Clinical and physical quality assurance for intensity modulated radiotherapy of prostate cancer

Some computer graphical user interfaces in radiation therapy

In this review, five graphical user interfaces (GUIs) used in radiation therapy practices and researches are introduced. They are: (1) the treatment time calculator, superficial X-ray treatment time calculator (SUPCALC) used in the superficial X-ray radiation therapy; (2) the monitor unit calculator, electron monitor unit calculator (EMUC) used in the electron radiation therapy; (3) the multileaf collimator machine file creator, sliding window intensity modulated radiotherapy (SWIMRT) used in generating fluence map for research and quality assurance in intensity modulated radiation therapy; (4) the treatment planning system, DOSCTP used in the calculation of 3D dose distribution using Monte Carlo simulation; and (5) the monitor unit calculator, photon beam monitor unit calculator (PMUC) used in photon beam radiation therapy. One common issue of these GUIs is that all user-friendly interfaces are linked to complex formulas and algorithms based on various theories, which do not have to be understood and noted by the user. In that case, user only needs to input the required information with help from graphical elements in order to produce desired results. SUPCALC is a superficial radiation treatment time calculator using the GUI technique to provide a convenient way for radiation therapist to calculate the treatment time, and keep a record for the skin cancer patient. EMUC is an electron monitor unit calculator for electron radiation therapy. Instead of doing hand calculation according to pre-determined dosimetric tables, clinical user needs only to input the required drawing of electron field in computer graphical file format, prescription dose, and beam parameters to EMUC to calculate the required monitor unit for the electron beam treatment. EMUC is based on a semi-experimental theory of sector-integration algorithm. SWIMRT is a multileaf collimator machine file creator to generate a fluence map produced by a medical linear accelerator. This machine file controls the multileaf collimator to deliver intensity modulated beams for a specific fluence map used in quality assurance or research. DOSCTP is a treatment planning system using the computed tomography images. Radiation beams (photon or electron) with different energies and field sizes produced by a linear accelerator can be placed in different positions to irradiate the tumour in the patient. DOSCTP is linked to a Monte Carlo simulation engine using the EGSnrc-based code, so that 3D dose distribution can be determined accurately for radiation therapy. Moreover, DOSCTP can be used for treatment planning of patient or small animal. PMUC is a GUI for calculation of the monitor unit based on the prescription dose of patient in photon beam radiation therapy. The calculation is based on dose corrections in changes of photon beam energy, treatment depth, field size, jaw position, beam axis, treatment distance and beam modifiers. All GUIs mentioned in this review were written either by the Microsoft Visual Basic.net or a MATLAB GUI development tool called GUIDE. In addition, all GUIs were verified and tested using measurements to ensure their accuracies were up to clinical acceptable levels for implementations.

Twin machines validation for VMAT treatments using electronic portal-imaging device: a multicenter study

Purpose

To verify the accuracy of volumetric arc therapy (VMAT) using the RapidArc™ device when switching patients from one single linear accelerator (linac) to a paired energy and mechanics "twin" linac without reoptimization of the original treatment plan.

Patients and Methods

Four centers using 8 linacs were involved in this study. Seventy-four patients previously treated with the 6MV photon RapidArc™ technique were selected for analysis, using 242 measurements. In each institution, all patients were planned on linac A, and their plans were verified both on linac A and on the twin linac B. Verifications were done using the amorphous silicium electronic portal imager (EPID) of the linacs and were analyzed with the EpiQa software (Epidos, Bratislavia, Slovakia). The gamma index formalism was used for validation with a double threshold of 3 % and 3 mm with a measurement resolution of 0.39 mm/pixel, and a smoothed resolution of approximately 2.5 mm.

Results

The number of points passing the gamma criteria between the measured and computed doses was 94.79 ± 2.57 % for linac A and 94.61 ± 2.46 % for linac B. Concerning the smoothed measurement analysis, 98.67 ± 1.26 % and 98.59 ± 1.20 % points passing the threshold were obtained for linacs A and B, respectively.

The difference between the 2 dose matrices acquired on the EPID was very small, with 99.92 ± 0.06 % of the points passing the criteria.

Conclusion

For linacs sharing the same mechanical and energy parameters, this study tends to indicate that patients may be safely switched from treatment with one linac to treatment with its twin linac using the same VMAT plan.

Implementation of intensity modulated radiotherapy for prostate cancer in a private radiotherapy service in Mexico

Intensity modulated radiation therapy (IMRT) allows physicians to deliver higher conformal doses to the tumour, while avoiding adjacent structures. As a result the probability of tumour control is higher and toxicity may be reduced. However, implementation of IMRT is highly complex and requires a rigorous quality assurance (QA) program both before and during treatment. The present article describes the process of implementing IMRT for localized prostate cancer in a radiation therapy department. In our experience, IMRT implementation requires careful planning due to the need to simultaneously implement specialized software, multifaceted QA programs, and training of the multidisciplinary team. Establishing standardized protocols and ensuring close collaboration between a multidisciplinary team is challenging but essential.

Eight years of IMRT quality assurance with ionization chambers and film dosimetry: experience of the Montpellier Comprehensive Cancer Center

Background

To present the results of quality assurance (QA) in IMRT of film dosimetry and ionization chambers measurements with an eight year follow-up.

Methods

All treatment plans were validated under the linear accelerator by absolute and relative measures obtained with ionization chambers (IC) and with XomatV and EDR2 films (Kodak).

Results

The average difference between IC measured and computed dose at isocenter with the gantry angle of 0° was 0.07 ± 1.22% (average ± 1 SD) for 2316 prostate, 1.33 ± 3.22% for 808 head and neck (h&n), and 0.37 ± 0.62% for 108 measurements of prostate bed fields. Pelvic treatment showed differences of 0.49 ± 1.86% in 26 fields for prostate cases and 2.07 ± 2.83% in 109 fields of anal canal.

Composite measurement at isocenter for each patient showed an average difference with computed dose of 0.05 ± 0.87% for 386 prostate, 1.49 ± 1.86% for 158 h&n, 0.37 ± 0.34% for 23 prostate bed, 0.80 ± 0.28% for 4 pelvis, and 2.31 ± 0.56% for 17 anal canal cases. On the first 250 h&n analyzed by film in absolute dose, the average of the points crossing a gamma index 3% and 3 mm was 93%. This value reached 99% for the prostate fields.

Conclusion

More than 3500 beams were found to be within the limits defined as validated for treatment between 2001 and 2008.

Statistical analysis of IMRT dosimetry quality assurance measurements for local delivery guideline

Purpose

To establish our institutional guideline for IMRT delivery, we statistically evaluated the results of dosimetry quality assurance (DQA) measurements and derived local confidence limits using the concept confidence limit of |mean|+1.96σ.

Materials and methods

From June 2006 to March 2009, 206 patients with head and neck cancer, prostate cancer, liver cancer, or brain tumor were treated using LINAC-based IMRT technique. In order to determine site specific DQA tolerances at a later stage, a hybrid plan with the same fluence maps as in the treatment plan was generated on CT images of a cylindrical phantom of acryl. Points of measurement using a 0.125 cm³ ion-chamber were typically located in the region of high and uniform doses. The planar dose distributions perpendicular to the central axis were measured by using a diode array in solid water with all fields delivered, and assessed using gamma criteria of 3%/3 mm. The mean values and standard deviations were used to develop the local confidence and tolerance limits. The dose differences and gamma pass rates for the different treatment sites were also evaluated in terms of total monitor uints (MU), MU/cGy, and the number of PTV's pieces.

Results

The mean values and standard deviations of ion-chamber dosimetry differences between calculated and measured doses were -1.6 ± 1.2% for H&N cancer, -0.4 ± 1.2% for prostate and abdominal cancer, and -0.6 ± 1.5% for brain tumor. Most of measured doses (92.2%) agreed with the calculated doses within a tolerance limit of ±3% recommended in the literature. However, we found some systematic under-dosage for all treatment sites. The percentage of points passing the gamma criteria, averaged over all treatment sites was 97.3 ± 3.7%. The gamma pass rate and the agreement of ion-chamber dosimetry generally decreased with increasing the number of PTV's pieces, the degree of modulation (MU/cGy), and the total MU beyond 700. Our local confidence limits were comparable to those of AAPM TG 119 and ESTRO guidelines that were provided as a practical baseline for center-to-center commissioning comparison. Thus, our institutional confidence and action limits for IMRT delivery were set into the same levels of those guidelines.

Discussion and Conclusions

The systematic under-dosage were corrected by tuning up the MLC-related factors (dosimetric gap and transmission) in treatment planning system (TPS) and further by incorporating the tongue-and groove effect into TPS. Institutions that have performed IMRT DQA measurements over a certain period of time need to analyze their accrued DQA data. We confirmed the overall integrity of our IMRT system and established the IMRT delivery guideline during this procedure. Dosimetric corrections for the treatment plans outside of the action level can be suggested only with such rigorous DQA and statistical analysis.

Practical guidelines for routine intensity-modulated radiotherapy verification: pre-treatment verification with portal dosimetry and treatment verification with in vivo dosimetry

The purpose of this work is to provide guidelines for the routine use of portal dosimetry and in vivo diode measurements to verify intensity-modulated radiotherapy (IMRT) treatments. To achieve tolerance levels that are sensitive enough to intercept problems, both the portal dosimetry and the in vivo procedure must be optimised. Portal dosimetry was improved by the introduction of an optimised two-dimensional (2D) profile correction, which also accounted for the effect of backscatter from the R-arm. The scaled score, indicating the fraction of points not meeting the desired gamma evaluation criteria within the field opening, was determined as the parameter of interest. Using gamma criteria of a 3% dose difference and 3 mm distance to agreement, a "scaled score" threshold value of 1.5% was chosen to indicate excessive tongue and groove and other problems. The pre-treatment portal dosimetry quality assurance (QA) does not encompass verification of the patient dose calculation or position, and so it is complemented by in vivo diode measurements. Diode positioning is crucial in IMRT, and so we describe a method for diode positioning at any suitable point. We achieved 95% of IMRT field measurements within ±5% and 99% within ±8%, with improved accuracy being achieved over time owing to better positioning. Although the careful preparation and setup of the diode measurements can be time-consuming, this is compensated for by the time efficiency of the optimised procedure. Both methods are now easily absorbed into the routine work of the department.

Radiobiological impact of reduced margins and treatment technique for prostate cancer in terms of tumor control probability (TCP) and normal tissue complication probability (NTCP)

Dose escalation in prostate radiotherapy is limited by normal tissue toxicities. The aim of this study was to assess the impact of margin size on tumor control and side effects for intensity-modulated radiation therapy (IMRT) and 3D conformal radiotherapy (3DCRT) treatment plans with increased dose. Eighteen patients with localized prostate cancer were enrolled. 3DCRT and IMRT plans were compared for a variety of margin sizes. A marker detectable on daily portal images was presupposed for narrow margins. Prescribed dose was 82 Gy within 41 fractions to the prostate clinical target volume (CTV). Tumor control probability (TCP) calculations based on the Poisson model including the linear quadratic approach were performed. Normal tissue complication probability (NTCP) was calculated for bladder, rectum and femoral heads according to the Lyman-Kutcher-Burman method. All plan types presented essentially identical TCP values and very low NTCP for bladder and femoral heads. Mean doses for these critical structures reached a minimum for IMRT with reduced margins. Two endpoints for rectal complications were analyzed. A marked decrease in NTCP for IMRT plans with narrow margins was seen for mild RTOG grade 2/3 as well as for proctitis/necrosis/stenosis/fistula, for which NTCP <7% was obtained. For equivalent TCP values, sparing of normal tissue was demonstrated with the narrow margin approach. The effect was more pronounced for IMRT than 3DCRT, with respect to NTCP for mild, as well as severe, rectal complications.

MRI before and after external beam intensity-modulated radiotherapy of patients with prostate cancer: the feasibility of monitoring of radiation-induced tissue changes using a dynamic contrast-enhanced inversion-prepared dual-contrast gradient echo sequence

PURPOSE

To identify and quantify suitable pharmacokinetic MRI parameters for monitoring tissue changes after external beam intensity-modulated radiotherapy of prostate cancer.

MATERIAL AND METHODS

Six patients with biopsy-proven prostate cancer (initial PSA, 6.0-81.4 ng/ml) underwent MRI at 1.5 T using a combined endorectal/body phased-array coil and a dynamic contrast-enhanced inversion-prepared dual-contrast gradient echo sequence (T1/T2(*)w; 1.65 s temporal resolution). MRI was performed before and immediately after radiotherapy, at 3 months and at 1 year. Perfusion, blood volume, mean transit time, delay, dispersion, interstitial volume, and extraction coefficient were calculated in prostate cancer and normal prostate for all four time points using a sequential 3-compartment model.

RESULTS

Prostate cancer and normal prostate tissue showed a statistically significant decrease in perfusion (p=0.006, p=0.001) and increase in extraction coefficient (p=0.004, p<0.001). For prostate cancer, there was also a decrease in vascular volume (p=0.034). The other parameters investigated showed no statistically significant changes. Statistically significant differences between prostate cancer and normal prostate tissue were only observed before radiotherapy, when prostate cancer showed significantly higher perfusion (1.84 vs. 0.12 ml/cm(3)min, p=0.028) and a smaller extraction coefficient (0.42 vs. 0.64, p=0.028).

CONCLUSIONS

Two pharmacokinetic parameters, perfusion and extraction coefficient, appear to be suitable candidates for monitoring the response to percutaneous intensity-modulated radiotherapy of prostate cancer.

[Serial tomotherapy vs. MLC-IMRT (multileaf collimator intensity modulated radiotherapy) for simultaneous boost treatment large intracerebral lesions]

INTRODUCTION

Recent data suggest that a radiosurgery boost treatment for up to three brain metastases in addition to whole brain radiotherapy (WBRT) is beneficial. Sequential treatment of multiple metastatic lesions is time-consuming and optimal normal tissue sparing is not trivial for larger metastases when separate plans are created and are only superimposed afterwards. Sequential Tomotherapy (see image I) with noncoplanar arcs and Multi-field IMRT may streamline the process and enable easy simultaneous treatment. We compared plans for 2-3 intracerebral targets calculated with Intensity Modulated Radiotherapy (IMRT) based on treatment with MLC or sequential Tomotherapy using the Peacock-System (see image II). Treatment time was not to exceed 90 min on a linac with standard dose rate. MIMiC plans without treatment-time restrictions were created as a benchmark.

MATERIALS AND METHODS

Calculations are based on a Siemens KD2 linac with a dose rate of 200 MU/min. Step-and-Shoot IMRT is performed with a standard MLC (2 x 29 leaves, 1 cm), serial Tomotherapy with the Multivane-Collimator MIMiC (NOMOS Inc. USA) (see image II). Treatment plans are created with Corvus 5.0. To create plans with good conformity we chose a noncoplanar beam- and arc geometry for each approach (IMRT 4-, MIMiC 5-couch angles). The benchmark MIMiC plans with maximally steep dose gradients had 9 couch angles. For plan comparison reasons, 10 Gy were prescribed to 90% of the PTV. Steepness of dose gradients, homogeneity and conformity were assessed by the following parameters: Volume encompassed by certain isodoses outside the target as well as homogeneity and conformity as indicated by Homogeneity- and Conformity-Index.

RESULTS

Plans without treatment-time restrictions had slightest dose to organ at risk (OAR), normal tissue and least Conformity-index. MIMiC- and MLC-IMRT based plans can be treated within the intended period of 90 min, all plans met the required dose (see Table 2). MLC based plans resulted in higher dose to organs at risk (OAR) (see table 1) and dose to tissue outside the targets (see table 3), as indicated by a higher CI (see image III). The HI was similar for all calculated plans (see image IV).

DISCUSSION

When treatment plans resulting in a similar treatment time were compared, serial Tomotherapy showed minor advantages over MLC based IMRT with regard to conformity, OAR sparing, and steepness of dose gradients. Both methods are inferior to serial Tomotherapy with ideal plan quality disregarding treatment efficiency. Treating multiple metastases in less than 1 h would therefore be possible on a LINAC with high dose rate and bidirectional rotation with minor compromises on gradient steepness.

Dose–Volume Comparison of Proton Therapy and Intensity-Modulated Radiotherapy for Prostate Cancer

PURPOSE

The contrast in dose distribution between proton radiotherapy (RT) and intensity-modulated RT (IMRT) is unclear, particularly in regard to critical structures such as the rectum and bladder.

METHODS AND MATERIALS

Between August and November 2006, the first 10 consecutive patients treated in our Phase II low-risk prostate proton protocol (University of Florida Proton Therapy Institute protocol 0001) were reviewed. The double-scatter proton beam plans used in treatment were analyzed for various dosimetric endpoints. For all plans, each beam dose distribution, angle, smearing, and aperture margin were optimized. IMRT plans were created for all patients and simultaneously analyzed. The IMRT plans were optimized through multiple volume objectives, beam weighting, and individual leaf movement. The patients were treated to 78 Gray-equivalents (GE) in 2-GE fractions with a biologically equivalent dose of 1.1.

RESULTS

All rectal and rectal wall volumes treated to 10-80 GE (percentage of volume receiving 10-80 GE [V(10)-V(80)]) were significantly lower with proton therapy (p < 0.05). The rectal V(50) was reduced from 31.3% +/- 4.1% with IMRT to 14.6% +/- 3.0% with proton therapy for a relative improvement of 53.4% and an absolute benefit of 16.7% (p < 0.001). The mean rectal dose decreased 59% with proton therapy (p < 0.001). For the bladder and bladder wall, proton therapy produced significantly smaller volumes treated to doses of 10-35 GE (p < 0.05) with a nonsignificant advantage demonstrated for the volume receiving < or =60 GE. The bladder V(30) was reduced with proton therapy for a relative improvement of 35.3% and an absolute benefit of 15.1% (p = 0.02). The mean bladder dose decreased 35% with proton therapy (p = 0.002).

CONCLUSION

Compared with IMRT, proton therapy reduced the dose to the dose-limiting normal structures while maintaining excellent planning target volume coverage.

[Organ-confined prostate cancer: treatment with high doses of radioterapy (intensity modulated radiotherapy)]

PURPOSE

To report toxicity and local control in patients with localized prostate cancer, treated with high dose radiotherapy.

MATERIALS AND METHODS

The records of 100 consecutive patients with clinically localized prostate cancer treated between june 2003 and may 2006 were reviewed. They received 80 Gy to the target volume with a biphasic technique (3DCRT + IMRT). The median pretreatment PSA was 9. The median follow-up time was 12 months.

RESULTS

Eighteen (18%) developed acute Grade 2 rectal toxicity, and no patient experienced acute grade 3 or higher rectal symptoms. Forty-four (44%) developed acute Grade 2 urinary symptoms while 34% of the patients experienced no GU symptoms (Grade 0) during treatment. Three patients (3%) developed late rectal toxicity grade 2 and eight patients (8%) experienced late urinary toxicity grade 2; any patients experienced more severe symptoms. We recorded biochemical relapse in two patients, both had poor prognostic factors at initial diagnosis of prostate cancer.

CONCLUSIONS

The data demonstrate the feasibility and safety of high dose radiotherapy for patients with localized prostate cancer and provide a proof that this method allow safe dose escalation with low severe toxicities to the normal tissues.

Evidence-based radiation oncology: Definitive, adjuvant and salvage radiotherapy for non-metastatic prostate cancer

The standard treatment options based on the risk category (stage, Gleason score, PSA) for localized prostate cancer include surgery, radiotherapy and watchful waiting. The literature does not provide clear-cut evidence for the superiority of surgery over radiotherapy, whereas both approaches differ in their side effects. The definitive external beam irradiation is frequently employed in stage T1b-T1c, T2 and T3 tumors. There is a pretty strong evidence that intermediate- and high-risk patients benefit from dose escalation. The latter requires reduction of the irradiated normal tissue (using 3-dimensional conformal approach, intensity modulated radiotherapy, image-guided radiotherapy, etc.). Recent data suggest that prostate cancer may benefit from hypofractionation due to relatively low alpha/beta ratio; these findings warrant confirmation though. The role of whole pelvis irradiation is still controversial. Numerous randomized trials demonstrated a clinical benefit in terms of biochemical control, local and distant control, and overall survival from the addition of androgen suppression to external beam radiotherapy in intermediate- and high-risk patients. These studies typically included locally advanced (T3-T4) and poor-prognosis (Gleason score >7 and/or PSA >20 ng/mL) tumors and employed neoadjuvant/concomitant/adjuvant androgen suppression rather than only adjuvant setting. The ongoing trials will hopefully further define the role of endocrine treatment in more favorable risk patients and in the setting of the dose escalated radiotherapy. Brachytherapy (BRT) with permanent implants may be offered to low-risk patients (cT1-T2a, Gleason score <7, or 3+4, PSA <or=10 ng/mL), with prostate volume of <or=50 ml, no previous transurethral prostate resection and a good urinary function. Some recent data suggest a benefit from combining external beam irradiation and BRT for intermediate-risk patients. EBRT after radical prostatectomy improves disease-free survival and biochemical and local control rates in patients with positive surgical margins or pT3 tumors. Salvage radiotherapy may be considered at the time of biochemical failure in previously non-irradiated patients.

Intensity modulated radiation therapy with irregular multileaf collimated field: a dosimetric study on the penumbra region with different leaf stepping patterns

Using a Varian 21 EX linear accelerator with a multileaf collimator (MLC) of 120 leaves, the penumbra regions of beam profiles within an irregular multileaf collimated fields were studied. MLC fields with different leaf stepping angles from 21.8 degrees to 68.2 degrees were used. Beam profiles in different directions: (1) along the cross-line and in-line axis, (2) along the leaf stepping edges of the field, and (3) parallel to the stepping edges but in the middle of the field, were measured and calculated using Kodak XV radiographic film and Pinnacle3 treatment planning system version 7.4f. These beam profiles were measured and calculated at source to axis distance= 100 cm with 5 cm of solid water slab on top. On the one hand, for both cross-line and in-line beam profiles, the penumbra widths of 20%-80% did not vary with the leaf stepping angles and were about 0.4 cm. On the other hand, the penumbra widths of 10%-90% of the above two profiles varied with the stepping angles and had maximum widths of about 1.9 cm (cross-line) and 1.65 cm (in-line) for stepping angles of 38.7 degrees and 51.3 degrees , respectively. For profiles crossing the "rippled" stepping edges of the field, the penumbra widths (10%-90%) at the regions between two opposite leaves (i.e., profile end at the Y1/Y2 jaw position) decreased with the stepping angles. At the penumbra regions between two leaf edges with the tongue-and-groove structure of the same bank (i.e., profile end at the X1/X2 jaw position), the penumbra widths increased with the stepping angles. When the penumbra widths were measured between two opposite leaf edges and at corners between two leaves, the widths first decreased with the stepping angles and then increased beyond the minimum width point at stepping angle of 45 degrees. The penumbra width (10%-90%) measured at the leaf edge was larger than that at the corner. For the beam profiles calculated using Pinnacle3, it is found that the results agreed well with the measurements along the cross-line and in-line axis, while there was a deviation for the profiles along the leaf stepping edge of the field compared to the film measurements. The measured results in this study can help us to understand the dosimetric effect of the leaf stepping (due to finite leaf width), tongue-and-groove and rounded leaf end structure in the penumbra region of an irregular MLC field. A more dedicated penumbra model can be developed for the treatment planning system.

Dosimetric verification of 6 and 18MV intensity modulated photon beams using a dedicated fluoroscopic electronic portal imaging device (EPID)

BACKGROUND AND PURPOSE

Intensity modulated radiation therapy offers a dose distribution improvement by modulating the two-dimensional X-ray fluence. This increases the treatment complexity from the planning phase to the verification one. Pre-treatment dosimetric verifications of each treatment field are strictly necessary, films and EPIDs are generally used for this purpose. In this paper we investigated the dosimetric characteristics of a commercially available fluoroscopic EPID "I'mRT QA" (Scanditronix-Wellhöfer) (I-QA) based on charge-coupled device camera (CCD) and we present a new method for optimising the EPID's response.

MATERIALS AND METHODS

I-QA is an optically sealed EPID and it is designed for on-line two-dimensional measurements of relative dose distribution. Dose profiles measured in water with diodes and dose distributions measured in water-equivalent phantoms with films were compared with those obtained with the I-QA for homogeneous and intensity modulated 6 and 18 MV photon beams.

RESULTS AND CONCLUSIONS

I-QA measurements depend on the field size and on the two-dimensional energy spectrum of the beam. The incoming beam was modified positioning a series of lead and plastic (RW3) slabs above the fluorescent screen to obtain a homogeneous response of the I-QA over the whole sensitive area. The thickness of lead and RW3 sheets was optimised to get the best matching between diodes, films and the I-QA measurements for each energy. Gamma index evaluations showed a correspondence between I-QA measurements and diode ones within 3% or 2 mm for homogeneous and simple modulated fields, and within 5% or 3mm for complex modulated fields.

Evaluation of dosimetric effect of leaf position in a radiation field of an 80 leaf multileaf collimator fitted to the LINAC head as tertiary collimator

This study evaluates changes in the dosimetric characteristics of a Varian Millennium 80‐leaf multileaf collimator (MLC) in a radiation field. In this study, dose rate, scatter factor, percentage depth dose, surface dose and dose in the buildup region, beam profile, flatness and symmetry, and penumbra width measurements were made for 6‐MV and 15‐MV photon beams. Analysis of widths between 50% dose levels of the beam profiles to reflect the field size at the level of profile measurement shows a significant difference between the fields defined by MLC and/or jaws and MLC (zero gap) and the fields defined by jaws only. The position of the MLC leaves in the radiation field also significantly affects scatter factors. A new relationship has, therefore, been established between the scatter factors and the position of the MLC, which will indeed be useful in the dose calculation for irregular fields. Penumbra widths increase with field size and were higher for fields defined by jaws and/or MLC than jaws and MLC (zero gap) by 1.5 mm to 4.2 mm and 3.8 mm to 5.0 mm, for 6‐MV, and 1.5 mm to 2.4 mm and 3.0 mm to 5.6 mm, for 15‐MV, at 20% to 80% and 10% to 90% levels, respectively. The surface dose and the dose in the buildup region were smaller for fields defined by jaws and MLC (zero gap) than the fields defined by jaws and/or MLC for both photon energies. No significant differences were found in percentage depth dose beyond dmax, beam profiles above 80% dose level, and flatness and symmetry for both energies. The results of this study suggest that while one collects linear accelerator beam data with a MLC, the effects of the positions of the MLC leaves play an important role in dosimetric characteristics of 3D conformal radiation therapy as well as intensity‐modulated radiotherapy.

PACS number: 87.53.Dq

Guidelines for primary radiotherapy of patients with prostate cancer

BACKGROUND AND PURPOSES

The appropriate application of 3-D conformal radiotherapy, intensity modulated radiotherapy or image guided radiotherapy for patients undergoing radiotherapy for prostate cancer requires a standardisation of target delineation as well as clinical quality assurance procedures.

PATIENTS AND METHODS

Pathological and imaging studies provide valuable information on tumour extension. In addition, clinical investigations on patient positioning and immobilisation as well as treatment verification data offer an abundance of information.

RESULTS

Target volume definitions for different risk groups of prostate cancer patients based on pathological and imaging studies are provided. Available imaging modalities, patient positioning and treatment preparation studies as well as verification procedures are collected from literature studies. These studies are summarised and recommendations are given where appropriate.

CONCLUSIONS

On behalf of the European Organisation for Research and Treatment of Cancer (EORTC) Radiation Oncology Group this article presents a common set of recommendations for external beam radiotherapy of patients with prostate cancer.

SWIMRT: a graphical user interface using sliding window algorithm to construct a fluence map machine file

A custom‐made computer program, SWIMRT, to construct “multileaf collimator (MLC) machine” file for intensity‐modulated radiotherapy (IMRT) fluence maps was developed using MATLAB® and the sliding window algorithm. The user can either import a fluence map with a graphical file format created by an external treatment‐planning system such as Pinnacle3 or create his or her own fluence map using the matrix editor in the program. Through comprehensive calibrations of the dose and the dimension of the imported fluence field, the user can use associated image‐processing tools such as field resizing and edge trimming to modify the imported map. When the processed fluence map is suitable, a “MLC machine” file is generated for our Varian 21 EX linear accelerator with a 120‐leaf Millennium MLC. This machine file is transferred to the MLC console of the LINAC to control the continuous motions of the leaves during beam irradiation. An IMRT field is then irradiated with the 2D intensity profiles, and the irradiated profiles are compared to the imported or modified fluence map. This program was verified and tested using film dosimetry to address the following uncertainties: (1) the mechanical limitation due to the leaf width and maximum traveling speed, and (2) the dosimetric limitation due to the leaf leakage/transmission and penumbra effect. Because the fluence map can be edited, resized, and processed according to the requirement of a study, SWIMRT is essential in studying and investigating the IMRT technique using the sliding window algorithm. Using this program, future work on the algorithm may include redistributing the time space between segmental fields to enhance the fluence resolution, and readjusting the timing of each leaf during delivery to avoid small fields. Possible clinical utilities and examples for SWIMRT are given in this paper.

PACS numbers: 87.53.Kn, 87.53.St, 87.53.Uv

Localized Prostate Cancer Treated with Intensity-Modulated Radiotherapy

Aims and background

The development and use of new radiotherapy techniques, especially 3D conformal radiotherapy or intensity-modulated radiotherapy, has allowed the safe application of high doses of external beam radiotherapy without increasing toxicity. The aim of this analysis was to describe the acute and when possible late toxicity and the feasibility on using intensity-modulated radiotherapy into our routine work.

Patients and methods

From June 2003 to December 2004, 60 patients with prostate cancer underwent high dose (80 Gy) radiotherapy treatment with intensity-modulated radiotherapy at the University of Florence. In the current analysis, we included patients without clinical or radiographic evidence of distant disease at the time of the first evaluation in the radiotherapy unit.

Results

Intensity-modulated radiotherapy treatments were delivered successfully without any interruption or technical problem. High-dose intensity-modulated radiotherapy was well tolerated acutely. Four patients (10%) developed grade 1 late rectal toxicity after completion of intensity-modulated radiotherapy and 8 patients (20%) developed grade 1 late urinary symptoms.

Conclusions

Intensity-modulated radiotherapy is the approach of choice for high-dose radiotherapy delivery. No patient had severe toxicity (grade 3) despite the high dose delivered. From a cost-benefit point of view, our experience shows that delivery of intensity-modulated radiotherapy requires only minor corrections to the ordinary activity schedule.

The role of whole pelvic radiotherapy in locally advanced prostate cancer

Routine PSA testing has led to diagnosis and treatment of prostate cancer at earlier stages than previously. Earlier and technically-improved treatment, together with escalation of dose has enhanced cure rates. Although, the incidence of nodal metastases is now lower than in pre-PSA days, more extended pelvic lymphadenectomies have shown the actual rate of lymph node involvement to be higher than had been determined from standard radical prostate surgery. As in cancers in other sites, especially in their earlier stages, lymph node metastases may exist in the absence of haematogenous dissemination. This, together with the improved rates of control of the primary prostate tumour, suggests that elective irradiation of early-stage lymph nodes from prostate cancer should enhance survival in a manner analogous to improvements seen with this approach in other cancers. Although, the absolute incidence of positive nodes in locally advanced prostate cancer warrants elective radiotherapy, it is relatively low and the modest improvements to be expected may be undetected in the results of a small trial.

Dosimetry limitations and a dose correction methodology for step-and-shoot IMRT

For the step-and-shoot intensity-modulated radiation therapy (IMRT) technique, the combination of high dose rate, multiple beam segments and low dose per segment can lead to significant differences between the planned dose and the dose delivered to the patient. In this technique, a dose delivery inaccuracy known as the 'overshoot' effect is caused by the dose servo control system. This typically occurs in the first and last beam segments and causes an over- and underdose, respectively. Some dose positional error in the segment sequence is also possible there. Commercial ionization chambers (RK-type) and radiographic Kodak films were used for the measurements. The reported results were obtained using the Pinnacle(3)-V6.2 treatment planning system and a Varian Clinac 21 EX linear accelerator equipped with a 120-leaf Millennium MLC. The dose inaccuracy measurements were based on the comparison of the dose and profiles for reference fields and fields irradiated with the step-and-shoot technique. For our linear accelerators, an 'overshoot' effect ranging from 0.1 to 0.6 MU was found, corresponding to a dose rate from 100 to 600 MU min(-1), respectively. For segments with off-axis distances from 0 to 5.5 cm with >3.5 MU per segment and all dose rates, a MLC leaf-position error of <1 mm was measured. For segments with an off-axis distance of 9.5 cm, a positional error >2 mm was measured for 600 MU min(-1) and 1 MU per segment. The purpose of this study was to find a correction method for segmental dose errors caused by the 'overshoot' effect when small monitor unit and high dose rate are used. To better represent the fluctuation of the segment doses in the beam, a dose ratio between reference and step-and-shoot irradiated fields was defined. A method for the correction of segment dose inaccuracies and a quality assurance programme for the 'overshoot' effect were developed. The ordering of the biggest segment shape in the segment sequence was studied for ten randomly selected prostate patients planned for IMRT. The results of this work can be used to improve the agreement between the planned and delivered doses for IMRT.

The impact of introducing intensity modulated radiotherapy into routine clinical practice

BACKGROUND AND PURPOSE

Intensity modulated radiotherapy (IMRT) at the Royal Marsden Hospital London was introduced in July 2001. Treatment delivery was dynamic using a single-phase technique. Concerns were raised regarding increased clinical workload due to introduction of new technology. The potential increased use of resources was assessed.

PATIENTS AND METHODS

IMRT patient selection was within guidelines of clinical trials and included patients undergoing prostate plus pelvic lymph node (PPN) irradiation and head and neck cancer (HNC) treatment. Patient planning, quality assurance and treatment times were collected for an initial IMRT patient group. A comparative group of patients with advanced HNC undergoing two- or three-phase conventional radiotherapy, requiring matched photon and electron fields, were also timed.

RESULTS

The median overall total planning time for IMRT was greater for HNC patients compared to the PPN cohort. For HNC the overall IMRT planning time was significantly longer than for conventional. The median treatment time for conventional two- or three-phase HNC treatments, encompassing similar volumes to those treated with IMRT, was greater than that for the IMRT HNC patient cohort. A reduction in radiographer man hours per patient of 4.8h was recorded whereas physics time was increased by 4.9h per patient.

CONCLUSIONS

IMRT currently increases overall planning time. Additional clinician input is required for target volume localisation. Physics time is increased, a significant component of this being patient specific QA. Radiographer time is decreased. For HNC a single phase IMRT treatment has proven to be more efficient than a multiple phase conventional treatment. IMRT has been integrated smoothly and efficiently into the existing treatment working day. This preliminary study suggests that IMRT could be a routine treatment with efficient use of current radiotherapy resources.

La radiothérapie de conformation avec et sans modulation d'intensité dans le traitement du cancer localisé de la prostate

Conformal radiation therapy has now to be considered as a standard treatment of localized prostatic adenocarcinomas. Using conformational methods and intensity modulated radiation therapy requires a rigorous approach for their implementation in routine, focused on the reproducibility of the treatment, target volume definitions, dosimetry, quality control, setup positioning. In order to offer to the largest number of patients high-dose treatment, the clinicians must integrate as prognostic factors accurate definition of microscopic extension as well as the tolerance threshold of critical organs. High-dose delivery is expected to be most efficient in intermediary risks and locally advanced diseases. Intensity modulated radiation therapy is specifically dedicated to dose escalation. Perfect knowledge of classical constraints of conformal radiation therapy is required. Using such an approach in routine needs a learning curve including the physicists and a specific quality assurance program.

Dosimetric effect of collimating jaws for small multileaf collimated fields

The dosimetric effects from the jaw positioned close to the small field (0.5 x 0.5, 1 x 1, and 2 x 2 cm2) side-edge generated by a single-focused multileaf collimator (MLC) were measured and studied. The measurement is important in intensity modulated radiotherapy (IMRT) because generally the jaw cannot perfectly cover all the leaf-ends in a segment of irregular field. This leads to additional dose contributed by (1) the end surface of the jaw, (2) the leaf-end, and (3) the inter- and intraleaf leakage/transmissions during the dosimetric measurement. Moreover, most of the conventional treatment planning systems ignore these effects in the dose calculation. In this study, measurements were made using a Varian 21 EX linear accelerator with 6 MV photon beam through a MLC containing 120 leaves. Percentage depth dose, beam profile, and output for small fields were measured by varying the jaw at different positions away from the leaf-ends in the field side-edge. Moving the jaw away from the leaf-ends increases the output and penumbra width for the small fields. Such increase is particularly significant when the field size is small (0.5 x 0.5 cm2) and the degree of increase changes quickly when the jaw-end is at about 1-2 cm from the leaf-end. It is suggested that measurements should be carried out in the IMRT commissioning to provide information to physicists in reviewing the treatment planning system's accuracy regarding leaf leakage/transmission and jaw effects.

Characterization of a 2D ion chamber array for the verification of radiotherapy treatments

In this study we investigated the characteristics of a commercial ion chamber array and its performance in the verification of radiotherapy plans. The device was the 2D Array Seven29 model (PTW, Freiburg, Germany). This is a two-dimensional detector array with 729 ionization chambers uniformly arranged in a 27 x 27 matrix with an active area of 27 x 27 cm(2). The detector short-, medium- and long-term reproducibility have been tested through an extensive set of repeated measurements. Short-term reproducibility was well within 0.2%. Medium- and long-term reproducibility were within 1%, including set-up reproducibility errors and linac output fluctuations. Dose linearity was also assessed. The system response to dose was verified to be linear within the range 2-500 MU. Output factors matched very well pinpoint chamber measurements performed in the same experimental conditions with a maximum local percentage difference of 0.4%. Furthermore, the 2D Array sensitivity to millimetric collimator positional changes and to perturbation effect of irradiated area was tested. The comparison with ion chamber data carried out in water was very satisfying. Finally, measurements of wedge-modulated fields and IMRT beam sequence matched very well ion chamber dose profiles acquired in a water tank. The extensive tests performed in this investigation show that the 2D Array Seven29 is a reliable and accurate dosimeter and that it could be a useful tool for the quality assurance and the verification of radiotherapy plans.

Theragnostic imaging for radiation oncology: dose-painting by numbers

Theragnostic imaging for radiation oncology is the use of molecular and functional imaging to prescribe the distribution of radiation in four dimensions-the three dimensions of space plus time-of radiotherapy alone or combined with other treatment modalities in an individual patient. Several new imaging targets for positron-emission tomography, single-photon-emission CT, and magnetic resonance spectroscopy allow variations in microenvironmental or cellular phenotypes that modulate the effect of radiation to be mapped in three dimensions. Dose-painting by numbers is a strategy by which the dose distribution delivered by inverse planned intensity-modulated radiotherapy is prescribed in four dimensions. This approach will revolutionise the way that radiotherapy is prescribed and planned and, at least in theory, will improve the therapeutic outcome in terms of local tumour control and side-effects to unaffected tissue.