Peter J. Biggs

AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, N(60Co)D,w which are traceable to national primary standards, and the equation D(Q)w = MkQN(60Co)D,w where Q is the beam quality of the clinical beam, D(Q)w is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a beam of quality Q. Values of kQ are presented as a function of Q for many ion chambers. The value of M is given by M = PionP(TP)PelecPpolMraw, where Mraw is the raw, uncorrected ion chamber reading and Pion corrects for ion recombination, P(TP) for temperature and pressure variations, Pelec for inaccuracy of the electrometer if calibrated separately, and Ppol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10)x, the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10 cm2 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R50, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size > or =10x10 cm2 on the surface of the phantom (> or =20x20 cm2 for R50>8.5 cm) at an SSD of 100 cm. R50 is determined directly from the measured value of I50, the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and 0.6R50-0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10x10 cm2 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of > or =10x10 cm2 (> or =20x20 cm2 for R50>8.5 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPMs TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.

Advanced prostate cancer: The results of a randomized comparative trial of high dose irradiation boosting with conformal protons compared with conventional dose irradiation using photons alone☆

PURPOSE Following a thorough Phase I/II study, we evaluated by a Phase III trial high versus conventional dose external beam irradiation as mono-therapy for patients with Stage T3-T4 prostate cancer. Patient outcome following standard dose radiotherapy or following a 12.5% increase in total dose to 75.6 Cobalt Gray Equivalent (CGE) using a conformal perineal proton boost was compared for local tumor control, disease-free survival, and overall survival. METHODS AND MATERIALS Stage T3-T4, Nx, N0-2, M0 patients received 50.4 Gy by four-field photons and were randomized to receive either an additional 25.2 CGE by conformal protons (arm 1--the high dose arm, 103 patients, total dose 75.6 CGE) or an additional 16.8 Gy by photons (arm 2--the conventional dose arm, 99 patients, total dose 67.2 Gy). Actuarial overall survival (OS), disease-specific survival (DSS), total recurrence-free survival (TRFS), (clinically free, prostate specific antigen (PSA) less than 4ng/ml and a negative prostate rebiopsy, done in 38 patients without evidence of disease) and local control (digital rectal exam and rebiopsy negative) were evaluated. RESULTS The protocol completion rate was 90% for arm 1 and 97% for arm 2. With a median follow-up of 61 months (range 3 to 139 months) 135 patients are alive and 67 have died, 20 from causes other than prostate cancer. We found no significant differences in OS, DSS, TRFS or local control between the two arms. Among those completing randomized treatment (93 in arm 1 and 96 in arm 2), the local control at 5 and 8 years for arm 1 is 92% and 77%, respectively and is 80% and 60%, respectively for arm 2 (p = .089) and there are no significant differences in OS, DSS, and TRFS. The local control for the 57 patients with poorly differentiated (Gleason 4 or 5 of 5) tumors at 5 and 8 years for arm 1 is 94% and 84% and is 64% and 19% on arm 2 (p = 0.0014). In patients whose digital rectal exam had normalized following treatment and underwent prostate rebiopsy there was a lower positive rebiopsy rate for arm 1 versus arm 2 patients (28 vs. 45%) and also for those with well and moderately differentiated tumors versus poorly differentiated tumors (32 and 50%). These differences were not statistically significant. Grade 1 and 2 rectal bleeding is higher (32 vs. 12%, p = 0.002) as may be urethral stricture (19 vs. 8%, p = 0.07) in the arm 1 versus arm 2. CONCLUSIONS An increase in prostate tumor dose by external beam of 12.5% to 75.6 CGE by a conformal proton boost compared to a conventional dose of 67.2 Gy by a photon boost significantly improved local control only in patients with poorly differentiated tumors. It has increased late radiation sequelae, and as yet, has not increased overall survival, disease-specific survival, or total recurrence-free survival in any subgroup. These results have led us to test by a subsequent Phase III trial the potential beneficial effect on local control and disease-specific survival of a 12.5% increase in total dose relative to conventional dose in patients with T1, T2a, and T2b tumors.

Long-term Results of Intraoperative Electron Beam Irradiation (IOERT) for Patients With Unresectable Pancreatic Cancer

Summary Background Data:To analyze the effects of a treatment program of intraoperative electron beam radiation therapy (IOERT) and external beam radiation therapy and chemotherapy on the outcome of patients with unresectable or locally advanced pancreatic cancer. Methods:From 1978 to 2001, 150 patients with unresectable and nonmetastatic pancreatic cancer received IOERT combined with external beam radiation therapy and 5-fluorouracil–based chemotherapy for definitive treatment. Results:The 1-, 2-, and 3-year actuarial survival rates of all 150 patients were 54%, 15%, and 7%, respectively. Median and mean survival rates were 13 and 17 months, respectively. Long-term survival has been observed in 8 patients. Five patients have survived beyond 5 years and 3 more between 3 and 4 years. There was a statistically significant correlation of survival to the diameter of treatment applicator (a surrogate for tumor size) used during IOERT. For 26 patients treated with a small-diameter applicator (5 cm or 6 cm), the 2- and 3-year actuarial survival rates were 27% and 17%, respectively. In contrast, none of the 11 patients treated with a 9-cm-diameter applicator survived beyond 18 months. Intermediate survival rates were seen for patients treated with a 7- or 8-cm-diameter applicator. Operative mortality was 0.6%, and postoperative and late complications were 20% and 15%, respectively. Conclusions:A treatment strategy employing IOERT has resulted in long-term survival in 8 of 150 patients with unresectable pancreatic cancer. Survival benefit was limited to patients with small tumors. Enrollment of selected patients with small tumors into innovative protocols employing this treatment approach is appropriate.

A new miniature x-ray device for interstitial radiosurgery: dosimetry.

A miniature, battery operated 40 kV x-ray device has been developed for the interstitial treatment of small tumors ( < 3 cm diam) in humans. X rays are emitted from the tip of a 10 cm long, 3 mm diameter probe that is stereotactically inserted into the tumor. The beam, characterized by half-value layer (HVL), spectrum analysis, and isodose contours, behaves essentially as a point isotropic source with an effective energy of 20 keV at a depth of 10 mm in water. The absolute output from the device was measured using a parallel plate ionization chamber, modified with a platinum aperture. The dose rate in water determined from these chamber measurements was found to be nominally 150 cGy/min at a distance of 10 mm for a beam current of 40 microA and voltage of 40 kV. The dose in water falls off approximately as the third power of the distance. To date, 14 patients have been treated with this device in a phase I clinical trial.

Electrons as the cause of the observed dmax shift with field size in high energy photon beams

For megavoltage x-ray beams, it is well known that the percent depth-dose increases considerably with field size in the buildup region, resulting in a significant shift in the apparent position of maximum dose, dmax. The nature of this increase has been investigated using a sweeping magnet placed just below the treatment head of a 25-MV linac. Measurements show that for increasing magnetic fields the dose in the buildup region is continually reduced, until a point is reached beyond which no additional reduction is observed. Here the buildup curve is essentially field size independent. These results clearly show that electrons are the primary cause of dose increase with field size in the buildup region, in contrast to a recent publication claiming that scattered photons are the cause. Further measurements were made by blocking out the primary beam at the level of the jaws and measuring the depth dose of the scattered electrons originating from the jaws. The results show that a thickness of approximately 1 gcm-2, of either polystyrene or lead, reduces the dose by a factor of two, providing further evidence that the scattered component of the beam consists of low energy electrons.

Intraoperative irradiation: A pilot study combining external beam photons with “boost” dose intraoperative electrons

Intraoperative “boost” dose electron beam therapy given in combination with 4500‐5000 rad (45–50 Gray) external beam irradiation has been demonstrated as a practical therapeutic modality at the MGH. This procedure has been employed thus far in 58 patients; the results in the initial 36 are analyzed in detail in this paper. Thirty‐four of the 36 patients had locally advanced lesions—unresectable, recurrent, or residual disease. Results achieved to date are in full agreement with our expectations: high radiation doses have been delivered to the primary intra‐abdominal and pelvic tumors, excluding the sensitive structures from irradiation. This has been accomplished by a truly multidisciplinary effort comprising surgery, anesthesiology, OR nursing, administration, engineers, physicists, therapy technologists, and radiation therapists. Although follow‐up is not yet sufficient to judge ultimate efficacy, acute and chronic severe morbidity is low and local control is good. There is justified enthusiasm for continuing the procedure.

Sources of electron contamination for the Clinac‐35 25‐MV photon beam

A detailed Monte Carlo approach has been employed to investigate the sources of electron contamination for the 25-MV photon beam generated by Varians Clinac-35. Three sources of contamination were examined: (a) the flattening filter and beam monitor chamber, (b) the fixed primary collimators downstream from the monitor chamber and the adjustable photon jaws, and (c) the air volume separating the treatment head from the observation point. Five source-to-surface distances (SSDs) were considered for a single field size, 28 cm in diameter at 80 cm SSD. It was found that for small SSDs (80-100 cm), the dominant sources of electron contamination were the flattening filter and the beam monitor chamber which accounted for 70% of the unwanted electrons. Thirteen percent of the remaining electrons originated in the downstream primary collimators and the photon jaws, and 17% were produced in air. At larger SSDs, the fraction of unwanted electrons originating in air increased. At 400 cm SSD, 61% of the contaminating electrons present in the beam were produced in air, 34% originated in the flattening filter and beam monitor chamber, and 5% were due to interactions in the fixed collimators downstream from the monitor chamber and the adjustable photon jaws. These calculated results are substantiated by recent experiments.

An investigation into the presence of secondary electrons in megavoltage photon beams (radiotherapy application)

The presence of secondary electrons in photon beams of 60Co and 4, 8, 10, 15 and 25 MV x-rays has been studied by measuring surface charge using thin window ionisation chambers. Measurements have been made for square fields from 4 X 4 cm2 to 35 X 35 cm2 for locations from the collimator head to a distance of 4 m from the target. In addition, measurements have been made for rectangular fields at 10 MV and 25 MV for fields of equivalent area from 5 X 5 cm2 to 25 X 25 cm2. By eliminating the inverse square effect, the presence of contaminants from the head and the effect of build-up in air are clearly seen and well separated. Comparison between the curves at different energies indicates an increasing effect of contamination from the head with energy and a decreasing effect of electron production in air with increasing energy.

Intraoperative electron beam radiation therapy: technique, dosimetry, and dose specification: report of task force 48 of the Radiation Therapy Committee, American Association of Physicists in Medicine.

Intraoperative radiation therapy (IORT) is a treatment modality whereby a large single dose of radiation is delivered to a surgically open, exposed cancer site. Typically, a beam of megavoltage electrons is directed at an exposed tumor or tumor bed through a specially designed applicator system. In the last few years, IORT facilities have proliferated around the world. The IORT technique and the applicator systems used at these facilities vary greatly in sophistication and design philosophy. The IORT beam characteristics vary for different designs of applicator systems. It is necessary to document the existing techniques of IORT, to detail the dosimetry data required for accurate delivery of the prescribed dose, and to have a uniform method of dose specification for cooperative clinical trials. The specific charge to the task group includes the following: (a) identify the multidisciplinary IORT team, (b) outline special considerations that must be addressed by an IORT program, (c) review currently available IORT techniques, (d) describe dosimetric measurements necessary for accurate delivery of prescribed dose, (e) describe dosimetric measurements necessary in documenting doses to the surrounding normal tissues, (f) recommend quality assurance procedures for IORT, (g) review methods of treatment documentation and verification, and (h) recommend methods of dose specification and recording for cooperative clinical trials.

Intraoperative radiation therapy using mobile electron linear accelerators: Report of AAPM Radiation Therapy Committee Task Group No. 72

Intraoperative radiation therapy (IORT) has been customarily performed either in a shielded operating suite located in the operating room (OR) or in a shielded treatment room located within the Department of Radiation Oncology. In both cases, this cancer treatment modality uses stationary linear accelerators. With the development of new technology, mobile linear accelerators have recently become available for IORT. Mobility offers flexibility in treatment location and is leading to a renewed interest in IORT. These mobile accelerator units, which can be transported any day of use to almost any location within a hospital setting, are assembled in a nondedicated environment and used to deliver IORT. Numerous aspects of the design of these new units differ from that of conventional linear accelerators. The scope of this Task Group (TG-72) will focus on items that particularly apply to mobile IORT electron systems. More specifically, the charges to this Task Group are to (i) identify the key differences between stationary and mobile electron linear accelerators used for IORT, (ii) describe and recommend the implementation of an IORT program within the OR environment, (iii) present and discuss radiation protection issues and consequences of working within a nondedicated radiotherapy environment, (iv) describe and recommend the acceptance and machine commissioning of items that are specific to mobile electron linear accelerators, and (v) design and recommend an efficient quality assurance program for mobile systems.