Dosimetric evaluation of patient setup errors due to uncertainties during IMRT for head and neck cancer cases

Received: 02-Feb-2021 Accepted: 25-Mar-2021 Published: 15-Apr-2021

Introduction

Radiation Therapy (RT) is commonly used as part of multiple modality treatment for prostate cancer. Intensity-Modulated Radiation Therapy (IMRT) has become increasingly popular because dose escalation to the target can be done while sparing adjacent normal tissues. Several factors such as the accuracy of the immobilization device change in body contours, and tumor regression could lead to setup uncertainties during RT, all of these factors need to be minimized with the use of special approaches. Image-Guided Radiation Therapy (IGRT) can be used to correct and quantify geometrical uncertainties for daily setup [1].

IMRT target contours in three dimensions, often with six independent values~anterior, posterior, medial, lateral, superior, inferior [2]. Intensity-modulated radiation therapy tends to produce steep absorbed-dose gradients between the target volume and the OAR. Having realistic margins for both the tumor volume and any OAR. Factors affecting margin requirements to define the PTV include uncertainty of patient positioning, mechanical uncertainty of the equipment (e.g. gantry sagging), dosimetric uncertainties (e.g. penetration of the beam), the use of motion management techniques such as gating, image transfer errors from CT and simulator to the treatment unit, and human factors. These factors will vary from center to center, and, within a given center, from machine to machine and from patient to patient. The use of patient immobilization devices, the application of quality-assurance programs, and the skill and experience of the radiographers/radiotherapists are also important and must be taken into account. Additionally, the use of different image-guidance systems or other uncertaintyreduction techniques can significantly alter the size of the required margins.

An error is defined as; The difference between the measured (observation) value and the actual (true) value [3-12].

Errors can be divided into three categories:

• Personal Error

• Systematic Error

• Random Error

Systematic Error

The type of error arises due to defect in the measuring device or its data handling system, or because the instrument is wrongly used by the experimenter. Generally, it is called "ZERO ERROR". It may be positive or negative error and can be removed by correcting measurement device. [13-14].

Systematic errors may be of four kinds:

• Instrumental: For example, a poorly calibrated instrument such as a thermometer that reads 1020C when immersed in boiling water and 20C when immersed in ice water at atmospheric pressure. Such a thermometer would result in measured values that are consistently too high.

• Observational: For example, parallax in reading a meter scale.

• Environmental: For example, an electrical power that causes measured currents to be consistently too low.

• Theoretical: Due to simplification of the model system or approximations in the equations describing it. For example, if your theory says that the temperature of the surrounding will not affect the readings taken when it actually does, then this factor will introduce a source of error [13].

Random Error

The error produced due to sudden change in experimental conditions. For example: During sudden change in temperature, change in humidity, fluctuation in potential difference (voltage). It is an accidental error and is beyond the control of the person making measurement. Random errors are positive and negative fluctuations that cause about one-half of the measurements to be too high and one-half to be too low. Sources of random errors cannot always be identified. Possible sources of random errors are as follows:

• Observational. For example, errors in judgment of an observer when reading the scale of a measuring device to the smallest division.

• Environmental. For example, unpredictable fluctuations in line voltage, temperature, or mechanical vibrations of equipment.

Random errors, unlike systematic errors, can often be quantified by statistical analysis; therefore, the effects of random errors on the quantity or physical law under investigation can often be determined. The precision is limited by the random errors. It may usually be determined by repeating the measurements.

Random errors are errors which fluctuate from one measurement to the next. They yield results distributed about some mean value. They can occur for a variety of reasons. They may occur due to lack of sensitivity. For a sufficiently a small change an instrument may not be able to respond to it or to indicate it or the observer may not be able to discern it. They may occur due to noise. There may be extraneous disturbances which cannot be taken into account. They may be due to imprecise definition. They may also occur due to statistical processes such as the roll of dice [9,12].

This study was done to determine the error during RT on positioning displacement for patients treated for Head and Neck. We tried to find the magnitude of daily setup errors to determine the set up error cases. The results can help physicians to determine the most suitable margin for H&N cases.

Methods

Patients

We used data from 20 patients treated for Head and Neck at Our radiotherapy center. All patients received IGRT with daily on-line kilovoltage imaging with weekly Cone Beam Computed Tomography (CBCT) to correct the treatment position. No patients had ART planning before a prescribed dose of (70) Gy.

Treatment Planning

To enhance the accuracy of the daily irradiated position, simulation using a Computed Tomographic (CT) scan simulator (GE ct) was made. The scans consisted of a protocol with a 2.5-mm-slice thickness, and Marks on the patients’ skin were drawn using setup lasers to facilitate an accurate daily position.

For patients receiving definitive RT, the Clinical Target Volume (CTV) was defined as the gross tumor volume plus a margin of 7 mm posteriorly, and 10 mm in all other directions. All patients underwent IMRT plans and all plans were carried out using a commercial radiation treatment planning system (Elekta).

Treatment verification

All patients were treated with IGRT with linear accelerator equipped with an on-line On-Board Imaging (OBI) function including two-dimensional (2D) kilovoltage (kV) images and three-dimensional (3D) CBCT. The technicians set up the patients on a couch in the simulation room according to the marks drawn on their bodies. On-line OBI images (2D kV images daily and 3D CBCT weekly) were taken and sent to the station where they could be registered to digitally reconstructed radiographs from the treatment planning images. Two technicians compared these paired images by correlating the bony anatomy and corrected the difference by shifting the couch translationally before treatment. Then, physician confirmed the corrected on-line images. Anatomic reference landmarks included at least three visible bony structures.

Setup displacement

After image registration, quantification of alignment data for daily OBI in the Superior-Inferior (SI), Anterior-Posterior (AP), and Medial-Lateral (ML) directions, and Couch Rotation (CR) for all patients were collected. For each direction, the recorded setup displacements were composed of two components, Systematic Errors (SE) and Random Errors (RE). The SE was the deviation between the simulated patient position and the average patient position, while the RE was that which occurred between different fractions.

Results

Setup errors are modelled as shifts of the beam isocenter. A shift of the beam isocenter leads to a non-rigid shift of the dose distribution. Small setup errors might thus lead to large displacements of spots that travel close to and in parallel with steep density gradients, such as along bone edges.in our study average data from 20 patient during all fractions calculated in table (1).The Right/Left direction as X direction, Anterior/Posterior direction as Y shift and Up/Down direction as Z shift as shown. Then drawing the relation between X, Y and Z shift as a function of no. of fractions then average shift for all fractions calculated to be (0.05) cm, ( 0.08) cm and (-0.02) cm as RT/LT , Ant./Post. And Up/Down shifts.

Discussion

Several studies have investigated setup uncertainty in H&N cancer patients [8]. According to the International Commission on Radiation Units and Measurements report 62 [5], an inappropriate definition of the CTV-PTV margin, accounting for organ motion and setup uncertainties, may yield an under-dose to the CTV. Organ motion could be neglected, while variability due to inadequate setup or deformity must be carefully considered. In clinical practice, use of daily IGRT is not always possible because of limited facilities in some countries as well as concerns about increased daily doses to patients [4]. Because of RT treatment for H and N cancer, anatomical modifications due to tumor regression led to geometric change of tumor volume and organs at risk and Margins in the three translation directions should be done.

Generally, our study record shifts during all fractions (shown in Tables 1-33).

Table 1. Bet Shifts and no. of fractions. Shifts were found that it range from 1 mm to 4 mm in X- direction and from 1 mm to 2 mm in Y and from 2 mm to 5 mm in Z direction (between + and in all directions) . Then we applied shifts to plan on planning system and recording differences occurred on DVH’s of targets and critical organs

Table 2. Changes occurred to dose delivered to 98% volume of PTV45 with different shifts

Table 3. Changes occurred to dose delivered to 98% volume of PTV60 with different shifts

Table 4. Changes occurred to dose delivered to 98% volume of PTV70 with different shifts

Table 5. Changes Max dose of LT lens with different shifts

Table 6. Changes occurred to Max dose of RT lens with different shifts

Table 7. Changes occurred to Max dose of Optic Chiasm with different shifts

Table 8. Changes occurred to Max dose of Brain stem with different shifts

Table 9. Changes occurred to Max dose of Spinal cord with different shifts

Table 10. Changes occurred to Max dose of optic chiasm PRV with different shifts

Table 11. Changes occurred to Max dose of Brain stem PRV with different shifts

Table 12. Changes occurred to Max dose S.C. PRV with different shifts

Table 13. Changes occurred to Max dose of LT. Parotid with different shifts

Table 14. Changes occurred to Max dose of RT. Parotid with different shifts

Table 15. Changes occurred to Mean dose of LT. Cochlea with different shifts

Table 16. Changes occurred to Mean dose of RT. Cochlea with different shifts

Table 17. Changes occurred to Max dose of Mandible with different shifts

Table 18. Changes occurred to Max dose of Lt.Temp.with different shifts

Table 19. Changes occurred to Max dose of RT. Temp with different shifts

Table 20. Changes occurred to Max dose of Lt .Eye with different shifts

Table 21. Changes occurred to Max dose of RT Eye with different shifts

Table 22. Changes occurred to max dose of LT optic nerve with different shifts

Table 23. Changes occurred to Max dose of RT Optic Nerve with different shifts

Table 24. Changes occurred to Max dose of LT TMJ with different shifts

Table 25. Changes occurred to Max dose of RT TMJ with different shifts

Table 26. Changes occurred to Mean dose of Thyroid gland with different shifts

Table 27. Changes occurred to Mean dose of Esophageal inlet with different shifts

Table 28. Changes occurred to Mean dose of Cervical Esophagus with different shifts

Table 29. Changes occurred to Mean dose of Base of tongue with different shifts

Table 30. Changes occurred to Mean dose of Lt Carotid with different shifts

Table 31. Changes occurred to Mean dose of Rt Carotid with different shifts

Table 32. Illustrate Changes occurred in critical organs according to shifts

Table 33. Illustrate changes occurred in critical organs according to shifts

This study recommends on-line IGRT for patients receiving RT to deliver more accurate dose to tumor and avoid extra dose to organs at risk due to anatomical change also according to (11-12) shifts in all direction reduced when using on line image guided leading to reduce margins in all direction surrounding the tumor and saving critical organs.

The primary objective of the study was to measure inter-fraction setup variation in head and neck cancer patients undergoing. Displacements of portal images from CT images, set as reference images, were measured for calculating errors are related to any accidental error during setup, due to mis-positioning of the patient in the mask, movements of the patient or organ motion in the period between positioning and start of irradiation or during irradiation. Naiyanet, N. et al [10] reported the L-R, S-I and A-P axes. While our study has shown the errors along the L-R, S-I and A-P axes) that 0.05 mm, 0.08 mm and -0.02 mm. Large systematic errors lead to a large under-dosage.

The secondary objective of the present study was to define adequate CTV-to-PTV margin for IMRT of head and neck cancer in our department. Ideally, the CTV-to-PTV margin should be determined solely by the magnitudes of the uncertainties involved. In practice, the clinician usually also considers abutting healthy tissues when deciding on the size of the CTV-to-PTV margin.

Generally, our study record shifts during all fractions (shown in Tables 1) & then illustrated the differences occurred in target and critical organs according to shifts that applied to plan on planning system. We found that all targets dose reduced with increasing shifts.

Conclusion

In this examine, the scientific effectiveness of planned and delivered dose distributions of IMRT technique for head-andneck cancer became evaluated the usage of both physical and dose constraints criteria. The distinction between the “one-toall” and “cascade” dose distributions became small, statistically insignificant, and really near the values of the corresponding treatment plans. However, for a fraction of the sufferers and given OAR, the differences among the added and deliberate doses had been mainly large. These findings aid the necessity of the correct affected person setup earlier than the treatment the usage of IGRT, as a result minimizing dose inaccuracy mistakes. We recommend reducing setup errors in patients with Head and Neck cancer receiving RT, the use of on-line image-guided radiotherapy is recommended to increase accuracy.

List of Abbreviations

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