J. J. Battista

Lung dose corrections for 6‐ and 15‐MV x rays

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.

Electron dose distributions in experimental phantoms: a comparison with 2D pencil beam calculations

Dose distributions were measured and computed within inhomogeneous phantoms irradiated with beams of electrons having initial energies of 10 and 18 MeV. The measurements were made with a small p-type silicon diode and the calculations were performed using the pencil beam algorithm developed originally at the M D Anderson Hospital (MDAH). This algorithm, which is available commercially on many radiotherapy planning computers, is based on the Fermi-Eyges theory of electron transport. The phantoms used in this work were composed of water into which two- and three-dimensional inhomogeneities of aluminum and air (embedded in wax) were introduced. This was done in order to simulate the small bones and the air cavities encountered clinically in radiation therapy of the chest wall or neck. Our intent was to test the adequacy of the two-dimensional implementation of the pencil beam approach. The agreement between measured and computed doses is very good for inhomogeneities which are essentially two-dimensional but discrepancies as large as 40% were observed for more complex three-dimensional inhomogeneities. We can only trace the discrepancies to the complex interplay of numerous approximations in the Fermi-Eyges theory of multiple scattering and its adaptation for practical computer-aided radiotherapy planning.

Experimental evaluation of a 2D and 3D electron pencil beam algorithm

Verification of electron beam treatment-planning algorithms in the presence of heterogeneities can be very difficult. Using controlled geometries to minimise physical uncertainties in geometric alignment and composition, a large number of measurements were made to test the performance of a 2D and 3D electron pencil beam algorithm. A Therados RFA-3 beam-scanning system interfaced to a microcomputer was used to measure the dose distributions. The geometric arrangement consisted of single and double rods 1 cm in diameter situated just below the surface of a unit density phantom. The electron densities (relative to water) of the rods ranged from 2.12 (aluminium) to 1.29 (soft bone analogue), and their length could be varied between 1 cm and 10 cm. Measured isodose distributions beyond the inhomogeneities were compared with those predicted theoretically. Calculations were performed on a VAX-11/780 using 2D and 3D implementations of the Hogstrom electron pencil beam algorithm. The authors report on the nature of this 3D implementation and assess the magnitude of discrepancies between calculation and measurement for 10 MeV and 18 MeV electron beams, and for the variety of phantom compositions and geometries identified above.

Improved lung dose calculation using tissue-maximum ratios in the Batho correction.

We have reexamined the Batho power law for computing the dose within and beyond lung irradiated with small and large fields of cobalt-60 and 6-MV x rays. Using slab phantoms consisting of two materials, agreement between calculated and measured doses was within 2% inside lung for 6-MV x irradiation, but much poorer (9%) for cobalt-60 irradiation. For cobalt-60 irradiation, tissue-air ratios (TARs) were used initially in the Batho equation, while for 6-MV x rays, tissue-maximum ratios (TMRs) were used. When we substituted TMR values instead of TAR values for cobalt-60, we found marked improvement by nearly 5% in the accuracy of dose calculated within lung. This was confirmed by numerical comparison of the Batho expression with an analytic solution of the primary and first-scattered radiation. We therefore encourage the use of TMRs for cobalt-60 radiation, especially for larger radiation fields, and provide measured data tables for field sizes up to 50 X 50 cm2, and depths up to 30 cm. In addition to unifying the dosimetry for all megavoltage irradiation, this approach improves the accuracy of doses calculated within lung.

The density of mouse lung in vivo following X irradiation

The lungs of mice were irradiated with single X radiation doses of 5 to 14 Gy. Six weeks after irradiation, computed tomographic (CT) scans of the mice were performed at two-week intervals. Beyond 14 weeks after irradiation, the animals were scanned at 1-week intervals. The mice irradiated to 5 and 7 Gy exhibited no change in lung density, in comparison with the unirradiated lungs of control mice up to times of 48 weeks. The mice irradiated to doses of greater than 10 Gy exhibited marked increases in lung density at 15 weeks after irradiation. Increases in density followed a similar time course for these doses, but the magnitude of the density increase was dependent on the radiation dose. An interpretation of these findings in terms of radiation pneumonitis is presented, and the possibility of using CT to monitor lung density in radiotherapy patients is discussed.

Schedule for CT image guidance in treating prostate cancer with helical tomotherapy

The aim of this study was to determine the effect of reducing the number of image guidance sessions and patient-specific target margins on the dose distribution in the treatment of prostate cancer with helical tomotherapy. 20 patients with prostate cancer who were treated with helical tomotherapy using daily megavoltage CT (MVCT) imaging before treatment served as the study population. The average geometric shifts applied for set-up corrections, as a result of co-registration of MVCT and planning kilovoltage CT studies over an increasing number of image guidance sessions, were determined. Simulation of the consequences of various imaging scenarios on the dose distribution was performed for two patients with different patterns of interfraction changes in anatomy. Our analysis of the daily set-up correction shifts for 20 prostate cancer patients suggests that the use of four fractions would result in a population average shift that was within 1 mm of the average obtained from the data accumulated over all daily MVCT sessions. Simulation of a scenario in which imaging sessions are performed at a reduced frequency and the planning target volume margin is adapted provided significantly better sparing of organs at risk, with acceptable reproducibility of dose delivery to the clinical target volume. Our results indicate that four MVCT sessions on helical tomotherapy are sufficient to provide information for the creation of personalised target margins and the establishment of the new reference position that accounts for the systematic error. This simplified approach reduces overall treatment session time and decreases the imaging dose to the patient.

Computed tomographic assessment of radiation induced damage in the lung of normal and WR 2721 protected LAF1 mice

LAF1 mice were irradiated with single, graded doses of X rays to the thorax in the range of 0 to 14 Gy unprotected, or 0 to 18 Gy after injecting the radioprotective aminothiol compound, WR 2721. Computed tomographic (CT) scanning of the thorax was performed at intervals for a period of 42 weeks after irradiation. The gravimetric density was determined for both left and right lungs by averaging the CT numerical data within lung slices traced on a magnified video image of the thorax. Significant elevations in CT density occurred at post-irradiation times corresponding to pneumonitis and late phase, as evidenced by the pneumopathic decline in survival. The threshold dose yielding a significant increase in CT density in the pneumonitis phase was 11 Gy, a dose at which only 3% of the animals died. A single peak of increased CT density was observed for the pneumonitis phase for unprotected animals, whereas a transient return of CT density toward control values at 21-22 weeks produced two peaks from the WR 2721 treated group. The CT density of lung increased in a stepwise manner in the dose range of 11-14 Gy. For the isoeffect dose that produced equal animal survival (14 Gy and 18 Gy + WR 2721), the lung density increased by approximately 27% over control values for both treatments, suggesting that CT density is related to survival. Periodic computed tomographic analysis of the lungs of patients sustaining radiotherapy to large pulmonary fields may be of value in assessing the degree and progression of pulmonary complications.

Accuracy of lung dose calculations for large-field irradiation with 6-MV x rays.

In large-field irradiations of the upper half-body, there is a potential for severe respiratory complications. The incidence of severe radiation damage to lung can be reduced by limiting the lung dose and the volume of lung irradiated. It is necessary to have an accurate method of calculating the dose actually delivered to lung, and this work deals with such calculations and measurements in large (half-body) fields of 6-MV x rays. The results of various lung dose calculations by the Batho, the modified Batho (Lulu and Bjärngard), the simple scaled tissue-maximum ratio and the detailed equivalent tissue-air ratio methods are compared with doses measured in phantoms representing the mediastinum and adjacent lungs.

Computerized tomography versus perfusion lung scanning in canine radiation lung injury

Computerized tomographic (CT) measurements of lung density were obtained before and serially after thoracic irradiation in dogs to detect the alterations caused by radiation therapy. Fourteen mongrel dogs were given either 2000 cGy (Group A, 10 dogs, right lower zone irradiation), 1000 cGy (Group B, 2 dogs, right lower zone irradiation), or 500 cGy (Group C, 2 dogs, right lung irradiation) in one fraction. Once before and bi-weekly after irradiation, the anesthetized dogs had thoracic CT scans. CT numbers for the irradiated area were compared to their preirradiation control values. Macro-aggregated albumin (MAA) perfusion lung scans were also obtained before and at weekly intervals after irradiation and were evaluated visually and quantitatively for abnormalities. When both these tests were abnormal, or at the end of the scheduled study, the dogs were sacrificed to confirm radiation lung injury histologically. Our results showed that CT numbers (as a measure of tissue density) were higher with higher doses of radiation. Among all the techniques used, only the quantitative assessment of macro-aggregated albumin perfusion scan detected abnormalities in all the dogs given 2000 cGy. Their abnormalities correlated well with the presence of radiation lung damage histologically, however, the applicability of these methods in the detection of early injury has to be further evaluated.

TU‐C‐BRB‐07: Medical Physics Staffing for Radiation Treatment: A Robust Algorithm with Trans‐Canada Validation

Purpose: To describe an algorithm for determining appropriate physics staffing for radiation treatment. Motivation for this work came from the age of current guidelines which predate the recent evolution in techniques and technology, and also several significant adverse incidents where a lack of physics staffing was identified as a contributing factor to excessive radiation exposure of patients. Methods: Guided by published times required per procedure, we developed an algorithm adaptable to local practice which estimates staffing requirements for medical physics with parameters derived from clinical procedures and service workload, equipment inventory, training, clinical development and administration. The predictive power was evaluated using data from 32 Canadian centres. This algorithm was used to model staffing requirements for the next 10 years to aid regional, institutional and educational program planning with consideration given to the “4Rs” of human resources planning: Requirements, Recruitment, Retention and Residency. Results: For centre‐specific human resource planning, we propose a grid of coefficients addressing specific workload factors for each group. For larger scale planning, case‐based ratios were determined at 260, 300 and 600 annual radiotherapy cases for medical physicists,dosimetrists and electronics technologists respectively. Assuming a 2.5% growth in incidence of cancer and stable utilisation, our supply and demand model predicts a requirement for an additional 39 medical physicists for Ontario by the year 2020. If an additional 3% annual growth in radiation therapy utilisation is included, the number rises to 87. Conclusions: We describe a robust algorithm to determine medical physics staffing levels adaptable to centre‐specific workload and evolving local radiation treatment practice. Although annual caseload has been used in the past as a major parameter for global physics staffing determination, our results indicate that local clinical services and equipment as well as academic activity cause significant deviations from predictions based solely on caseload.