factor in the conformity of 3D-conformal boluses in our setting was the immobilization mask, which was produced primarily for the 3D boluses. The mask itself limited tight contact of subsequently produced 3D-conformal boluses to the mask-covered body areas. In this respect, bolus adjustment before mask fabrication will be done in the future setting.

Accuracy Evaluation of Collapsed Cone Convolution Superposition Algorithms for the Nasopharynx Interface in the Early Stage of Nasopharyngeal Carcinoma

This study combined the use of radiation dosimeteric measurements and a custom-made anthropomorphic phantom in order to evaluate the accuracy of therapeutic dose calculations at the nasopharyngeal air-tissue interface. The doses at the nasopharyngeal air-tissue interface obtained utilizing the Pinnacle and TomoTherapy TPS, which are based on collapsed cone convolution superposition (CCCS) algorithms, were evaluated and measured under single $10\times 10\text{c}{\text{m}}^2$ , $2\times 2\text{c}{\text{m}}^2$ , two parallel opposed $2\times 2\text{c}{\text{m}}^2$ and clinical fields for early stage of nasopharyngeal carcinoma by using EBT3, GR-200F, and TLD 100. At the air-tissue interface under a $10\times 10\text{c}{\text{m}}^2$ field, the TPS dose calculation values were in good agreement with the dosimeter measurement with all differences within 3.5%. When measured the single field $2\times 2\text{c}{\text{m}}^2$ , the differences between the average dose were measured at the distal interface for EBT3, GR-200F, and TLD-100 and the calculation values were -15.8%, -16.4%, and -4.9%, respectively. When using the clinical techniques such as IMRT, VMAT, and tomotherapy, the measurement results at the interface for all three techniques did not imply under dose. Small-field sizes will lead to dose overestimation at the nasopharyngeal air-tissue interface due to electronic disequilibrium when using CCCS algorithms. However, under clinical applications of multiangle irradiation, the dose errors caused by this effect were not significant.

Dosimetric effect of intensity-modulated radiation therapy for postoperative non-small cell lung cancer with and without air cavity in the planning target volume

To evaluate the dosimetric effect of intensity-modulated radiation therapy (IMRT) for postoperative non-small cell lung cancer (NSCLC), with and without the air cavity in the planning target volume (PTV). Two kinds of IMRT plans were made for 21 postoperative NSCLC patients. In Plan-0: PTV included the tracheal air cavity, and in Plan-1: the air cavity was subtracted from the PTV. For PTV, the dosimetric parameters, including D_mean, D₉₈, D₉₅, D₂, D_0.2, conformity index (CI), and homogeneity index (HI) were evaluated. For organs at risk (OARs), the evaluation indexes, included the V₅, V₂₀ and the mean lung dose (MLD) of total lung, the V₃₀, V₄₀, and the mean heart dose (MHD) of heart, the spinal cord D_max, and the V₃₅ and the mean esophageal dose (MED) of esophagus. The number of segments and MUs were also recorded. Additionally, the correlation between the Plan-1 dosimetric change value relative to Plan-0, the size of air cavity, and the volume proportion of the cavity in the PTV was also analyzed. The D_mean of PTV, D₂, D_0.2, HI and CI in Plan-1 decreased compared with those in Plan-0. For OARs, the V₃₀, MHD, and MED also decreased. The CI change value of Plan-1 relative to Plan-0 had a significantly negative correlation with the size and the volume proportion of air cavity, and the MED change value also had a significantly negative correlation with the air cavity size. The IMRT plans for patients with postoperative NSCLC can achieve a better target dose distribution and offer an improved sparing of the heart and esophagus by removing the PTV air cavity, while reducing the target conformity. The change value of CI and MED had a significantly negative correlation with the air cavity size.

Evaluation of camera settings for photogrammetric reconstruction of humanoid phantoms for EBRT bolus and HDR surface brachytherapy applications

The fabrication of brachytherapy surface moulds is considered laborious and time consuming that often result in repeated attempts due to incorrect catheter positioning or the presence of air gaps. 3-dimensional printing using low-cost and reliable materials has allowed the rapid creation of patient-specific surface mould applicators to be achieved using patient imaging data obtained via CT scan. In this study we investigate whether an alternative approach using photogrammetry techniques can improve this process and how camera settings and object texture affect the reconstructions. Two humanoid phantoms, an anthropomorphic RANDO phantom and a Laerdal Little Anne CPR training manikin were used in this study. Both were imaged using a Nikon D5600 DSLR and Nokia 3.1 smartphone camera and reconstructed using Agisoft Metashape software. CT scans of both phantoms were taken as references for comparing the photogrammetry reconstructions. Models were reconstructed from different photo sets and assessed by distance to agreement with the CT models. Both phantoms were effectively reconstructed for most experiments. Increasing the number of photos used produced the better reconstructions while in general, reconstructions using video data were poor. The two phantoms were reconstructed at a similar quality. Background light that caused undesirable reflections significantly reduced reconstruction quality. Applying a non-reflective tape to the affected regions provided a suitable method for reducing their effects. Photogrammetry techniques were effectively able to reconstruct 3-dimensional models of both phantom. The camera settings and lighting did have a profound effect on the reconstruction quality and should be chosen appropriately depending on the scene.

Investigating the Effect of Air Cavities of Sinuses on the Radiotherapy Dose Distribution Using Monte Carlo Method

BACKGROUND

Considering that some vital organs exist in the head and neck region, the treatment of tumors in this area is a crucial task. The existence of air cavities, namely sinuses, disrupt the radiotherapy dose distribution. The study aims to analyze the effect of maxillary, frontal, ethmoid and sphenoid sinuses on radiotherapy dose distribution by Monte Carlo method.

MATERIAL AND METHODS

In order to analyze the effect of the cavities on dose distribution, the maxillary, frontal, ethmoid and sphenoid sinus cavities were simulated with (3×3.2×2) cm³, (2×2×3.2) cm³, (1×1×1.2) cm³ and (1×1×2) cm³ dimensions.

RESULTS

In the analysis of the dose distribution caused by cavities, some parameters were observed, including: inhomogeneity of dose distribution in the cavities, inhomogeneity of dose on the edges of the air cavities and dispersion of the radiations after the air cavity. The amount of the dose in various situations showed differences: before the cavity a 0.64% and a 2.76% decrease, a 12.06% and a 17.17% decrease in the air zone, and a 2.25% and a 5.9% increase after the cavity.

CONCLUSION

The results indicate that a drop in dose before the air cavities and in the air zone occurs due to the lack of scattered radiation. Furthermore, the rise in dose was due to the passage of more radiation from the air cavity and dose deposition after the air cavity. The changes in dose distribution are dependent on the cavity size and depth. As a result, this has to be noted in the treatment planning and MU calculations of the patient.

Characterisation of small photon field outputs in a heterogeneous medium using X-ray voxel Monte Carlo dose calculation algorithm

Aim

To characterise small photon beams using the Monte Carlo dose calculation algorithm for small field ranges in a heterogeneous medium.

Materials and method

An in-house phantom constructed with three different mediums, foam, polymethyl methacrylate and delrin resembling the densities of lung, soft tissue and bone respectively, was used in this study. Photon beam energies of 6 and 15 MV and field sizes of 8×8, 16×16, 24×24, 32×32 and 40×40 mm using X-ray voxel Monte Carlo (XVMC) algorithm using different detectors were validated. The relative output factor was measured in three different mediums having six different tissue interfaces; at the depth of 0, 1, 2 and 3 cm. The planar dose verification was undertaken using gafchromic films and considered dose at the lung and bone medium interfaces. For all the measurements, 104×104 mm was taken as the reference field size. The relative output factor for all other field sizes was taken and compared with planning system calculated values.

Results

From field size 16×16 mm and above, the relative output factors were analysed in bone and soft tissue medium having lung as first medium. The maximum deviations were observed as 1·8 and 1·3% for 6 MV and 2·5 and 1·1% for 15 MV photon beams for bone and soft tissue, respectively. For lung as measurement medium, the maximum deviation of 14·8 and 19·2% were observed and having bone as first medium with 8×8 mm for 6 and 15 MV photon beams, respectively. The fluence verification of dose spectrum for the lung–bone interface scenarios with smaller field sizes were found within 2% of deviation with treatment planning system (TPS).

Conclusion

The accuracy of dose calculations for small field sizes in XVMC-based treatment planning algorithm was studied in different inhomogeneous mediums. It was found that the results correlated with measurement data for field size 16×16 mm and above. Noticeable deviation was observed for the smallest field size of 8×8 mm with interfaces of significant change in density. The observed results demands further analysis of work with smaller field sizes.

Clinical applications of 3-dimensional printing in radiation therapy

Three-dimensional (3D) printing is suitable for the fabrication of complex radiotherapy bolus. Although investigated from dosimetric and feasibility standpoints, there are few reports to date of its use for actual patient treatment. This study illustrates the versatile applications of 3D printing in clinical radiation oncology through a selection of patient cases, namely, to create bolus for photon and modulated electron radiotherapy (MERT), as well as applicators for surface high-dose rate (HDR) brachytherapy. Photon boluses were 3D-printed to treat a recurrent squamous cell carcinoma (SCC) of the nasal septum and a basal cell carcinoma (BCC) of the posterior pinna. For a patient with a mycosis fungoides involving the upper face, a 3D-printed MERT bolus was used. To treat an SCC of the nose, a 3D-printed applicator for surface brachytherapy was made. The structures' fit to the anatomy and the radiotherapy treatment plans were assessed. Based on the treatment planning computed tomography (CT), the size of the largest air gap at the interface of the 3D-printed structure was 3 mm for the SCC of the nasal septum, 3 mm for the BCC of the pinna, 2 mm for the mycosis fungoides of the face, and 2 mm for the SCC of the nose. Acceptable treatment plans were obtained for the SCC of the nasal septum (95% isodose to 99.8% of planning target volume [PTV]), the BCC of the pinna (95% isodose to 97.7% of PTV), and the mycosis fungoides of the face (90% isodose to 92.5% of PTV). For the latter, compared with a plan with a uniform thickness bolus, the one featuring the MERT bolus achieved relative sparing of all the organs at risk (OARs) distal to the target volume, while maintaining similar target volume coverage. The surface brachytherapy plan for the SCC of the nose had adequate coverage (95% isodose to 95.6% of clinical target volume [CTV]), but a relatively high dose to the left eye, owing to its proximity to the tumor. 3D printing can be implemented effectively in the clinical setting to create highly conformal bolus for photon and MERT, as well as applicators for surface brachytherapy.

Post-Prostatectomy Image-Guided Radiotherapy: The Invisible Target Concept

In the era of intensity-modulated radiation therapy, image-guided radiotherapy (IGRT) appears crucial to control dose delivery and to promote dose escalation while allowing healthy tissue sparing. The place of IGRT following radical prostatectomy is poorly described in the literature. This review aims to highlight some key points on the different IGRT techniques applicable to prostatic bed radiotherapy. Furthermore, methods used to evaluate target motion and to reduce planning target volume margins will also be explored.

Correlation between tumor regression grade and rectal volume in neoadjuvant concurrent chemoradiotherapy for rectal cancer

PURPOSE

To determine whether large rectal volume on planning computed tomography (CT) results in lower tumor regression grade (TRG) after neoadjuvant concurrent chemoradiotherapy (CCRT) in rectal cancer patients.

MATERIALS AND METHODS

We reviewed medical records of 113 patients treated with surgery following neoadjuvant CCRT for rectal cancer between January and December 2012. Rectal volume was contoured on axial images in which gross tumor volume was included. Average axial rectal area (ARA) was defined as rectal volume divided by longitudinal tumor length. The impact of rectal volume and ARA on TRG was assessed.

RESULTS

Average rectal volume and ARA were 11.3 mL and 2.9 cm². After completion of neoadjuvant CCRT in 113 patients, pathologic results revealed total regression (TRG 4) in 28 patients (25%), good regression (TRG 3) in 25 patients (22%), moderate regression (TRG 2) in 34 patients (30%), minor regression (TRG 1) in 24 patients (21%), and no regression (TRG0) in 2 patients (2%). No difference of rectal volume and ARA was found between each TRG groups. Linear correlation existed between rectal volume and TRG (p = 0.036) but not between ARA and TRG (p = 0.058).

CONCLUSION

Rectal volume on planning CT has no significance on TRG in patients receiving neoadjuvant CCRT for rectal cancer. These results indicate that maintaining minimal rectal volume before each treatment may not be necessary.

How air influences radiation dose deposition in multiwell culture plates: a Monte Carlo simulation of radiation geometry

Radiation of experimental culture cells on plates with various wells can cause a risk of underdosage as a result of the existence of multiple air-water interfaces. The objective of our study was to quantify this error in culture plates with multiple wells. Radiation conditions were simulated with the GAMOS code, based on the GEANT4 code, and this was compared with a simulation performed with PENELOPE and measured data. We observed a slight underdosage of ∼ 4% on the most superficial half of the culture medium. We believe that this underdosage does not have a significant effect on the dose received by culture cells deposited in a monolayer and adhered to the base of the wells.

Dosimetric effect by shallow air cavities in high energy electron beams

This study evaluates the dosimetric impact caused by an air cavity located at 2 mm depth from the top surface in a PMMA phantom irradiated by electron beams produced by a Siemens Primus linear accelerator. A systematic evaluation of the effect related to the cavity area and thickness as well as to the electron beam energy was performed by using Monte Carlo simulations (EGSnrc code), Pencil Beam algorithm and Gafchromic EBT2 films. A home-PMMA phantom with the same geometry as the simulated one was specifically constructed for the measurements. Our results indicate that the presence of the cavity causes an increase (up to 70%) of the dose maximum value as well as a shift forward of the position of the depth-dose curve, compared to the homogeneous one. Pronounced dose discontinuities in the regions close to the lateral cavity edges are observed. The shape and magnitude of these discontinuities change with the dimension of the cavity. It is also found that the cavity effect is more pronounced (6%) for the 12 MeV electron beam and the presence of cavities with large thickness and small area introduces more significant variations (up to 70%) on the depth-dose curves. Overall, the Gafchromic EBT2 film measurements were found in agreement within 3% with Monte Carlo calculations and predict well the fine details of the dosimetric change near the cavity interface. The Pencil Beam calculations underestimate the dose up to 40% compared to Monte Carlo simulations; in particular for the largest cavity thickness (2.8 cm).

Acute gastrointestinal and genitourinary toxicity of image-guided intensity modulated radiation therapy for prostate cancer using a daily water-filled endorectal balloon

Background

Our purpose was to report acute gastrointestinal (GI) and genitourinary (GU) toxicity rates for prostate cancer patients undergoing image-guided intensity modulated radiation therapy (IG-IMRT) with a daily endorectal water-filled balloon (ERB_H2O), and assess associations with planning parameters and pretreatment clinical characteristics.

Methods

The first 100 patients undergoing prostate and proximal seminal vesicle IG-IMRT with indexed-lumen 100 cc ERB_H2O to 79.2 Gy in 1.8 Gy fractions at our institution from 12/2008- 12/2010 were assessed. Pretreatment characteristics, organ-at-risk dose volume histograms, and maximum GU and GI toxicities (CTCAE 3.0) were evaluated. Logistic regression models evaluated univariate association between toxicities and dosimetric parameters, and uni- and multivariate association between toxicities and pretreatment characteristics.

Results

Mean age was 68 (range 51–88). Thirty-two, 49, and 19 patients were low, intermediate, and high-risk, respectively; 40 received concurrent androgen deprivation. No grade 3 or greater toxicities were recorded. Maximum GI toxicity was grade 0, 1, and 2 in 69%, 23%, and 8%, respectively. Infield (defined as 1 cm above/below the CTV) rectal mean/median doses, D75, V30, and V40 and hemorrhoid history were associated with grade 2 GI toxicity (Ps < 0.05). Maximum acute GU toxicity was grade 0, 1, and 2 for 17%, 41%, and 42% of patients, respectively. Infield bladder V20 (P = 0.03) and pretreatment International Prostate Symptom Scale (IPSS) (P = 0.003) were associated with grade 2 GU toxicity.

Conclusion

Prostate IG-IMRT using a daily ERB_H2O shows low rates of acute GI toxicity compared to previous reports of air-filled ERB IMRT when using stringent infield rectum constraints and comparable GU toxicities.

Dosimetry of interface region near closed air cavities for Co-60, 6 MV and 15 MV photon beams using Monte Carlo simulations

Underdosing of treatment targets can occur in radiation therapy due to electronic disequilibrium around air-tissue interfaces when tumors are situated near natural air cavities. These effects have been shown to increase with the beam energy and decrease with the field size. Intensity modulated radiation therapy (IMRT) and tomotherapy techniques employ combinations of multiple small radiation beamlets of varying intensities to deliver highly conformal radiation therapy. The use of small beamlets in these techniques may therefore result in underdosing of treatment target in the air-tissue interfaces region surrounding an air cavity. This work was undertaken to investigate dose reductions near the air-water interfaces of 1×1×1 and 3×3×3 cm³ air cavities, typically encountered in the treatment of head and neck cancer utilizing radiation therapy techniques such as IMRT and tomotherapy using small fields of Co-60, 6 MV and 15 MV photons. Additional investigations were performed for larger photon field sizes encompassing the entire air-cavity, such as encountered in conventional three dimensional conformal radiation therapy (3DCRT) techniques. The EGSnrc/DOSXYZnrc Monte Carlo code was used to calculate the dose reductions (in water) in air-water interface region for single, parallel opposed and four field irradiations with 2×2 cm² (beamlet), 10×2 cm² (fan beam), 5×5 and 7×7 cm² field sizes. The magnitude of dose reduction in water near air-water interface increases with photon energy; decreases with distance from the interface as well as decreases as the number of beams are increased. No dose reductions were observed for large field sizes encompassing the air cavities. The results demonstrate that Co-60 beams may provide significantly smaller interface dose reductions than 6 MV and 15 MV irradiations for small field irradiations such as used in IMRT and tomotherapy.

Endo-rectal balloon cavity dosimetry in a phantom: performance under IMRT and helical tomotherapy beams

BACKGROUND AND PURPOSE

The use of endo-rectal balloons as immobilisation devices in external beam radiotherapy for prostate cancer has led to improved target position reproducibility and a decrease in rectal toxicity. The air cavity created by an endo-rectal balloon in photon radiotherapy perturbs the dose distribution. In this study, the effect of the balloon cavity on the dose distribution and the accuracy to which two treatment planning systems calculate the dose distribution were investigated.

MATERIALS AND METHODS

Single beams as well as 3D conformal, conventional IMRT and helical tomotherapy treatment plans were investigated using a specifically constructed phantom. Radiochromic film was used to measure the cavity wall doses and cavity wall DVHs.

RESULTS

For a 70 Gy prescription dose both the Pinnacle and TomoTherapy TPSs over-predicted the anterior cavity wall dose by 1.43 Gy, 3.92 Gy and 2.67 Gy for 3D conformal, conventional IMRT and helical tomotherapy, respectively. The posterior cavity wall dose was under-predicted by 2.62 Gy, 2.01 Gy and 4.79 Gy for 3D conformal, conventional IMRT and helical tomotherapy, respectively. An over-prediction by the Pinnacle RTPS of the V50, V60, V65 and V70 values for the cavity wall DVH was measured for the 3D conformal and conventional IMRT cases. These reductions may lead to a less than expected rectal toxicity. The TomoTherapy RTPS under-predicted the V50, V60, V65 and V70 values which may lead to higher rectal toxicity than predicted.

CONCLUSION

Calculation of dose around an air cavity created by an endo-rectal balloon provides a challenge for radiotherapy planning systems. Various electronic disequilibrium situations exist due to the cavity, which can lead to a lower anterior rectal wall and higher posterior rectal wall dose than that calculated by planning systems. This has consequences for comparisons of dose volume constraints between different modalities.

High-Dose-Rate Intracavitary Brachytherapy Boost for Early T Stage Nasopharyngeal Carcinoma{PRIVATE}

PURPOSE

To investigate any possible therapeutic gain from dose escalation with brachytherapy for early T stage nasopharyngeal carcinoma (NPC).

METHODS AND MATERIALS

One hundred forty-five patients with T1-2b N0-3 NPC were boosted with high-dose-rate intracavitary brachytherapy after completion of two-dimensional external radiotherapy (ERT) during the period from 1999 to 2003. To compare the efficacy of brachytherapy boost, another 142 patients with T1-2b N0-3 disease who were treated with ERT alone during 1994 to 1999 were evaluated. All patients were treated with ERT to a total dose of 66 Gy in 6.5 weeks. The brachytherapy boost group was given 10-12 Gy in 2 weekly fractions.

RESULTS

Dose escalation beyond 66 Gy with brachytherapy boost was shown to improve local control and survival. The 5-year actuarial local failure-free survival, regional failure-free survival, distant metastasis-free survival, progression-free survival, cancer-specific survival, and overall survival rates for the brachytherapy group and the control group were 95.8% and 88.3% (p = 0.020), 96% and 94.6% (p = 0.40), 95% and 83.2% (p = 0.0045), 89.2% and 74.8% (p = 0.0021), 94.5% and 83.4% (p = 0.0058), and 91.1% and 79.6% (p = 0.0062), respectively. The 5-year major-complication-free survival rate was 89.5% for the brachytherapy group and 85.6% for the control group (p = 0.23).

CONCLUSIONS

For patients who are treated with two-dimensional treatment techniques, dose escalation with brachytherapy boost improves local control and overall survival of patients with T1-T2a and possibly non-bulky T2b disease.

A finite size pencil beam algorithm for IMRT dose optimization: density corrections

For beamlet-based IMRT optimization, fast and less accurate dose computation algorithms are frequently used, while more accurate algorithms are needed to recompute the final dose for verification. In order to speed up the optimization process and ensure close proximity between dose in optimization and verification, proper consideration of dose gradients and tissue inhomogeneity effects should be ensured at every stage of the optimization. Due to their speed, pencil beam algorithms are often used for precalculation of beamlet dose distributions in IMRT treatment planning systems. However, accounting for tissue heterogeneities with these models requires the use of approximate rescaling methods. Recently, a finite size pencil beam (fsPB) algorithm, based on a simple and small set of data, was proposed which was specifically designed for the purpose of dose pre-computation in beamlet-based IMRT. The present work describes the incorporation of 3D density corrections, based on Monte Carlo simulations in heterogeneous phantoms, into this method improving the algorithm accuracy in inhomogeneous geometries while keeping its original speed and simplicity of commissioning. The algorithm affords the full accuracy of 3D density corrections at every stage of the optimization, hence providing the means for density related fluence modulation like penumbra shaping at field edges.

Dose build-up behind air cavities for Co-60, 4, 6 and 8 MV. Measurements and Monte Carlo simulations

It has been shown in several studies that the build-up in photon beams behind air cavities (such as in the head and neck) increases with energy. In this study this effect is investigated over a broad range of energies that have been used for treating head and neck tumours. The study addresses the question of whether an energy lower than 6 MV is desirable and is based on measurements and Monte Carlo (MC) simulations. In a PMMA phantom containing an air cavity (3 x 16 x 3 cm3 at 3 cm depth) an ionization chamber (Capintec PS-033) was used to measure the dose build-up behind the cavity for 4, 6 and 8 MV beam qualities for different field sizes (from 3 x 6 cm2 to 8 x 8 cm2). MC simulations were made using the EGSnrc code for the same geometry and energies as well as for Co-60. Measurements and MC simulations agree well when the fixed-separation plane-parallel chamber measurements have been corrected for the expected over-response in the build-up region. This work demonstrates that the build-up effect of 6 MV is 'closer' to the build-up effect of 8 MV than to that of 4 MV. This suggests that if the build-up effect is of concern when the target volume is in the vicinity of air cavities, 4 MV should be preferred over both 6 MV and 8 MV. This work also shows that the build-up effect for Co-60 is significantly smaller than that of 4 MV. Moreover, the build-up effect increases as the field size decreases. With the increasing use of IMRT (and radiosurgery), small fields are used more frequently making these issues even more relevant. This should be taken into consideration when choosing the accelerator energies for a radiotherapy department.

Direct aperture optimization for IMRT using Monte Carlo generated beamlets

This work introduces an EGSnrc-based Monte Carlo (MC) beamlet does distribution matrix into a direct aperture optimization (DAO) algorithm for IMRT inverse planning. The technique is referred to as Monte Carlo-direct aperture optimization (MC-DAO). The goal is to assess if the combination of accurate Monte Carlo tissue inhomogeneity modeling and DAO inverse planning will improve the dose accuracy and treatment efficiency for treatment planning. Several authors have shown that the presence of small fields and/or inhomogeneous materials in IMRT treatment fields can cause dose calculation errors for algorithms that are unable to accurately model electronic disequilibrium. This issue may also affect the IMRT optimization process because the dose calculation algorithm may not properly model difficult geometries such as targets close to low-density regions (lung, air etc.). A clinical linear accelerator head is simulated using BEAMnrc (NRC, Canada). A novel in-house algorithm subdivides the resulting phase space into 2.5 X 5.0 mm2 beamlets. Each beamlet is projected onto a patient-specific phantom. The beamlet dose contribution to each voxel in a structure-of-interest is calculated using DOSXYZnrc. The multileaf collimator (MLC) leaf positions are linked to the location of the beamlet does distributions. The MLC shapes are optimized using direct aperture optimization (DAO). A final Monte Carlo calculation with MLC modeling is used to compute the final dose distribution. Monte Carlo simulation can generate accurate beamlet dose distributions for traditionally difficult-to-calculate geometries, particularly for small fields crossing regions of tissue inhomogeneity. The introduction of DAO results in an additional improvement by increasing the treatment delivery efficiency. For the examples presented in this paper the reduction in the total number of monitor units to deliver is approximately 33% compared to fluence-based optimization methods.

Evaluation of underdosage in the external photon beam radiotherapy of glottic carcinoma: Monte Carlo study

PURPOSE

Underdosage in the human larynx may be the true factor behind the decrease in local control rates.

PATIENTS AND METHODS

To evaluate underdosage with Monte Carlo a CT-based geometrical model of the patient's neck (mathematical neck) was created. Dose was calculated for a pair of 6 Me V parallel-opposed photon beams modulated with 15 degree steel wedges.

RESULTS

At least 5% of volume of 3.5 cm(3) hypothetical tumor near the air wall of the larynx receives less than 86% of the maximum tumor dose. The same volume received less than 91% of the maximum tumor dose when the mathematical neck had no air cavities.

CONCLUSIONS

We conclude the significant underdosage at the air-tissue interface in the larynx occurs in traditional radiotherapy treatments, especially in the glottic part of the larynx.

Rectal wall sparing by dosimetric effect of rectal balloon used during intensity-modulated radiation therapy (IMRT) for prostate cancer

The use of an air-filled rectal balloon has been shown to decrease prostate motion during prostate radiotherapy. However, the perturbation of radiation dose near the air-tissue interfaces has raised clinical concerns of underdosing the prostate gland. The aim of this study was to investigate the dosimetric effects of an air-filled rectal balloon on the rectal wall/mucosa and prostate gland. Clinical rectal toxicity and dose-volume histogram (DVH) were also assessed to evaluate for any correlation. A film phantom was constructed to simulate the 4-cm diameter air cavity created by a rectal balloon. Kodak XV2 films were utilized to measure and compare dose distribution with and without air cavity. To study the effect in a typical clinical situation, the phantom was computed tomography (CT) scanned on a Siemens DR CT scanner for intensity-modulated radiation therapy (IMRT) treatment planning. A target object was drawn on the phantom CT images to simulate the treatment of prostate cancer. Because patients were treated in prone position, the air cavity was situated superiorly to the target. The treatment used a serial tomotherapy technique with the Multivane Intensity Modulating Collimator (MIMiC) in arc treatment mode. Rectal toxicity was assessed in 116 patients treated with IMRT to a mean dose of 76 Gy over 35 fractions (2.17-Gy fraction size). They were treated in the prone position, immobilized using a Vac-Loktrade mark bag and carrier-box system. Rectal balloon inflated with 100 cc of air was used for prostate gland immobilization during daily treatment. Rectal toxicity was assessed using modifications of the Radiation Therapy Oncology Group (RTOG) and late effects Normal Tissue Task Force (LENT) scales systems. DVH of the rectum was also evaluated. From film dosimetry, there was a dose reduction at the distal air-tissue interface as much as 60% compared with the same geometry without the air cavity for 15-MV photon beam and 2x2-cm field size. The dose beyond the interface recovered quickly and the dose reductions due to air cavity were 50%, 28%, 11%, and 1% at 2, 5, 10, and 15 mm, respectively, from the distal air-tissue interface. Evaluating the dose profiles of the more clinically relevant situation revealed the dose at air-tissue interface was approximately 15% lower in comparison to that without an air cavity. The dose built up rapidly so that at 1 and 2 mm, there was only an 8% and 5% differential, respectively. The dosimetric coverage at the depth of the posterior prostate wall was essentially equal with or without the air cavity. The median follow-up was 31.3 months. Rectal toxicity profile was very favorable: 81% (94/116) patients had no rectal complaint while 10.3% (12/116), 6.9% (8/116), and 1.7% (2/116) had grade 1, 2, and 3 toxicity, respectively. There was no grade 4 rectal toxicity. DVH analysis revealed that none of the patients had more than 25% of the rectum receiving 70 Gy or greater. Rectal balloon has rendered anterior rectal wall sparing by its dosimetric effects. In addition, it has reduced rectal volume, especially posterior and lateral rectal wall receiving high-dose radiation by rectal wall distension. Both factors may have contributed to decreased rectal toxicity achieved by IMRT despite dose escalation and higher than conventional fraction size. The findings have clinical significance for future very high-dose escalation trials whereby radiation proctitis is a major limiting factor.

Monte Carlo- versus pencil-beam-/collapsed-cone-dose calculation in a heterogeneous multi-layer phantom

The aim of this work was to evaluate the accuracy of dose predicted in heterogeneous media by a pencil beam (PB), a collapsed cone (CC) and a Monte Carlo (MC) algorithm. For this purpose, a simple multi-layer phantom composed of Styrofoam and white polystyrene was irradiated with 10 x 10 cm2 as well as 20 x 20 cm2 open 6 MV photon fields. The beam axis was aligned parallel to the layers and various field offsets were applied. Thereby, the amount of lateral scatter was controlled. Dose measurements were performed with an ionization chamber positioned both in the central layer of white polystyrene and the adjacent layers of Styrofoam. It was found that, in white polystyrene, both MC and CC calculations agreed satisfactorily with the measurements whereas the PB algorithm calculated 12% higher doses on average. By studying off-axis dose profiles the observed differences in the calculation results increased dramatically for the three algorithms. In the regions of low density CC calculated 10% (8%) lower doses for the 10 x 10 cm2 (20 x 20 cm2) fields than MC. The MC data on the other hand agreed well with the measurements, presuming that proper replacement correction for the ionization chamber embedded in Styrofoam was performed. PB results evidently did not account for the scattering geometry and were therefore not really comparable. Our investigations showed that the PB algorithm generates very large errors for the dose in the vicinity of interfaces and within low-density regions. We also found that for the used CC algorithm large deviations for the absolute dose (dose/monitor unit) occur in regions of electronic disequilibrium. The performance might be improved by better adapted parameters. Therefore, we recommend a careful investigation of the accuracy for dose calculations in heterogeneous media for each beam data set and algorithm.

Monte Carlo study of TLD measurements in air cavities

Thermoluminescent dosimeters (TLDs) are used for verification of the delivered dose during IMRT treatment of head and neck carcinomas. The TLDs are put into a plastic tube, which is placed in the nasal cavities through the treated volume. In this study, the dose distribution to a phantom having a cylindrical air cavity containing a tube was calculated by Monte Carlo methods and the results were compared with data from a treatment planning system (TPS) to evaluate the accuracy of the TLD measurements. The phantom was defined in the DOSXYZnrc Monte Carlo code and calculations were performed with 6 MV fields, with the TLD tube placed at different positions within the cylindrical air cavity. A similar phantom was defined in the pencil beam based TPS. Differences between the Monte Carlo and the TPS calculations of the absorbed dose to the TLD tube were found to be small for an open symmetrical field. For a half-beam field through the air cavity, there was a larger discrepancy. Furthermore, dose profiles through the cylindrical air cavity show, as expected, that the treatment planning system overestimates the absorbed dose in the air cavity. This study shows that when using an open symmetrical field, Monte Carlo calculations of absorbed doses to a TLD tube in a cylindrical air cavity give results comparable to a pencil beam based treatment planning system.

Clinical experience with intensity-modulated radiation therapy (IMRT) for prostate cancer with the use of rectal balloon for prostate immobilization

The implementation of intensity-modulated radiation therapy (IMRT) is the result of advances in imaging, radiotherapy planning technologies, and computer-controlled linear accelerators. IMRT allows both conformal treatment of tumors and conformal avoidance of the surrounding normal structures. The first patient treated with Peacock IMRT at Baylor College of Medicine took place in March 1994. To date, more than 1500 patients have been treated with IMRT; more than 700 patients were treated for prostate cancer. Our experience in treating prostate cancer with IMRT was reviewed. Patient and prostate motions are important issues to address in delivering IMRT. The Vac-Lok bag-and-box system, as well as rectal balloon for immobilization of patient and prostate gland, respectively, are employed. Treatment planning also plays a very important role. IMRT as a boost after conventional external beam radiotherapy is not our treatment strategy. To derive maximal benefits with this new technology, all patients received full course IMRT. Three separate groups of patients receiving (1) primary IMRT, (2) combined radioactive seed implant and IMRT, and (3) post-prostatectomy IMRT were addressed. Overall, toxicity profiles in these patients were very favorable. IMRT has the potential to improve treatment outcome with dose escalation while minimizing treatment-related toxicity.

The use of rectal balloon during the delivery of intensity modulated radiotherapy (IMRT) for prostate cancer: more than just a prostate gland immobilization device?

PURPOSE

The purpose of this study was to investigate the role of a rectal balloon for prostate immobilization and rectal toxicity reduction in patients receiving dose-escalated intensity-modulated radiotherapy for prostate cancer.

PATIENTS AND METHODS

Patients with localized prostate cancer who were undergoing intensity-modulated radiotherapy were treated in a prone position, immobilized with a customized Vac-Lok bag (MED-TEC, Orange City, IA). A rectal balloon with 100 cc of air was used to immobilize the prostate. The prostate displacements were measured using computed tomography (CT)-CT fusion on 10 patients who received radioactive seed implant before intensity-modulated radiotherapy. They were scanned twice weekly during 5 weeks of intensity-modulated radiotherapy, and breathing studies were also performed. Rectal toxicity was evaluated by use of Radiation Therapy Oncology Group scoring in 100 patients. They were treated to a mean dose of 76 Gy over 35 fractions (2.17-Gy fraction size). Dose-volume histogram of the rectum was assessed. A film phantom was constructed to simulate the 4-cm diameter air cavity that was created by the rectal balloon. Kodak XV2 films (Rochester NY) were used to measure and compare dose distribution with and without the air cavity. A fraction of 1.25 Gy was delivered to the phantom at isocenter with 15-MV photons by use of the NOMOS Peacock system and the MIMiC treatment delivery system (Sewickley, PA).

RESULTS

The anterior-posterior and lateral prostate displacements were minimal, on the order of measurement uncertainty (approximately 1 mm). The standard deviation of superior-inferior displacement was 1.78 mm. Breathing studies showed no organ displacement during normal breathing when the rectal balloon was in place. The rectal toxicity profile was very favorable: 83% (83/100) patients had no rectal complaint, and 11% and 6% had grade 1 and 2 toxicity, respectively. Dose-volume histogram analysis revealed that in all of the patients, no more than 25% of the rectum received 70 Gy or greater. As visualized by film dosimetry, the dose at air-tissue interface was approximately 15% lower than that without an air cavity. The dose built up rapidly so that at 1 and 2 mm, the differential was approximately 8% and 5%, respectively. The dosimetric coverage at the depth of the posterior prostate wall was essentially equal, with or without the air cavity.

DISCUSSION

The use of a rectal balloon during intensity-modulated radiotherapy significantly reduces prostate motion. Prostate immobilization thus allows a safer and smaller planning target volume margin. It has also helped spare the anterior rectal wall (by its dosimetric effects) and reduced the rectal volume that received high-dose radiation (by rectal wall distension). All these factors may have further contributed to the decreased rectal toxicity achieved by intensity-modulated radiotherapy, despite dose escalation and higher-than-conventional fraction size.

Underdosage of the upper-airway mucosa for small fields as used in intensity-modulated radiation therapy: a comparison between radiochromic film measurements, Monte Carlo simulations, and collapsed cone convolution calculations

Head-and-neck tumors are often situated at an air-tissue interface what may result in an underdosage of part of the tumor in radiotherapy treatments using megavoltage photons, especially for small fields. In addition to effects of transient electronic disequilibrium, for these small fields, an increased lateral electron range in air will result in an important extra reduction of the central axis dose beyond the cavity. Therefore dose calculation algorithms need to model electron transport accurately. We simulated the trachea by a 2 cm diameter cylindrical air cavity with the rim situated 2 cm beneath the phantom surface. A 6 MV photon beam from an Elekta SLiplus linear accelerator, equipped with the standard multileaf collimator (MLC), was assessed. A 10 x 2 cm2 and a 10 x 1 cm2 field, both widthwise collimated by the MLC, were applied with their long side parallel to the cylinder axis. Central axis dose rebuild-up was studied. Radiochromic film measurements were performed in an in-house manufactured polystyrene phantom with the films oriented either along or perpendicular to the beam axis. Monte Carlo simulations were performed with BEAM and EGSnrc. Calculations were also performed using the pencil beam (PB) algorithm and the collapsed cone convolution (CCC) algorithm of Helax-TMS (MDS Nordion, Kanata, Cahada) version 6.0.2 and using the CCC algorithm of Pinnacle (ADAC Laboratories, Milpitas, CA, USA) version 4.2. A very good agreement between the film measurements and the Monte Carlo simulations was found. The CCC algorithms were not able to predict the interface dose accurately when lateral electronic disequilibrium occurs, but were shown to be a considerable improvement compared to the PB algorithm. The CCC algorithms overestimate the dose in the rebuild-up region. The interface dose was overestimated by a maximum of 31% or 54%, depending on the implementation of the CCC algorithm. At a depth of 1 mm, the maximum dose overestimation was 14% or 24%.

Evaluation of dose calculation algorithms for dynamic arc treatments of head and neck tumors

BACKGROUND AND PURPOSE

To investigate if the Pencil Beam (PB) algorithm takes the disturbance of the dose distribution due to tissue inhomogeneities sufficiently into account in dynamic field shaping rotation therapy (called the dynamic arc treatment modality) for fractionated stereotactic radiation therapy of head and neck tumors.

MATERIAL AND METHODS

A treatment plan using the dynamic arc treatment modality of an oropharynx lesion on a humanoid phantom was evaluated. The same plan was calculated with three different calculation algorithms: the Clarkson and the PB algorithm (both available on the planning system of the Novalis system used for dynamic arc treatments), and the Collapsed Cone Convolution Superposition (CC) algorithm (used by the Pinnacle planning system). The three resulting plans are compared using isodose distributions and cumulative dose volume histograms (CDVHs). An intercomparison of the results of the three algorithms was performed to investigate how accurately each of them takes the influence of tissue inhomogeneities into account such as bony structures and air cavities often appearing in the head and neck region. Additionally, the resulting plans were compared with absolute and relative dosimetric measurements of the treatment plan on the humanoid phantom with thermoluminescent detectors and radiographic film, respectively.

RESULTS

All calculated dose distributions show a good agreement with the measured distribution except in the planning target volume (PTV) in and at the border of the air cavity. All three algorithms overestimate the dose in the PTV at the boundary with the low-density tissue, with 12, 10 and 7% for the Clarkson, the PB and the CC algorithm, respectively. The correspondence between the calculated dose distributions is reflected in the graphs of the CDVHs. They show similar curves for the PTV and the structures except for the left parotic gland and the myelum.

CONCLUSIONS

The PB algorithm of the Novalis system calculates a treatment plan for the dynamic arc treatment modality adequately for fractionated stereotactic radiation therapy of head and neck tumors, except in the PTV in and at the border of the air cavity where the actual dose is overestimated. Care needs to be taken in clinical cases where it is critical to irradiate the air-tissue boundary to a sufficient dose.

Monte Carlo dose calculations in homogeneous media and at interfaces: a comparison between GEPTS, EGSnrc, MCNP, and measurements

Three Monte Carlo photon/electron transport codes (GEPTS, EGSnrc, and MCNP) are bench-marked against dose measurements in homogeneous (both low- and high-Z) media as well as at interfaces. A brief overview on physical models used by each code for photon and electron (positron) transport is given. Absolute calorimetric dose measurements for 0.5 and 1 MeV electron beams incident on homogeneous and multilayer media are compared with the predictions of the three codes. Comparison with dose measurements in two-layer media exposed to a 60Co gamma source is also performed. In addition, comparisons between the codes (including the EGS4 code) are done for (a) 0.05 to 10 MeV electron beams and positron point sources in lead, (b) high-energy photons (10 and 20 MeV) irradiating a multilayer phantom (water/steel/air), and (c) simulation of a 90Sr/90Y brachytherapy source. A good agreement is observed between the calorimetric electron dose measurements and predictions of GEPTS and EGSnrc in both homogeneous and multilayer media. MCNP outputs are found to be dependent on the energy-indexing method (Default/ITS style). This dependence is significant in homogeneous media as well as at interfaces. MCNP(ITS) fits more closely the experimental data than MCNP(DEF), except for the case of Be. At low energy (0.05 and 0.1 MeV), MCNP(ITS) dose distributions in lead show higher maximums in comparison with GEPTS and EGSnrc. EGS4 produces too penetrating electron-dose distributions in high-Z media, especially at low energy (<0.1 MeV). For positrons, differences between GEPTS and EGSnrc are observed in lead because GEPTS distinguishes positrons from electrons for both elastic multiple scattering and bremsstrahlung emission models. For the 60Co source, a quite good agreement between calculations and measurements is observed with regards to the experimental uncertainty. For the other cases (10 and 20 MeV photon sources and the 90Sr/90Y beta source), a good agreement is found between the three codes. In conclusion, differences between GEPTS and EGSnrc results are found to be very small for almost all media and energies studied. MCNP results depend significantly on the electron energy-indexing method.

Monte Carlo dose verification for intensity-modulated arc therapy

Intensity-modulated arc therapy (IMAT), a technique which combines beam rotation and dynamic multileaf collimation, has been implemented in our clinic. Dosimetric errors can be created by the inability of the planning system to accurately account for the effects of tissue inhomogeneities and physical characteristics of the multileaf collimator (MLC). The objective of this study is to explore the use of Monte Carlo (MC) simulation for IMAT dose verification. The BEAM/DOSXYZ Monte Carlo system was implemented to perform dose verification for the IMAT treatment. The implementation includes the simulation of the linac head/MLC (Elekta SL20), the conversion of patient CT images and beam arrangement for 3D dose calculation, the calculation of gantry rotation and leaf motion by a series of static beams and the development of software to automate the entire MC process. The MC calculations were verified by measurements for conventional beam settings. The agreement was within 2%. The IMAT dose distributions generated by a commercial forward planning system (RenderPlan. Elekta) were compared with those calculated by the MC package. For the cases studied, discrepancies of over 10% were found between the MC and the RenderPlan dose calculations. These discrepancies were due in part to the inaccurate dose calculation of the RenderPlan system. The computation time for the IMAT MC calculation was in the range of 20-80 min on 15 Pentium-Ill computers. The MC method was also useful in verifying the beam apertures used in the IMAT treatments.

Evaluating the effectiveness of a longitudinal magnetic field in reducing underdosing of the regions around upper respiratory cavities irradiated with photon beams--a Monte Carlo study

The problem of underdosing lesions adjacent to upper respiratory cavities and a proposal to correct it are presented in this work. The EGS4 Monte Carlo code was used to simulate a 6 MV x-ray beam passing through a block of tissues with air cavities 2, 4, and 6 cm wide. The geometry used approximates the tracheal geometry used by previous researchers who investigated the underdosing phenomenon. A uniform longitudinal magnetic field of 0.5 T strength is used to reduce secondary electron outscatter caused by the presence of an air gap, and thus improving the dose at the distal surface of air cavities. We introduce the term "percent dose reduction" (PDR), which is defined as the difference between the dose after the air cavity and the dose at the same depth in soft-tissue phantom normalized to the dose in the tissue phantom, to quantify the reduction in dose after an air gap. We also introduce the term dose improvement ratio (DIR), which is defined as the dose ratio with magnetic field to the dose, at the same point, without magnetic field, to quantify the improvement in dose when the magnetic field is applied. For 2 x 2 x 20 cm3 and 4 x 4 x 20 cm3 air cavities irradiated by 2 x 2 cm2 beams, we found PDRs of 38% and 52%, respectively. This means that for these cavities, there is a 38% and a 52% reduction in dose at the cavity edge compared to the same dose in tissue at the same depth for each cavity. The dose improved by 30% (DIR= 1.3) and 87% (DIR= 1.87), respectively, when applying the magnetic field. The worst effect on dose at the distal side came from larger cavities irradiated with small fields. In these situations, the improvement in dose due to the presence of magnetic field was the largest. This article deals with "ideal" head and neck geometries with a uniform magnetic field. In a paper to follow we will use a CT-based phantom to study the effect in realistic geometries with the presence of a magnetic field from a Helmholtz coil pair.

Reducing loss in lateral charged-particle equilibrium due to air cavities present in x-ray irradiated media by using longitudinal magnetic fields

The underdosing of lesions distal to air cavities, such as those found in upper respiratory passages, occurs due to the loss in lateral charged-particle equilibrium (CPE). The degree of underdosing worsens for smaller field sizes, resulting in more frequent recurrence of the cancer treated. Higher photon energies further aggravate the outcome by producing longer second build-up regions beyond the cavity. Besides underdosing, the larger lateral spread of secondary electron fluence in the air cavity produces diffuse dose distributions at the tissue-air interface for shaped or intensity modulated fields. These disequilibrium effects create undesirable deviations from the intended treatment. The clinical concern is further intensified by the failure of traditional treatment planning systems to even account for such defects. In this work, the use of longitudinal magnetic fields on the order of 0.5 T is proposed for alleviating lateral electronic disequilibrium due to the presence of air cavities in the irradiated volume. The magnetic field enforces lateral CPE by restricting the lateral range of electrons in the air cavity. The problem is studied in a simple water-air-water slab geometry using EGS4 Monte Carlo simulations for 6 MV photons. Electronic disequilibrium is evaluated for beams of various sizes, shapes and intensity distributions constructed by linear superposition of the dose distributions for 0.5 x 0.5 cm2 beamlets. Comparison is also made with 60Co irradiation. The results indicate that the lateral confinement of secondary electrons in the air cavity by sub-MRI strength longitudinal fields is effective in reducing deterioration of dose distributions near tissue-air interfaces. This can potentially reduce recurrence rates of cancers such as the larynx carcinoma.

Intensity modulated radiation therapy (IMRT) following prostatectomy: more favorable acute genitourinary toxicity profile compared to primary IMRT for prostate cancer

PURPOSE

To report our initial experience on postprostatectomy IMRT (PPI), addressing acute genitourinary (GU) toxicity in comparison to primary IMRT (PI) for prostate cancer.

METHODS AND MATERIALS

From April 1998 to December 1999, 40 postprostatectomy patients were treated with intensity modulated radiation therapy (IMRT) to a median prescribed dose of 64 Gy (mean dose of 69 Gy). The Radiation Therapy Oncology Group (RTOG) scoring system was used to assess acute GU toxicity. Target volume and maximum and mean doses were evaluated. The mean doses to the bladder and irradiated bladder volume receiving >65 Gy were assessed. These were compared to those of 125 patients treated with PI to a prescribed dose of 70 Gy (mean dose of 76 Gy).

RESULTS

The acute GU toxicity profile is more favorable in the PPI group with 82.5% of Grade 0-1 and 17.5% of Grade 2 toxicity compared to 59.2% and 40.8%, respectively, in the PI group (p < 0.001). There was no Grade 3 or higher toxicity in either group. The target volume was larger in the PPI group, while the maximum and mean doses to the target were higher in the PI group. The mean dose delivered to the bladder was higher in the PPI group. The irradiated bladder volume receiving >65 Gy was significantly larger in the PI group (p < 0.001).

CONCLUSIONS

PPI can be delivered with acceptable ute GU toxicity. The larger PPI target volume may be related to the difficulty in delineating prostatic fossa. Despite a larger target volume and a higher mean dose to the bladder, PPI produced a more favorable acute GU toxicity profile. This may be related to a combination of lower mean and maximum doses and smaller bladder volumes receiving >65 Gy in the PPI group, as well as urethral rather than bladder irradiation. The findings have implications in the evaluation of IMRT treatment plan for prostate cancer, whereby the irradiated bladder volumes above 65 Gy may be more meaningful than the mean dose to the bladder. Longer term toxicity results are awaited.