In vivo dosimetry during external photon beam radiotherapy

In-vivo dosimetry by diode semiconductors in combination with portal films during TBI: reporting a 5-year clinical experience

BACKGROUND AND PURPOSE

In-vivo dosimetry is vital to assure an accurate delivery of total body irradiation (TBI). In-vivo lung dosimetry is strongly recommended because of the risk of radiation-induced interstitial pneumonia (IP). Here we report on our 5-year experience with in-vivo dosimetry using diodes in combination with portal films and assessing the effectiveness of in-vivo dosimetry in improving the accuracy of the treatment. Moreover, we wished to investigate in detail the possibility of in-vivo portal dosimetry to yield individual information on the lung dose and to evaluate the impact of CT planning on the correspondence between stated and in-vivo measured doses.

MATERIALS AND METHODS

From March 1994 to March 1999, 229 supine-positioned patients were treated at our Institute with TBI, using a 6 MV X-rays opposed lateral beam technique. 146 patients received 10 Gy given in three fractions, once a day (FTBI), shielding the lungs by the arms; 70 received 12-13.2 Gy, given in 6-11 fractions, 2-3 fractions per day (HFTBI): in this case about 2/3 of the lungs were shielded by moulded blocks (mean shielded lung dose equal to 9 or 9.5 Gy). Thirteen patients received 8 Gy given in a single fraction (SFTBI, lung dose: 7 Gy). For all HFTBI and FTBI patients, midline in-vivo dosimetry was performed at the first fraction by positioning two diodes pairs (one at entrance and one at the exit side) at the waist (umbilicus) and at the pelvis (ankles). If at least one of the two diodes doses (waist-pelvis) was outside +/-5% from the prescribed dose, actions could be initiated, together with possible checks on the following fractions. Transit dosimetry by portal films was performed for most patients; for 165 of them (117 and 48, respectively for FTBI and HFTBI) the midline in-vivo dose distribution of the chest region was derived and mean lung dose assessed. As a CT plan was performed for all HFTBI patients, for these patients, the lung dose measured by portal in-vivo dosimetry was compared with the expected value.

RESULTS

Concerning all diodes data, 528 measurements were available: when excluding the data of the first fraction(s) of the patients undergoing corrections (n = 392), mean and SD were respectively 0.0% and 4.5% (FTBI: -0.3 +/- 4.8%; HFTBI: 0.4 +/- 3.9%). In total 105/229 patients had a change after the first fraction and 66/229 were controlled by in-vivo dosimetry for more than one fraction. Since January 1998 a CT plan is performed for FTBI patients too: when comparing the diodes data before and after this date, a significant improvement was found (i.e. rate of deviations larger than 5% respectively equal to 30.7% and 13.1%, P = 0.007). When considering only the patients with a CT plan, the global SD reduced to 3.5%. Concerning transit dosimetry data, for FTBI, the mean (midline) lung dose was found to vary significantly from patient to patient (Average 9.13 +/- 0.81 Gy; range 7.4-11.4 Gy); for the HFTBI patients the mean deviation between measured and expected lung dose was 0.0% (1 SD = 3.8%).

CONCLUSIONS

In vivo dosimetry is an effective tool to improve the accuracy of TBI. The impact of CT planning for FTBI significantly improved the accuracy of the treatment delivery. Transit dosimetry data revealed a significant inter-patient variation of the mean lung dose among patients undergoing the same irradiation technique. For patients with partial lung shielding (HFTBI), an excellent agreement between measured and expected lung dose was verified.

[Conformal radiotherapy: principles and classification]

'Conformal radiotherapy' is the name fixed by usage and given to a new form of radiotherapy resulting from the technological improvements observed during, the last ten years. While this terminology is now widely used, no precise definition can be found in the literature. Conformal radiotherapy refers to an approach in which the dose distribution is more closely 'conformed' or adapted to the actual shape of the target volume. However, the achievement of a consensus on a more specific definition is hampered by various difficulties, namely in characterizing the degree of 'conformality'. We have therefore suggested a classification scheme be established on the basis of the tools and the procedures actually used for all steps of the process, i.e., from prescription to treatment completion. Our classification consists of four levels: schematically, at level 0, there is no conformation (rectangular fields); at level 1, a simple conformation takes place, on the basis of conventional 2D imaging; at level 2, a 3D reconstruction of the structures is used for a more accurate conformation; and level 3 includes research and advanced dynamic techniques. We have used our personal experience, contacts with colleagues and data from the literature to analyze all the steps of the planning process, and to define the tools and procedures relevant to a given level. The corresponding tables have been discussed and approved at the European level within the Dynarad concerted action. It is proposed that the term 'conformal radiotherapy' be restricted to procedures where all steps are at least at level 2.

In vivo dosimetry during conformal radiotherapy: requirements for and findings of a routine procedure

PURPOSE

Conformal radiotherapy requires accurate knowledge of the actual dose delivered to a patient. The impact of routine in vivo dosimetry, including its special requirements, clinical findings and resources, has been analysed for three conformal treatment techniques to evaluate its usefulness in daily clinical practice.

MATERIALS AND METHODS

Based on pilot studies, routine in vivo dosimetry quality control (QC) protocols were implemented in the clinic. Entrance and exit diode dose measurements have been performed during two treatment sessions for 378 patients having prostate, bladder and parotid gland tumours. Dose calculations were performed with a CT-based three-dimensional treatment planning system. In our QC-protocol we applied action levels of 2.5% for the prostate and bladder tumour group and 4.0% for the parotid gland patients. When the difference between the measured dose at the dose specification point and the prescribed dose exceeded the action level the deviation was investigated and the number of monitor units (MUs) adjusted. Since an accurate dose measurement was necessary, some properties of the on-line high-precision diode measurement system and the long-term change in sensitivity of the diodes were investigated in detail.

RESULTS

The sensitivity of all diodes decreased by approximately 7% after receiving an integrated dose of 10 kGy, for 4 and 8 MV beams. For 34 (9%) patients the difference between the measured and calculated dose was larger than the action level. Systematic errors in the use of a new software release of the monitor unit calculation program, limitations of the dose calculation algorithms, errors in the planning procedure and instability in the performance of the accelerator have been detected.

CONCLUSIONS

Accurate in vivo dosimetry, using a diode measurement system, is a powerful tool to trace dosimetric errors during conformal radiotherapy in the range of 2.5-10%, provided that the system is carefully calibrated. The implementation of an intensive in vivo dosimetry programme requires additional staff for measurements and evaluation. The patient measurements add only a few minutes to the total treatment time per patient and guarantee an accurate dose delivery, which is a prerequisite for conformal radiotherapy.

Build-up modification of commercial diodes for entrance dose measurements in 'higher energy' photon beams

BACKGROUND AND PURPOSE

Several commercially available p-type diodes do not provide sufficient build-up for in-vivo dosimetry in 'higher' energy photon beams, and only limited information could be found in the literature describing the correction factor variation and/or the achievable accuracy for in-vivo dosimetry methods in this energy range. The first aim of this study is to assess and analyze the variation of diode correction factors for entrance dose measurements at higher photon energies. In a second step the total build up thickness of the diode has been modified in order to minimize the correction factor variation.

MATERIALS AND METHODS

Diode correction factors accounting for non-reference conditions (field size, source surface distance, tray, wedge, and block) are determined in 18-25 MV photon beams provided by different treatment units for Scanditronix p-type diodes recommended for higher energy photon beams: old type and new type EDP-20, and EDP-30 diodes. Hemispherical build-up caps of different materials (copper, iron, lead) are used to increase the total build-up thickness. Perturbation effects with and without additional build-up caps are assessed for the three diode types.

RESULTS

For unmodified diodes field size correction factors (C(FS)) vary between 1.7% and 6%, dependent on diode type and treatment unit. For example, for an old type EDP-20 the C(FS) variation at 18 MV is much higher on a GE linac (5%) as compared to the Philips machine (1.7%). Depending on diode type, this variation can be reduced to 1-2% when adding additional build-up. The variation of source to surface distance correction factors is almost independent of build-up thickness. By adding additional build-up the influence of trays and blocks can be almost eliminated.

CONCLUSIONS

The correction factor variation of unmodified diodes reflects the variation of the electron contamination with treatment geometry. A total build-up thickness of 30 mm is found to be the 'best compromise' for the three types of diodes investigated when measuring entrance doses in the energy range between 18 and 25 MV.