In vivo estimation of midline dose maps by transit dosimetry in head and neck radiotherapy

Radiochromic film based transit dosimetry for verification of dose delivery with intensity modulated radiotherapy

PURPOSE

To evaluate the transit dose based patient specific quality assurance (QA) of intensity modulated radiation therapy (IMRT) for verification of the accuracy of dose delivered to the patient.

METHODS

Five IMRT plans were selected and utilized to irradiate a homogeneous plastic water phantom and an inhomogeneous anthropomorphic phantom. The transit dose distribution was measured with radiochromic film and was compared with the computed dose map on the same plane using a gamma index with a 3% dose and a 3 mm distance-to-dose agreement tolerance limit.

RESULTS

While the average gamma index for comparisons of dose distributions was less than one for 98.9% of all pixels from the transit dose with the homogeneous phantom, the passing rate was reduced to 95.0% for the transit dose with the inhomogeneous phantom. Transit doses due to a 5 mm setup error may cause up to a 50% failure rate of the gamma index.

CONCLUSIONS

Transit dose based IMRT QA may be superior to the traditional QA method since the former can show whether the inhomogeneity correction algorithm from TPS is accurate. In addition, transit dose based IMRT QA can be used to verify the accuracy of the dose delivered to the patient during treatment by revealing significant increases in the failure rate of the gamma index resulting from errors in patient positioning during treatment.

Application of a practical method for the isocenter pointin vivodosimetry by a transit signal

This work reports the results of the application of a practical method to determine the in vivo dose at the isocenter point, D(iso), of brain thorax and pelvic treatments using a transit signal S(t). The use of a stable detector for the measurement of the signal S(t) (obtained by the x-ray beam transmitted through the patient) reduces many of the disadvantages associated with the use of solid-state detectors positioned on the patient as their periodic recalibration, and their positioning is time consuming. The method makes use of a set of correlation functions, obtained by the ratio between S(t) and the mid-plane dose value, D(m), in standard water-equivalent phantoms, both determined along the beam central axis. The in vivo measurement of D(iso) required the determination of the water-equivalent thickness of the patient along the beam central axis by the treatment planning system that uses the electron densities supplied by calibrated Hounsfield numbers of the computed tomography scanner. This way it is, therefore, possible to compare D(iso) with the stated doses, D(iso,TPS), generally used by the treatment planning system for the determination of the monitor units. The method was applied in five Italian centers that used beams of 6 MV, 10 MV, 15 MV x-rays and (60)Co gamma-rays. In particular, in four centers small ion-chambers were positioned below the patient and used for the S(t) measurement. In only one center, the S(t) signals were obtained directly by the central pixels of an EPID (electronic portal imaging device) equipped with commercial software that enabled its use as a stable detector. In the four centers where an ion-chamber was positioned on the EPID, 60 pelvic treatments were followed for two fields, an anterior-posterior or a posterior-anterior irradiation and a lateral-lateral irradiation. Moreover, ten brain tumors were checked for a lateral-lateral irradiation, and five lung tumors carried out with three irradiations with different gantry angles were followed. One center used the EPID as a detector for the S(t) measurement and five pelvic treatments with six fields (many with oblique incidence) were followed. These last results are reported together with those obtained in the same center during a pilot study on ten pelvic treatments carried out by four orthogonal fields. The tolerance/action levels for every radiotherapy fraction were 4% and 5% for the brain (symmetric inhomogeneities) and thorax/pelvic (asymmetric inhomogeneities) irradiations, respectively. This way the variations between the total measured and prescribed doses at the isocenter point in five fractions were well within 2% for the brain treatment, and 4% for thorax/pelvic treatments. Only 4 out of 90 patients needed new replanning, 2 patients of which needed a new CT scan.

Image-guided in vivo dosimetry for quality assurance of IMRT treatment for prostate cancer

PURPOSE

In external beam radiotherapy (EBRT) and especially in intensity-modulated radiotherapy (IMRT), the accuracy of the dose distribution in the patient is of utmost importance. It was investigated whether image guided in vivo dosimetry in the rectum is a reliable method for online dose verification.

METHODS AND MATERIALS

Twenty-one dose measurements were performed with an ionization chamber in the rectum of 7 patients undergoing IMRT for prostate cancer. The position of the probe was determined with cone beam computed tomography (CBCT). The point of measurement was determined relative to the isocenter and relative to an anatomic reference point. The dose deviations relative to the corresponding doses in the treatment plan were calculated. With an offline CT soft-tissue match, patient positioning after ultrasound was verified.

RESULTS

The mean magnitude +/- standard deviation (SD) of patient positioning errors was 3.0 +/- 2.5 mm, 5.1 +/- 4.9 mm, and 4.3 +/- 2.4 mm in the left-right, anteroposterior and craniocaudal direction. The dose deviations in points at corresponding positions relative to the isocenter were -1.4 +/- 4.9% (mean +/- SD). The mean dose deviation at corresponding anatomic positions was 6.5 +/- 21.6%. In the rare event of insufficient patient positioning, dose deviations could be >30% because of the close proximity of the probe and the posterior dose gradient.

CONCLUSIONS

Image-guided dosimetry in the rectum during IMRT of the prostate is a feasible and reliable direct method for dose verification when probe position is effectively controlled.

Changes in lateral dimensions of irradiated volume and their impact on the accuracy of dose delivery during radiotherapy for head and neck cancer

BACKGROUND AND PURPOSE

To assess changes in lateral dimensions of irradiated volume during head and neck cancer radiotherapy and to determine their impact on the accuracy of dose delivery.

PATIENTS AND METHODS

Lateral dimensions of irradiated volumes were measured in five predefined points prior to treatment and then bi-weekly. For each measurement, midline dose was calculated and verified using in vivo dosimetry. Early radiation reactions, patient weight changes and the need to modify radiotherapy accessories were also recorded. The study included 33 head and neck cancer patients irradiated using parallel opposed megavoltage fields.

RESULTS

Body mass changes during radiotherapy ranged from -18 to +4 kg (median -5). Lateral dimension changes >5 mm (range -37 to +16) occurred in 32 patients (97%). For axis measurements, the degree of lateral dimension changes were correlated with treatment field size (P = 0.022) and degree of mucositis (P = 0.017). Axis doses calculated for changed dimensions varied from those prescribed by -2.5 to +6% (median +2%). Differences larger than 5% were present in 4.8% of calculations. In 17 patients (52%), radiotherapy accessories had to be modified during treatment. The need to modify radiotherapy accessories correlated with larger treatment portals (P = 0.004), more weight loss during treatment (P = 0.01) and higher initial N stage (P = 0.04).

CONCLUSIONS

Changes of irradiated volume lateral dimensions during head and neck cancer radiotherapy may lead to considerable dose delivery inaccuracies. Watchful monitoring, corrections to calculated dose when changes observed are significant and radiotherapy accessories modification during the course of treatment are strongly recommended.

High-tech in radiation oncology: should there be a ceiling?

PURPOSE

To analyze some of the limitations to improvement of the outcome of radiotherapy (RT) expected from the introduction of sophisticated treatment planning and delivery technology.

METHODS AND MATERIALS

Several recent examples from the literature were analyzed in some detail. Mathematical modeling techniques were used to assess the likely clinical impact of new technologies or biologic principles. The findings of recent randomized controlled trials of RT for prostate, breast, and rectal cancer were analyzed from the perspective of cost-effectiveness and therapeutic gain.

RESULTS

The main findings of the analyses may be summarized as follows. Dosimetric precision should aim for a <2% patient-to-patient variability in the delivered dose. Imprecision in clinical target volume definition remains an obstacle for high-precision RT. Functional imaging and novel biologic assays may facilitate a move from a clinical target volume to the real target volume. Improved target volume coverage is mainly important if RT has high effectiveness. Radiation oncology is increasingly becoming evidence based. However, there is still a long way to go. Hypofractionation in adjuvant RT for breast cancer may represent a favorable balance between cost and benefit. Treatment complications are potentially associated with both suffering and high cost. The identification of high-risk patients would improve the cost-effectiveness of high-tech RT aimed at avoiding complications. Conformal RT may allow the introduction of hypofractionation, which, again, could potentially save resources. With improvement in surgery and more screening-detected cancer cases, the number needed to treat increases, and this will directly affect the cost-effectiveness of high-tech RT unless efficient patient selection can be developed.

CONCLUSION

Sustained technological refinement is only likely to be cost-effective if the clinical and biologic understanding of patient-to-patient variability in the risk of specific types of failure and the optimal multimodality approach to handle these risks is developed at the same time. Mathematical modeling together with methods from health technology assessment and health economics are useful complements to standard methods from evidence-based medicine. Progress in functional imaging and in basic and clinical cancer biology is likely to provide the tools required for individualized risk-adapted RT.

IMRT: delivery techniques and quality assurance

Intensity modulated radiotherapy (IMRT) is a major development in the delivery of radiation therapy that has the potential to improve patient outcome by reducing morbidity or increasing local tumour control. Delivery techniques include those based on purpose built devices and treatment machines together with those utilizing the capabilities of computer controlled multileaf collimators which are more widely available. The complexity of IMRT techniques demands a high level of quality control both in the operation of the equipment and in the delivery of treatment to individual patients. The purpose of this paper is therefore to review the techniques available, concentrating on the use of multileaf collimators, and to consider the necessary quality control requirements for clinical application. It demonstrates that the technology is mature and sufficiently well understood so that IMRT can be safely implemented in the general clinical environment rather than being limited to application in the research environment.